SECTION III.. The Nervous System.

THE NERVOUS SYSTEM.

THE NERVOUS SYSTEM.

LITERATURE.

*Ahlborn, F.*, Ueber die Bedeutung der Zirbeldrüse. Zeit. f. wiss. Zool. 1884. Vol. XL, pp. 331–337.

*Allen, H.*, The Spinal Cord in Batrachia and Reptilia. Proc. Acad. Nat. Sci. Philadelphia. 1883, pp. 56–57.

*Arndt, A. W.*, Untersuchungen über die Ganglienkörper des Nervus sympathicus. Arch. f. mikrosk. Anat. 1874. Vol. X, pp. 208–241.

*Arndt, R.*, Untersuchungen über die Endigung der Nerven in den quergestreiften Muskelfasern. Arch. f. mikrosk. Anat. 1873. Vol. IX, p. 481.

*Arndt, R.*, Untersuchungen über die Ganglienkörper der Spinalganglien. Arch. f. mikrosk. Anat. 1875. Vol. XI, p. 140.

*Arnold, J.*, Zur Histologie der Lungen des Frosches. Virchow’s Arch. 1863. Vol. XXVIII, p. 433.

*Arnold, J.*, Histologische Verhältnisse des Frosch-Sympathicus. Centralbl. f. d. med. Wiss. 1864, p. 657.

*Arnold, J.*, Ueber die feineren histologischen Verhältnisse der Ganglienzellen in dem Sympathicus des Frosches. Virchow’s Arch. 1865. Vol. XXXII, p. 1.

*Arnold, J.*, Die Spinalfasern im Sympathicus des Frosches. Arch. f. Anat. und Physiol. 1866, p. 398.

*Arnold, J.*, Das Gewebe der organischen Muskeln. Leipzig, 1869; and Chap. IV in Stricker’s Handbook.

*Arnstein, C.*, and *Gonjaew*, *K.*, Ueber die Nerven des Verdauungskanals. Bericht. f. Physiol. u. Histologie. Mittheilung. aus. d. 4 Vers. wissensch. Naturforsch. zu Kasan. Pflüger’s Arch. d. ges. Physiol. 1874. Vol. VIII, pp. 614–615.

*Aubert, H.*, Die Innervation der Kreislaufsorgane, Hermann’s Handbuch der Physiologie. Leipzig, 1880. Vol. IV, Pt. I, p. 377.

*Axmann*, De Gangliorum systematicis structura persitiori eiusque functionibus. Berolini, 1847.

*Axmann*, Beiträge zur mikroskopischen Anatomie und Physiologie des Ganglien-nervensystems. 1853, p. 20.

*Baculo, B.*, Nuove ricerche intorno l’apparechio ganglionare intrinseco dei cuori linfatici. Naples, 1885.

*Balfour, F. M.*, Treatise on Comparative Embryology. London, 1880.

*Balfour, F. M.*, Handbuch der vergleichenden Embryologie. Aus dem Englischen von B. Vetter. Jena, 1880.

*Beale, L. S.*, On the Structure and Formation of the so-called Apolar, Unipolar, and Bipolar Nerve-cells of the Frog. Phil. Trans. 1863, p. 543. (Hyla arborea.)

*Beale, L. S.*, Further observations in favour of the view that nerve-fibres never end in voluntary muscle. Proc. Roy. Soc. 1863. Abstract in Quart. Journ. Micros. Sci. 1863. Vol. XII, p. 668.

*Beale, L. S.*, New observations upon the structure, etc. of certain nervous centres. Proc. Roy. Soc. 1860. Vol. III.

*Beale, L. S.*, On the Distribution of Nerves to the elementary fibres of Striped Muscle. Phil. Trans. 1864, p. 611.

*Beale, L. S.*, Of very fine nerve-fibres ramifying in certain fibrous tissues and trunks, and plexuses consisting entirely of very fine nerve-fibres in the bladder of the frog. Beale’s Archives of Medicine, 1864. Vol. IV, pp. 19–251.

*Beard, J.*, The Ciliary or Motor-oculi ganglion and the Ganglion of the ophthalmicus profundus in Sharks. Anatom. Anzeiger. 1887. Vol. II, p. 565.

*Beck, K.*, Zur Kenntniss der Herznerven. Arch. f. mik. Anat. 1885. Vol. XXIV, pp. 11–19.

*Bellonci, G.*, Intorno alla struttura e alle connessioni dei lobi olfattorii negli arthropodi superiori e nei vertebrati. Atti Accad. Lincei Ann. 279 Mem. Accad. Bologna, Vol. XIII, pp. 555–564: and in Arch. Ital. Biol. 1883; Vol. III, pp. 191–196.

*Bellonci, G.*, Sulla terminazione centrale del nervo ottico nei mammiferi. Mem. Accad. Bologna, Vol. VI, pp. 199–205: and in Arch. Ital. Biol. 1884, Vol. VI, pp. 405–411.

*Bidder, F. H.*, Die Endigungsweise der Herzzweige des N. vagus beim Frosch. Arch. f. Anat. u. Physiol. 1839, p. 1.

*Bidder, F. H.*, Zur Lehre von dem Verhältniss der Ganglienkörper zu den Nervenfasern. Nebst einem Anhange von Dr. A. W. Volkmann. Leipzig, 1847.

*Bidder, F. H.*, Zur Lehre vom Verhalten der Ganglienkörper, u. s. w. Leipzig, 1847.

*Bidder, F. H.*, Ueber functionelle verschiedene und räumlich getrennte Nerven-centra im Froschherzen. Arch. f. Anat. u. Physiol. 1852, p. 163.

*Bidder, F. H.*, Endigungsweise der Herzzweige des N. vagus beim Frosch. Arch. f. Anat. und Physiol. 1868, p. 1.

*Bidder, F. H.*, Zur näheren Kenntniss des Froschherzens und seiner Nerven. Arch. f. Anat. u. Physiol. 1866, p. 1.

*Bidder, F. H.*, and *Kupffer*, Untersuchungen über die Textur des Rückenmarks. Leipzig, 1857.

*Bidder, F. H.*, and *Volkmann*, *A. W.*, Die Selbständigkeit des sympathischen Nervensystems, durch anatomische Untersuchungen nachgewiesen. Leipzig, 1842.

*Bidder* and *Gregory*, Beiträge zur Physiologie der Herzbewegung beim Frosche. Dorpat, 1865.

*Bikfalvi, K.*, Ueber die Hornscheide der markhaltigen Nervenfasern. Orvoster-meszettudomanyi Ertesito, 1884, p. 133. Abstract in Centralbl. f. d. med. Wiss. 1886, p. 34.

*Biroege, E. A.*, Die Zahl der Nervenfasern u. der motor. Ganglienzellen im Rückenmark des Frosches. Arch. f. Anat. u. Physiol. 1882. Physiol. Abth., p. 435.

*Bischoff*, Nervi accessorii Willisii anatomia et physiologia. Heidelberg, 1832.

*Blasius, G.*, Gerardi Blasii Anatomia Animalium. Amstelodami, 1681.

*Blattmann, A.*, Mikrosk. anatom. Darstellung der Centralorgane des Nervensystems bei den Batrachiern. Dissert. Zürich, 1850.

*Boll, F.*, Studj sulle immagini microscopiche della fibra nervosa midollare. Atti della R. accademia dei Lincei, 1876–1877. Vol. II.

*Boll, F.*, Ueber Zersetzungsbilder der markhaltigen Nervenfasern. Arch. f. Anat. u. Physiol. 1877, p. 288.

*Bojanus*, Testudinis anatomia. 1819.

*Bosse*, De ganglior. spinal. vi in nutr. radic. poster. nervor. spinal. Dissert. 1859.

*Boveri, T.*, Beiträge zur Kenntniss der Nervenfasern. Abhandl. d. bayr. Akad. d. Wiss. 1885. Vol. XV, p. 421.

*Bremer, D. L.*, Die Nerven der Capillaren, der kleinen Arterien und Venen. Arch. f. mik. Anat. Vol. XXI. 1882, p. 663.

*Bremer, L.*, Ueber die Muskelspindeln sowie über Structur, Neubildung u. Innervation der quergestreiften Muskelfaser. Waldeyer’s Arch. 1883. Vol. XXII, p. 2.

*Bremer, L.*, Ueber die Endigungen der markhaltigen und marklosen Nerven im quergestreiften Muskel. Arch. f. mikrosk. Anat. 1882. Vol. XXI, p. 165.

*Brenner, A.*, Ueber das Verhältniss des nervus laryngeus inferior vagi zu einigen Aortenvarietäten des Menschen und zu dem Aortensystem der durch Lungen athmenden Wirbelthiere überhaupt. His u. Braune’s Arch. 1883, pp. 373–397.

*Budge, J.*, Ueber den Verlauf der Nervenfasern im Rückenmark des Frosches. Arch. f. Anat. u. Physiol. 1844, p. 160.

*Budge, J.*, Ueber die Bewegung der Iris. (Concerning ganglia of motor-oculi, p. 36.)

*Budge, J.*, Wagner’s Handwörterb. d. Physiologie. 1846. Vol. III, p. 451.

*Burdon-Sanderson, J.*, Circulation of the Blood. Handbook for the Physiological Laboratory. London, 1873.

*Calberla, E.*, Ueber die Endigungsweise der Nerven in den quergestreiften Muskeln bei Amphibien. Zeit. f. wiss. Zool. 1874. Vol. XXIV, p. 164.

*Calberla, E.*, Studien über die Entwicklung der quergestreiften Muskeln und Nerven der Amphibien und Reptilien. Arch. f. mik. Anat. 1875. Vol. XI, p. 442.

*Carus*, Versuch einer Darstellung des Nervensystems und Gehirns. Leipzig, 1814.

*Champness, F.*, The septum atrium of the Frog and Rabbit. Journ. of Anat. and Physiol. 1874.

*Christmas-Dirckinck-Holmfeld, J.*, Experimentaluntersuchungen über den Bau der Regio olfactoria. Nord. med. ark. 1883. Vol. XV, No. 3.

*Ciaccio, G. V.*, On the nerves of the Cornea, and their distribution in the corneal tissue of man and animals. Proc. Roy. Soc. 1863. Abstract in Trans. Micros. Soc. London, 1863. Vol. XI, pp. 77–93.

*Ciaccio, G. V.*, On the distribution of nerves to the skin of the frog, with physiological remarks on the ganglia connected with the cerebro-spinal nerves. Quart. Journ. Micros. Sci. 1864. Vol. IV, pp. 15–31.

*Ciaccio, G. V.*, Intorno alla minuta fabbrica delle pelle Rana esculenta. Giornale di Scienze naturali ed economiche. Palermo, 1866. Vol. II, p. 103.

*Clarke, J. Lockhart*, Ueber den Bau des Bulbus olfactorius und der Geruchsschleimhaut. Zeit. f. wiss. Zool. 1862. Vol. XI, p. 31.

*Cornil* and *Ranvier*, A Manual of Pathological Histology. London, 1880, p. 359, Fig. 199 (Ganglia of heart).

*Courvoisier, G.*, Beobacht. über den sympathischen Gränzstrang. Arch. f. mik. Anat. 1866. Vol. II, p. 13.

*Courvoisier, L. G.*, Ueber die spinalen und sympathischen Zellen des Frosches. Centralbl. f. d. med. Wiss. 1867, p. 897.

*Courvoisier, G.*, Ueber die Zellen der Spinalganglien, sowie des Sympathicus beim Frosch. Arch. f. mikrosk. Anat. 1868. Vol. IV, p. 125.

*Cuvier*, Leçons d’anatomie comparée. 2nd Edit. Vol. II.

*Cyon*, Ueber die Nerven des Peritoneum. Berichte über die Verhandlungen d. königl. sächs. Gesellsch. d. Wissensch. zu Leipzig. Math.-physik. Classe. 1868. Vol. XX, p. 119.

*Czermak, J. N.*, Ueber die Hautnerven des Frosches. Arch. f. Anat. u. Physiol. 1849, p. 252. Leipzig, 1869.

*Czermak, J. N.*, Ueber die Herznerven des Frosches, Rana temporaria. Arch. f. Anat. u. Physiol. 1851, p. 160.

*v. Darkschewitsch, L.*, Zur Anatomie der Glandula pinealis. Neurol. Cent. Bl. 1886. Vol. V, p. 29.

*v. Deen*, De differentia et nexu inter nervos vitae animalis et vitae organicae. Diss. inaugur. Lugduni-Bat., 1834.

*v. Deen*, Over de zijdelingsche Takken dar swervende zenuw van den Proteus anguineus, aus Bijdragen tot de Naturkundige. Wettenschappen, 1834.

*v. Deen*, Ueber den Ramus lateralis n. vagi bei den Batrachiern. Arch. f. Anat. u. Physiol. 1834, p. 477 (abstract from Diss. inaugur. De differentia et nexu inter nervos vitae animalis et vitae organicae). Lugduni-Bat.

*Deiters, O.*, Untersuchungen über Gehirn und Rückenmark. Braunschweig, 1865.

*Denissenko, G.*, Zur Frage über den Bau der Kleinhirnrinde bei verschiedenen Klassen von Wirbelthieren. Arch. f. mikrosk. Anat. 1877. Vol. XIV, p. 203.

*Dietl, M. J.*, Beobachtungen über Theilungsvorgänge an Nervenzellen. Wiener Sitzungsber. 1874. Vol. LXIX, Pt. III, p. 71.

*Dietl, M. J.*, Casuistische Beiträge zur Morphologie der Nervenzellen. Wiener Sitzungsber. 1874. Vol. LXIX, Pt. III, p. 80.

*Dogiel, J.*, Die Ganglienzellen und Nerven des Herzventrikels beim Frosche. Arch. f. mikrosk. Anat. 1877. Vol. XIV, p. 470.

*Dogiel, J.*, Die Ganglienzellen des Herzens bei verschiedenen Thieren und beim Menschen. Arch. f. mikrosk. Anat. 1877. Vol. XVII, p. 471.

*Dogiel, J.*, Die Nervenzellen und Nerven des Herzventrikels beim Frosche. Arch. f. mikrosk. Anat. 1882. Vol. XXI, p. 21.

*Duméril, A. M. C.*, and *Bibron, G.*, Erpétologie générale ou histoire naturelle complète des reptiles. Paris, 1841.

*Eberth*, Untersuchungen zur normalen und pathologischen Anatomie der Froschhaut. Leipzig, 1869.

*Ecker, A.*, Icones physiologicae. Leipzig, 1851 to 1859.

*Eckhard, C.*, Beiträge zur Anatomie und Physiologie. Giessen, 1860. Vol. II.

*Eckhard, C.*, Experimentale Physiologie des Nervensystems. Giessen, 1867, p. 208 (Ganglion cells of heart).

*Edinger, L.*, Nachtrag zu dem Bericht über Leistungen auf dem Gebiete der Anatomie des Centralnervensystems. Schmidt’s Jahresb. 1887.

*Egorow, W.*, Ueber die Nervenzellen der Lungen. Centralbl. f. med. Wiss. 1879. Vol. XVII, p. 305.

*Eichholtz*, De piscium atque amphibiorum nudorum lobis opticis atque olfactoriis. Berolini, 1841.

*Engelmann, T. W.*, Zur Physiologie des Ureter. Pflüger’s Arch. f. d. ges. Physiol. 1869. Vol. II, p. 243. (Treats also of histological details of frog’s bladder.)

*Engelmann, T. W.*, Ueber die Discontinuität des Axencylinders und den fibrillären Bau der Nervenfasern. Pflüger’s Arch. f. d. ges. Physiol. 1880. Vol. XXII, p. 1.

*Engelmann, T. W.*, Der Bulbus aortae des Froschherzens. Pflüger’s Arch. f. d. ges. Physiol. 1882. Vol. XXIX, p. 425.

*Fischer, E.*, Ueber die Endigung der Nerven im quergestreiften Muskel der Wirbelthiere. Arch. f. mik. Anat. 1876. Vol. XIII, pp. 365–390.

*Fischer, J. G.*, Amphibiorum nudorum neurologiae specimen primum. Berlin, 1843; and Arch. f. Anat. u. Physiol. 1844, p. 57.

*Fowelin, C.*, De causa mortis post nervos vagos dissectos instantis. Inaug. Dissert. Dorpati, 1851.

*Frenkel, S.*, Nerv und Epithel am Froschlarvenschwanze. Arch. f. Anat. u. Physiol. 1886. Physiol. Abth., p. 405.

*Frey*, Handb. d. Histologie u. Histochemie d. Menschen. 3rd Edit., p. 341. (Ganglia of heart.)

*Friedländer*, Ueber die nervösen Centralorgane des Froschherzens. Untersuch. aus d. physiol. Laborat. zu Würzburg. 1867.

*Frommann*, Ueber Structur der Nervenzellen. Virchow’s Arch. 1864. Vol. XXXI, p. 129.

*Fubini, S.*, Gewicht des centralen Nervensystems im Vergleich zu dem Körpergewicht der Thiere, bei R. esculenta u. R. temporaria, Moleschott’s Untersuchungen zur Naturlehre d. Menschen. 1881. Vol. XII, pp. 455–461.

*Gad, J.*, Ueber Centrum und Leitungsbahnen im Rückenmark des Frosches. Verhandl. der Physiol. Gesellsch. zu Berlin, No. 10, 1884. Arch. f. Anat. u. Physiol. 1884, p. 304. Verhandl. d. physick. med. Gesellsch. zu Würzburg. 1884. Vol. XVIII.

*Gaskell, W. H.*, On the augmentor (accelerator) nerves of the heart of cold-blooded animals. Journ. of Physiol. 1884. Vol. V, p. 46.

*Gaskell, W. H.*, and *Gadow*, On the anatomy of the cardiac nerves in certain cold-blooded animals. Journ. of Physiol. 1884. Vol. V, p. 362.

*Gaskell, W. H.*, The Structure, Distribution, and Function of the Nerves which innervate the Visceral and Vascular System. Journ. of Physiol. 1886. Vol. VII, p. 1.

*Gegenbaur, C.*, Grundzüge der vergleichenden Anatomie. 1870. 2nd Edition, p. 728.

*Gerlach, J.*, Ueber das Verhalten der Nerven in den quergestreiften Muskelfäden der Wirbelthiere. Sitzungsber. d. phys.-med. Societät zu Erlangen. 1873. Vol. V, p. 97.

*Gerlach, J.*, Das Verhältniss der Nerven zu den willkürlichen Muskeln der Wirbelthiere. Leipzig, 1874.

*Gerlach, J.*, Ueber die Nervenendigungen in der Muskulatur des Froschherzens. Virchow’s Arch. 1876. Vol. LXVI, pp. 187–223.

*Gerlach, J.*, Das Verhältniss der nervösen und contractilen Substanz d. querge-streiften Muskels. Arch. f. mikrosk. Anat. 1877. Vol. XIII, p. 399.

*Giuliani*, Sulla struttura del midollo spinale della Lacerta viridis. Memorie della R. academia dei Lincei. Roma, 1878.

*Goette, A.*, Kurze Mittheilungen aus der Entwicklungsgeschichte der Unke. Arch. f. mik. Anat. 1873. Vol. IX, p. 396.

*Goette, A.*, Entwickelungsgeschichte der Unke. Leipzig, 1875.

*Golgi, C.*, Sulla fina anatomia degli organi centrali del sistema nervoso. Milano, 1866.

*Golgi, C.*, Sui Nervi dei Tendini dell’ Uomo e di altri Vertebrati e di un nuovo organo nervoso terminale muscolo-tendineo. Estr. dalle Memoire della Reale Acc. di Torino. 1880. Vol. XXXII, p. 29.

*Gonjaew, K.*, Die Nerven des Nahrungsschlauches. Arch. f. mikrosk. Anat. 1875. Vol. XI, pp. 479–496.

*de Graaf, H. W.*, Zur Anatomie u. Entwicklung d. Epiphyse bei Amphibien u. Reptilien. Zool. Anz. 1886. Vol. IX, No. 219.

*de Graaf, H. W.*, Over den bouw der epiphyse bij de amphibien. Tijdschr. der Nederl. dierkundige Vereeniging. 1886. Vol. II, p. 1.

*de Graaf, H. W.*, Bijdrage tot de kennis van der bouw ende entwickkeling der epiphyse bij amphibiën en reptiliën. Inaug.-Diss. Leiden, 1886. Abstract in Centralbl. f. d. med. Wiss. 1886, p. 706.

*Grant, R.*, Comparative Anatomy. London, 1839.

*Grant, R.*, Umrisse der vergleichenden Anatomie von R. E. Grant. Aus dem Englischen von C. C. Schmidt, M.D. Leipzig, 1842.

*Gruenhagen, A.*, Ueber ein Endothelialelement der Nervenprimitivscheide. Arch. f. mik. Anat. 1884. Vol. XXIII, p. 380.

*Gscheidlen, R.*, Ueber Nervenendigung in den glatten Muskelfasern. Arch. f. mik. Anat. 1877. Vol. XIV, pp. 320–332.

*Guillot*, Exposition anatomique de l’organisation des centres nerveux dans les quatres classes d’animaux vertébrés. Paris, 1844.

*Hannover, A.*, Die Chromsäure, ein vorzügliches Mittel bei mikroskopischen Unter-suchungen. Arch. f. Anat. u. Physiol. 1840, pp. 549–556. (Describes the connection between fine fibres and small ganglia in the central nervous system of fish, frogs, and birds.)

*Hannover, A.*, Recherches microscopiques sur le système nerveux. Recherches sur le cerveau de la grenouille, pp. 20–22. Kopenhagen, Paris, and Leipzig, 1844.

*Harless, Ern.*, Ueber die Functionen verschiedener Parthien des Rückenmarks der Amphibien. Arch. f. Anat. u. Physiol. 1846, p. 74.

*Hartmann, D. R.*, Ueber die Endigungsweise der Nerven in den Papillae fungi-formes der Froschzunge. Arch. f. Anat. u. Physiol. 1863, p. 634.

*Heidenhain*, Disquisitiones de nervis organisque centralibus cordis. Dissert. inaug. Berol. 1854; and in Arch. f. Anat. u. Physiol. 1858, p. 479.

*Hénoque*, Du mode de distribution et de la terminaison des nerfs dans les muscles lisses. Arch. de l’Anat. et de la Physiol. 1870.

*Hensen, V.*, Ueber die Entwicklung des Gewebes und der Nerven im Schwanze der Froschlarve. Virchow’s Arch. 1864. Vol. XXXI, p. 51.

*Hensen, V.*, Ueber die Nerven im Schwanz der Froschlarven. Arch. f. mik. Anat. 1868. Vol. IV, p. 111.

*Hesse, F.*, Zur Kenntniss der peripherischen markhaltigen Nervenfaser. His u. Braune’s Arch. f. Anat. u. Physiol. 1879. Vol. VI, p. 89. Abstract in Centralbl. f. d. med. Wiss. 1880, p. 324.

*His, W.*, Ueber die Endigung der Gefässnerven. Virchow’s Arch. 1863. Vol. XXVIII, p. 427.

*Hjelt*, De nervis cerebralibus parteque cephalica nervi sympathici Bufonis cinerei adnotata quaedam. Helsingforsiae, 1852.

*Hoffmann, C. K.*, in Bronn’s Klassen und Ordnungen des Thierreichs. Leipzig and Heidelberg, 1873–1878. Vol. VI. Amphibia.

*Holl, M.*, Ueber den Bau der Spinalganglien. Wiener Sitzungsber. 1876. Vol. LXXII, Pt. III, p. 31.

*Horn, H.*, Ueber die Endschlingen des Geruchsnerven (Nervus olfactorius) in Rana temporaria. Arch. f. Anat. und Physiol. 1850.

*Huizinga, D.*, Untersuchungen über die Innervation der Gefässe in der Schwimm-haut des Frosches. Pflüger’s Arch. f. d. ges. Physiol. 1875. Vol. XI, pp. 207–221.

*Huxley, T. H.*, Article Amphibia. Encyclopaedia Britannica, 9th Edition.

*Isquierdo, V.*, Ueber die Endigungsweise der sensiblen Nerven. Arch. f. mikrosk. Anat. 1880. Vol. XVII, p. 367.

*Jacubowitsch, M. N.*, Terminaisons des nerfs à la périphérie et dans les différents organes ou terminaisons périphériques du système nerveux en général. Comptes rendus. 1860. Vol. L, p. 859.

*Jantschitz, J.*, Materialien zur Anatomie der Nerven des Pericardiums. Rudneff’s Journ. f. normale u. patholog. Histologie u. klinische Med. St. Petersburg, 1874, pp. 417–442. (In Russian, Abstract in Centralbl. f. d. med. Wiss. 1874.)

*Johnson, A.*, and *Sheldon, L.*, Note on the Development of the Newt (Triton cristatus). Quart. Journ. Micros. Sci. 1886. Vol. XXVI, p. 573.

*Kandarazki, M.*, Ueber die Nerven der Respirationswege. His u. Braune’s Arch. f. Anat. u. Entwickelungsgesch. 1881, pp. 1–11.

*Kanhel, G.*, Studien über Innervation der Lymphherzen. Medic. Jahrb. 1886, pp. 392–420.

*Karabanowitsch, D.*, Ueber den Bau des Rückenmarkes vom Frosche. Arbeiten der St. Petersburger Gesellsch. der Naturforscher. St. Petersburg, 1872, pp. 402–421.

*Key, A.*, and *Retzius, G.*, Studien in der Anatomie des Nervensystems. Arch. f. mikrosk. Anat. 1873. Vol. IX, p. 308.

*Klebs, E.*, Die Nerven der organischen Muskeln. Centralbl. f. wiss. Med. 1863, p. 561.

*Klebs, E.*, Die Nerven der organischen Muskelfasern. Virchow’s Arch. 1865. Vol. XXXII, p. 168.

*Klein, E.*, Beiträge zur Anatomie der ungeschwänzten Batrachier. Württemberger Jahreshefte. 1850, pp. 1–84.

*Klein, E.*, Auerbach’s Plexus in the Intestine of Frog and Toad. Quart. Journ. Microsc. Sci. 1873. Vol. XIII, pp. 377–380.

*Klein, E.*, Some remarks on the finer nerves of the Cornea. Monthly Micros. Journ. 1872. Vol. VII, pp. 156–164.

*Klein, E.*, On the peripheral distribution of non-medullated nerve-fibres. Quart. Journ. Microsc. Sci. 1871. Vol. XI. New Series, p. 405.

*Klug, F.*, Ueber die Herznerven des Frosches. Arch. f. Anat. u. Entwicklungsgesch. 1881, pp. 330–346.

*Klug, F.*, Ueber die Beschleunigungsnerven des Froschherzens. Centralbl. f. med. Wissensch. Berlin, 1881, pp. 945–948.

*v. Kölliker, A.*, Neurologische Bemerkungen. Zeit. f. wiss. Zool. 1849. Vol. I, p. 135.

*v. Kölliker, A.*, Handbuch der Gewebelehre. 5th Edit. 1861, p. 163. (On nerve-fibres of frog’s heart.)

*v. Kölliker, A.*, Vorläufige Mittheil. über den Bau des Rückenmarks bei niederen Wirbelthieren. Zeit. f. wiss. Zool. 1858. Vol. IX, p. 1.

*v. Kölliker, A.*, Ueber die letzten Endigungen der Nerven in Muskeln des Frosches. Würzburger naturwissensch. Zeitschrift. Vol. III. Sitzungen, 8th and 22nd March, 1862.

*v. Kölliker, A.*, Untersuchungen über die letzten Endigungen der Nerven. Zeit. f. wiss. Zool. 1863. Vol. XII, p. 149.

*v. Kölliker, A.*, Histologische Studien an Batrachierlarven. Zeit. f. wiss. Zool. 1886. Vol. XII, pp. 1–4.

*v. Kölliker, A.*, Ueber die Vitalität der Nervenrohren der Frösche. Würzburg. Verhandl. 1857. Vol. VII, p. 145.

*Kollmann, J.*, Ueber den Verlauf des Lungenmagennerven in der Bauchhöhle. Zeit. f. wiss. Zool. 1859–1860. Vol. X, p. 413.

*Kollmann, J.*, and *Arnstein*, Die sympathischen Ganglienzellen des Frosches. Zeitschr. f. Biologie. 1866. Vol. II, p. 271.

*Königstein, L.*, Beobachtungen über die Nerven der Cornea und ihre Gefasse. Wiener Sitzungsber. 1878. Vol. LXXVI, Pt. III, p. 37.

*Köppen, M.*, Zur Anatomie des Froschgehirns. Neurologisches Centralblatt. 1888.

*Köppen, M.*, Zur Anatomie des Froschgehirns. Arch. f. Anat. u. Physiologie, 1888.

*Korybutt-Daszkiewicz, W.*, Ueber die Entwicklung der Nerven aus Plasmazellen beim Frosche. Arch. f. mikrosk. Anat. 1878. Vol. XV, p. 1.

*Krause, W.*, Die Nervenendigungen in den Froschmuskeln. Internat. Monats. f. Anat. u. Histologie. 1884. Vol. I, p. 194.

*Krause, W.*, Ueber die Drüsennerven. Zeitschr. f. rat. Medicin. Dritte Reihe. Vol. XXIII, p. 60.

*Krohn, A.*, Ueber den Ramus lateralis n. vagi bei niedrigen Amphibien. Fror. Not. 1836. Vol. XLVIII, p. 1043.

*Krohn, A.*, Ergänzungen der Nachricht über den n. lateralis der Froschlarven. Fror. Not. 1838. Vol. VII, n. 137.

*Kühne, W.*, Die Endigungsweise der Nerven in den Muskeln und das doppelsinnige Leitungsvermögen der motorischen Nervenfaser. Monatsbericht der Königl. Akademie der Wissenschaften zu Berlin, 19 May, 1859.

*Kühne, W.*, Die peripher. Endorgane der motorischen Nerven. Leipzig, 1862.

*Kühne, W.*, Zur Lehre von den Endplatten der Nervenhügel. Virchow’s Arch. 1865. Vol. XXXIV, p. 412.

*Kühne, W.*, Zur Histologie der motorischen Nerven. W. Kühne’s Heidelberger Untersuchungen, 1879. Vol. II, p. 187. Abstract in Centralbl. f. d. med. Wiss. 1879, p. 405.

*Kühne, W.*, Ueber Nervenendigungen in den Muskeln nach Beobachtungen von M. B. von Syckel. Verhandl. des naturf. Vereins z. Heidelberg, 1884. Vol. III, p. 238.

*Kühne, W.*, Widerlegen der Bemerkung E. du Bois-Reymonds über mehrfache Nervenendigungen in einer Muskelfaser. Zeitschr. f. Biol. 1884. Vol. XX, pp. 531–539.

*Kühne, W.*, Ueber das doppelsinnige Leitungsvermögen der Nerven. Zeitschr. f. Biol. 1886. Vol. XXII, p. 305.

*Kuhnt, J. H.*, Die peripherischen markhaltigen Nervenfasern. Arch. f. mik. Anat. 1877. Vol. XIII, p. 427.

*Kupffer*, De medullae spinalis textura in Ranis. 1854. Dissert. inaug.

*Kutschin*, Zur Structur des Nervengewebes. Centralbl. f. d. med. Wiss. 1865, p. 561.

*Küttner, C.*, De origine n. sympathici ranarum, ex nervorum dissectorum mutationibus dijudicatur. Dorpat, 1854. Dissert. inaug.

*Lahousse, P.*, Die Structur des Nervenplexus in der Vorhofscheidewand des Frosch-herzens. Arch. f. Anat. u. Physiol. 1886, p. 19.

*Langerhans, P.*, Zur Histologie des Herzens. Virchow’s Arch. 1873. Vol. LVIII, p. 65.

*Lantermann, A. J.*, Bemerkungen über den feineren Bau der markhaltigen Nervenfasern. Centralbl. f. d. med. Wiss. 1874, p. 706.

*Lantermann, A. J.*, Ueber den feineren Bau der markhaltigen Nervenfasern. Arch. f. mikrosk. Anat. 1877. Vol. XIII, p. 1.

*Lavdowsky, N.*, Die feinere Structur und die Nervenendigungen der Froschharn-blase. Arch. f. Anat. und Physiol. 1872, p. 55.

*Lavdowsky, N.*, Das Saugadersystem und die Nerven der Cornea. Arch. f. mikrosk. Anat. 1872. Vol. VIII, p. 538.

*Lavdowsky, N.*, Zum Nachweis der Axencylinderstructurbestandtheile von mark-haltigen Nervenfasern. Centralbl. f. d. med. Wiss. 1879, pp. 865, 881.

*Lavdowsky, N.*, Ueber die Fortsätze der Nervenzellen in den Herzganglien. Arch. f. mik. Anat. 1887. Vol. XXIX, p. 609.

*Lehmann, J. C.*, Ueber die Nervenendigungen und das Vorkommen von mikroskopischen Ganglien in den Gefasswandungen. Zeit. f. wiss. Zool. 1864. Vol. XIV, pp. 346–352.

*Lenhossek, M.*, Untersuchungen über die Spinalganglien des Frosches. Arch. f. mikrosk. Anat. 1886. Vol. XXVI, pp. 370–453.

*Leuret*, Anatomie comparée du système nerveux de l’homme et des animaux vertébrés. Paris. Vols. I, II.

*Lipmann*, Die Nerven der organischen Muskeln. Dissert. Berlin, 1869.

*Lominsky, T.*, Zur Frage über die Teilung der Nervenzellen. Centralbl. f. d. med. Wiss. 1882, p. 434.

*Longet*, Anatomie et Physiologie du système nerveux de l’homme et des animaux vertébrés. Paris, 1842.

*Löwit, M.*, Die Nerven der glatten Muskulatur. Wiener Sitzungsb. 1875. Vol. LXXI, Pt. III, p. 355.

*Löwit, M.*, Beiträge zur Kenntniss der Innervation des Froschherzens. Pflüger’s Arch. f. d. ges. Physiol. Vol. XXIII, p. 313; Vol. XXV, p. 399; Vol. XXVIII, p. 312; Vol. XXIX, p. 469.

*Löwit, M.*, Ueber die Gegenwart von Ganglienzellen im Bulbus aortae des Frosch-herzens. Pflüger’s Arch. f. d. ges. Physiol. 1883. Vol. XXXI, pp. 88–94.

*Löwit, M.*, Beiträge zur Kenntniss der Innervation des Herzens. Pflüger’s Arch. f. d. ges. Physiol. 1881. Vol. XXV, pp. 399–496.

*Luchsinger, B.*, Zur Innervation d. Lymphherzen. Pflüger’s Arch. f. d. ges. Physiol. 1880. Vol. XXIII, p. 304.

*Ludwig, C.*, Ueber die Herznerven des Frosches. Arch. f. Anat. u. Physiol. 1848, p. 139.

*Macallum, A. B.*, The Nerve Terminations in the Cutaneous Epithelium of the Tadpole. Quart. Journ. Micros. Sci. 1885. Vol. XXVI, p. 53.

*Maddox*, On the apparent relation of Nerve to Connective-tissue Corpuscles. Monthly Microsc. Journ. Vol. IX, p. 109.

*Maier, R.*, Die Ganglien in den harnabführenden Wegen des Menschen und einiger Thiere. Arch. f. pathol. Anat. u. Physiol. 1881. Vol. LXXXV, p. 49.

*Marchi, P.*, Beobachtungen über Wimper-Epithel. Arch. f. mik. Anat. 1866. Vol. II, p. 467.

*Marshall, A. M.*, The segmental value of the Cranial Nerves. Journ. of Anat. and Physiol. Vol. XVI, pp. 305–354.

*Marshall, A. M.*, The Frog: an introduction to Anatomy and Physiology. Manchester and London, 1885.

*Masius* and *Vaulair*, De la situation et de l’étendue des centres réflexes de la moelle épinière chez la grenouille. Bruxelles, 1870.

*Mason, J. J.*, A new group of Nerve-cells in the Spinal Cord of the Frog. New York Med. Journ. 1879.

*Mason, J. J.*, Microscopic Studies on the Central Nervous System of Reptiles and Batrachia. Journ. of Nerv. and Mental Diseases. New York. 1880, Vol. VII, p. 8; 1881, Vol. VIII, p. 7.

*Mason, J. J.*, Minute Structure of the Central Nervous System of certain Reptiles and Amphibia of America. Newport, 1884.

*Mayer, A. F. J. C.*, Ueber das Gehirn der Amphibien und ein zoologisches Gesetz. Fror. Notizen, 1833. Vol. XXXVII, pp. 49–53.

*Mayer, S.*, Zur Lehre der Struktur der Spinalganglien und der peripherischen Nerven in Rana esculenta. Arch. f. Anat. u. Physiol. 1858, p. 274.

*Mays, K.*, Histo-physiol. Untersuchungen über die Verbreitung der Nerven in den Muskeln. Zeitschr. f. Biol. Vol. XX, p. 449.

*Mays, K.*, Ueber die Nervatur des Musculus rectus abdominis des Frosches. Heidelberg, 1886.

*Mays, K.*, Nervenfasertheilungen in den Nervenstämmen der Froschmuskeln. Zeitschr. f. Biologie. 1886. Vol. XXII, p. 354.

*Meckel, A.*, System der vergleichenden Anatomie. Halle, 1833.

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*Michel*, Ueber den Bau des chiasma nervorum opticum. v. Gräfe’s Arch. 1864. Vol. XIX, p. 59.

*Müller, J.*, Beiträge zur Anatomie und Naturgeschichte der Amphibien. Tiedemann’s Zeitschr. 1831. Vol. IV, pp. 190–275.

*Namias, M.*, Sui ganglii miocardici della Rana. Osservazioni ed esperienze in Lo Spallanzani. 1881. Vol. X, pp. 402–419.

*Niccolucci, G.*, Sul sistema nervoso e circolatorio della Salamandra aquaivola. Napoli, 1842.

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*Odenias, M. W.*, Undersökungar öfver de sensibla muskelnervena. Nord. Medic. Arch. Vol. IV, No. 18.

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*v. Openchowski, Th.*, Beitrag zur Kenntniss d. Nervenendigungen im Herzen. Arch. f. mik. Anat. 1883. Vol. XXII, pp. 408–419.

*v. Openchowski, Th.*, Histologisches zur Innervation der Drüsen. Pflüger’s Arch. f. d. Ges. Physiol. 1882. Vol. XXVII, p. 223.

*Osborne, H. F.*, Observations upon the Urodele Amphibian Brain. Zool. Anz. 1884. Vol. VII, p. 679.

*Osborne, H. F.*, Observations on the presence of the corpus callosum in the brains of Amphibians and Reptiles. Zool. Anz. 1886. Vol. IX, p. 200.

*Osborne, H. F.*, Note upon the cerebral commissures in the lower vertebrata and a probable fornix rudiment in the brain of Tropidonotus. Zool. Anz. 1886. Vol. IX, p. 577.

*Partsch, C.*, Beiträge zur Kenntniss des Vorderdarmes einiger Amphibien und Reptilien. Arch. f. mik. Anat. 1877. Vol. XIV, p. 179.

*Pertik, O.*, Untersuchungen über Nervenfasern. Arch. f. mik. Anat. 1881. Vol. XIX, p. 183.

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*Popoff, N.*, Die Nerven der Gallenblase. Rudneff’s Journ. f. normale und pathol. Histologie u. klin. Med. St. Petersburg, 1872. (In Russian. Abstract in Virchow and Hirschfeld’s Arch.)

*Priestley, J.*, An account of the anatomy and physiology of the batrachian lymph-hearts. Journ. of Physiol. 1878. Vol. I, p. 1.

*Ranvier, L.*, Recherches sur l’histologie et la physiologie des nerfs. Arch. de physiol. normale et pathologique. 1872. Vol. IV, pp. 427–446.

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*Sachs, C.*, Die Nerven der Sehnen. Reichert u. du Bois-Reymond’s Arch. 1875, p. 402.

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*Sanders, A.*, Contributions to the Anatomy of the Central Nervous System in Vertebrate Animals. Phil. Trans. 1882. Pt. III, p. 927.

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*Schwalbe, G.*, Handbuch der Neurologie. Erlangen, 1881.

*Schweiger-Seidel*, Das Herz. Stricker’s Gewebelehre, p. 177. (Ganglion cells of heart.)

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*Stieda, L.*, Zur vergleichenden Anatomie und Histologie des Cerebellum. Reichert u. du Bois-Reymond’s Arch. 1865, pp. 407–433.

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*Stirling, W.*, and *Macdonald, F.*, The minute structure of the palatine nerves of the frog and the termination of nerves in blood vessels and glands. Journ. of Anat. and Physiol. 1884. Vol. XVII, p. 293.

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*Tiedemann*, Anatomie und Bildungsgeschichte des Hirns. 1816.

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*Török, A.*, Beiträge zur Kenntniss der ersten Anlagen der Sinnesorgane und der primären Schädelformation bei den Batrachiern. Wiener Sitzungsber. 1886. Vol. LII, Pt. I, p. 646.

*Traugott*, Ein Beitrag zur feineren Anatomie des Rückenmarks von Rana temporaria. Dissert. Dorpat, 1861.

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*Treviranus, G. R.*, Bemerkungen über das Nervensystem des Fisches und über einige bisher unbeachtete Theile dieses Thieres in G. R. and L. Chr. Treviranus. Verm. Schriften Anat. u. Physiol. Vol. I, 1816, p. 94.

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*Valentin*, Die letzten Enden der Nerven. Nova acta academ. caesar. Leop. 1836. Vol. XVIII, p. 51.

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*Vogt, C.*, Zoologische Briefe. Frankfurt a. M., 1851, p. 197, etc.

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*Wagner, R.*, Icones Zootomicae. Leipzig, 1841.

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x*Wagner, R.*, Lehrbuch der vergleichenden Anatomie. Leipzig, 1834–35, pp. 400–403.

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*Wolff, W.*, Ueber freie, sensible Nervenendigungen. Arch. f. mik. Anat. 1882. Vol. XX, p. 377.

*Wyman, J.*, Anatomy of the nervous system of Rana pipiens. Smithsonian Contributions to Knowledge. 1853. Vol. V.

*Wyman, J.*, Results of microscopical examination of the structure of the brain and spinal cord in Frogs. Boston, Nat. Hist. Soc. 1851–1854. Vol. V, p. 107.

THE NERVOUS SYSTEM.

I. THE CENTRAL NERVOUS SYSTEM.

(Re-written by the translator.)

A. THE SPINAL CORD (Fig. 97).

*1.* *External form.* The spinal cord is, in comparison with the brain, somewhat small; the two organs are directly continuous, and present no distinct line of demarcation: the point of origin of the first spinal nerve is, therefore, arbitrarily accepted as the anterior limit of the spinal cord; posteriorly it terminates in the filum terminale.

The spinal cord is flattened dorso-ventrally, and is constricted at a point (pars media, Reissner) somewhat anterior to its middle: in consequence of this constriction the cord has two enlargements; an anterior smaller, and a posterior larger (intumescentiae anterior v. posterior), from which arise the nerves of the brachial and lumbar plexuses respectively. At about the sixth or seventh vertebra, the hinder enlargement diminishes rather abruptly to form the so-called conus medullaris; this is continued into the filum terminate, which enters the cavity of the urostyle. The hinder portion of the lumbar nerves forms a cauda equina, the constituent nerves of which surround the filum terminale.

A *dorsal longitudinal fissure* (sulcus longitudinalis superior) is well marked in the middle line of the dorsal surface of the posterior enlargement; anteriorly and posteriorly it rapidly fades away, its position being merely indicated by a small amount of connective-tissue and a blood-vessel.

The *ventral longitudinal fissure* (sulcus longitudinalis inferior) is well marked throughout the length of the cord. Neither fissure can be traced in the hinder part of the conus medullaris or in the filum terminale.

Ten pairs of nerves arise from the spinal cord, each nerve arising by two roots, a ventral and a dorsal (anterior and posterior), from points near the ventral and dorsal longitudinal sinuses respectively: the two roots unite at their point of exit from the vertebral canal through the intervertebral foramen; each dorsal root possesses a ganglion.

The nervous system of Rana esculenta, from the ventral surface. (From Icones physiologicae by A. Ecker. Pl. XXIV, Fig. 1.)

F Facial nerve. G Ganglion of pneumogastric nerve. He Cerebral hemisphere. Lc Optic tract. Lop Optic lobe. M Boundary between medulla oblongata and spinal cord. M1–10 Spinal nerves. MS Connection between fourth spinal nerve and sympathetic chain. N Nasal sac. Ni Sciatic nerve. No Crural nerve. o Eyeball. S Trunk of sympathetic. S1–10 Sympathetic ganglia. Sm Rami communicantes of sympathetic. Sp Continuation of sympathetic into head. I Olfactory nerve. II Optic nerve. III Motor oculi nerve. IV Trochlear nerve. V Trigeminal and facial nerves. Va Ramus ophthalmicus of trigeminal. Vc Ramus maxillo-mandibularis of trigeminal. Vd Mandibular branch of trigeminal. Ve Hyomandibular branch of facial. Vg Gasserian ganglion. Vs Upper end of sympathetic trunk in connection with Gasserian ganglion. VI Abducens nerve. VII Facial nerve. VIII Auditory nerve. X Glossopharyngeal and pneumogastric nerves. X1 Ramus anterior of glossopharyngeal. X2 Ramus Posterior of glossopharyngeal. X3–4 Branches of pneumogastric. ]

*2.* *Internal structure.* As in other vertebrates, the spinal cord consists of white and grey matter, the latter being surrounded by the former; the relative amount of the one to the other varies in different parts of the cord.

A *transverse* section of the cord presents the same general characteristics as a similar section from a bird or mammal. The grey matter lies in the middle, surrounding the central canal, and is prolonged into each half of the section by ventral and dorsal horns or cornua. The grey matter is surrounded by the white, but is not so sharply marked off from this in amphibia as in birds and mammals.

A section through the *anterior enlargement* is almost quadrangular in outline, and somewhat broader below than above; the ventral longitudinal fissure is well marked and deep, while the dorsal is indistinct. The dorsal horns are narrow and short, the ventral longer and much broader. The space between the dorsal horns is narrow and deep, that between the ventral shallow; the outer boundary of the grey matter is almost straight, and the central canal is placed above the centre of the section.

In the *constricted portion* of the cord (pars media) the relative proportion of the white matter is increased at the expense of the grey; the horns are less distinct, the dorsal being directed outwards and the ventral somewhat flattened. The central canal is in the centre of the section.

The *posterior enlargement* shows the ventral horns projecting outwards, very much enlarged, and approaching the periphery; the space between them is semilunar. The dorsal horns are well developed, especially at their upper parts; the space between them is narrow and deep. The outer margin of the grey matter is again nearly a straight line. The central canal is below the centre of the section.

The arrangement of the parts in the conus medullaris approaches that in the pars media: the horns are much diminished in size, and posteriorly they entirely disappear, the ventral horns persisting longer than the dorsal. The central canal approaches the lower surface.

No cornua can be recognised in the filum terminale behind the origin of the last pair of spinal nerves. The grey matter has here a circular outline, with the exception of a very slight indentation below; the white matter is almost absent: the central canal is on the lower border of the section in the anterior part, while posteriorly it occupies almost the entire space below the pia mater.

The *Central Canal* lies in the median line, and always presents a distinct lumen; in the two enlargements this lumen forms a vertical ellipse, while in the pars media it is circular. The canal is lined by a single layer of ciliated columnar epithelium, the cells of which are usually conical, with their bases directed towards the lumen; but other forms may be noticed. These cells average about 0·040 mm. in length and about 0·002 mm. in width at their broadest part. The peripheral extremities of the cells have processes, which, in the case of the cells above and below the cord, may often be traced as far as the pia mater; the processes of the lateral cells are less distinct and seem to be shorter. Anteriorly the central canal opens into the floor of the fourth ventricle. The termination of the central canal, posteriorly, has been described by Masius and Vanlair. According to these observers, the canal towards the hinder end of the filum terminale increases very gradually in width, until its epithelial cells are immediately underneath the pia mater, its walls then converge somewhat more sharply to close the cavity. From the point where the canal commences to increase in width, a change takes place in its epithelial wall, which, instead of having a single layer of cells, now possesses two to three layers; towards the posterior end it is again thinned out to a single layer.

The *grey matter* varies in different parts of the cord and in different parts of the same section. In a section through the anterior enlargement a portion of the grey matter, placed immediately above the central canal, is easily distinguished by its vertically-placed, elliptical outline, and by its transparency: it is known as the substantia reticularis. It consists of a mesh-work of branched cells, through which course distinctly contoured fine commisural fibres which are derived chiefly from the dorsal cornua, the vertical from the epithelial cells of the roof of the central canal. In a series of sections the substantia reticulosa may be seen to originate indistinctly in the most anterior part of the cord; in the anterior enlargement it already possesses the elliptical form mentioned, and in the pars media it commences to send off lateral processes to either side. In the posterior enlargement it reaches its greatest absolute development, as it here surrounds the central canal; while further backwards it encroaches until, together with the remaining grey matter, it constitutes almost the whole of the filum terminale.

Immediately underneath the central canal is a narrow band, which immediately connects the grey matter of the two sides; above it is bounded by the epithelium of the central canal, below by medullated fibres. This septum medium (Reissner) may be traced backwards from the anterior extremity of the cord to the posterior enlargement. It consists chiefly of transverse fibres, together with vertical fibres derived from the epithelial cells on the floor of the canal, and like the substantia reticulosa, is distinguished by its greater transparency from the rest of the grey matter.

The grey matter consists of connective-tissue and nervous elements, which are very intimately united; each of these elements including its special cells and fibres. The *nerve-cells* vary much in size and appearance. A prominent group of large cells in the ventral cornua, the *lateral group* (Stieda), is very conspicuous. The individual cells are seldom rounded, but are usually spindle-shaped or angular, and each possesses one to five processes, which may often be traced through considerable distances. These cells have an average length of 0·040 mm., and are about 0·016 mm. broad; they are somewhat larger in the anterior enlargement than in the posterior, and are also more numerous in a section from the anterior enlargement than in a section from the posterior. In the terminal filament they gradually disappear. Some of their processes are continued into the lateral columns (Köppen).

Smaller, spindle-shaped or triangular cells are scattered irregularly throughout the grey matter, without forming distinct groups, though for descriptive purposes those of the centre have been named the *central group*. In a stained, transverse section of the cord are seen numerous *nuclei*; these undoubtedly belong both to small nerve-cells and to connective-tissue cells, but except under the most favourable conditions the two are indistinguishable. The larger nerve-cells present the usual characters of nerve-cells: more or less granular contents, nucleus, well-marked nucleolus, more or less marked fibrillation; their processes are usually more homogeneous. The cells are frequently pigmented.

The *processes* of the cells belonging to the lateral group radiate in all possible directions, but certain well-marked processes directed towards the middle line can always be made out. From the lowest part of this group a few processes may be traced into the ventral roots. The small cells seem to be chiefly arranged vertically, although their processes radiate in all directions (Stieda).

The *Fibres of the grey substance* are nearly all non-medullated. They may be traced in all directions, but the best-marked groups are either vertical or transverse. The vertical fibres appear to arise from the central grey matter, and to ascend in larger or smaller bundles towards the periphery. The transverse fibres are arranged chiefly in the two commissures: the dorsal commissure (commissura superior) is the smaller; the fibres are parallel, and show no trace of decussation; externally they radiate in various directions to the dorsal horns. The ventral commissure is composed of two layers, the upper grey (commissura inferior grisea), the lower white (commissura inferior alba): both are interrupted by the septum medium. A well-marked decussation of the fibres is seen in the middle line; the exact mode of termination of these fibres has not been made out, but many appear to communicate with the large cells of the ventral horns.

The *white matter* consists chiefly of longitudinal, medullated fibres, in which various columns may be distinguished. The dorsal columns are separated from the lateral by a process of neuroglia, continued from the general investment of neuroglia lying under the pia mater. The line of separation between the lateral and ventral columns is not well marked; it is about the line which would be formed by prolonging the ventral horns to the surface.

The fibres vary in size, and fibres of all sizes may be found in any particular part of a transverse section; still the fibres of the ventral columns have an average greater diameter than those of the lateral columns, and the fibres of the dorsal columns are finer than those of the lateral. The largest fibres of the ventral column are placed near the ventral fissure and on its lower border; they attain their greatest development in the posterior enlargement; these fibres frequently contain two or three axis-cylinders each. Köppen suggests that they may represent the formatio reticularis of higher animals.

In the lateral columns the larger fibres are placed close to the grey matter, these columns also receive fibres from the cells of the lateral group.

In the dorsal columns the radiating root-fibres never reach the median plane, but leave an area of purely longitudinal fibres on each side of the dorsal fissure; these represent Goll’s columns, and have a club-shaped outline in transverse section.

The white matter is pierced in all directions by fine connective-tissue fibres and bundles of fibres which radiate from the grey matter; some branch and join with others to form a network, others pass almost uninterruptedly to the periphery. From the connective-tissue cells of the pia mater, processes pass into the white matter and assist in completing the connective-tissue matrix for the nervous elements.

*Dorsal roots* of the spinal nerves. Each root consists of a single bundle of nerve fibres, which suddenly bends at the periphery of the cord in order to descend vertically through the white matter towards the dorsal horn; the fibres divide into three sets, one to the dorsal commissure, a second to the upper horn itself, and the third helps to form the dorsal columns.

*Ventral roots* of the spinal nerves. Each ventral root consists of three or four delicate threads, which may be traced to the ventral cornua, which they reach after a vertical or very slightly oblique course through the white matter. Other fibres arise from the ventral columns, but these oblique fibres never extend to the median plane.

*Pigment* is found distributed irregularly through a section from any part of the spinal cord; it is increased in amount in those parts in which there is an increased amount of grey matter. The pigmentation is always found more marked in the lower parts of the ventral horns than in other parts.

B. THE BRAIN (Figs. 98, 102, 103, and 105).

*1.* *General description.* From behind forwards, the *dorsal* surface of the brain presents the following parts for examination: the medulla oblongata, the cerebellum, the optic lobes, the thalamencephalon, the cerebral hemispheres, and the olfactory lobes.

The medulla oblongata is a direct continuation of the spinal cord; it is wider anteriorly than posteriorly, and is separated in front from the optic lobes by a vertical plate of nervous matter, the cerebellum. The optic lobes are two symmetrical ovoid bodies touching each other in the median plane, and together forming the widest part of the brain. In front of the optic lobes is the thalamencephalon, with a thick vascular membrane, the choroid plexus, lying on its upper surface, and connected with the pineal gland; the thalamencephalon extends forwards between the posterior ends of the cerebral hemispheres. The cerebral hemispheres are two symmetrical ovoid bodies, narrow in front, wider and slightly diverging behind: each hemisphere is prolonged forwards to form an olfactory lobe.

The *ventral* surface has in front the olfactory lobes, then the cerebral hemispheres, behind these the lamina terminalis, the tuber cinereum, the optic chiasma, the pituitary body, the crura cerebri, and lastly the medulla oblongata, in the order here given.

The various *cranial nerves* (Figs. 102 and 103) may be seen to arise as follows:--The olfactory nerve (I, I′, L.ol′) arises directly from the anterior end and outer side of the corresponding olfactory lobe, and from the cerebral hemisphere. The optic nerve (To and II) arises, as the optic tract, from the side of the brain below the optic lobe, whence it passes to the chiasma on the under surface of the brain. The oculo-motor (III) takes its origin from the ventral surface close to the median line and between the crura cerebri. The pathetic or trochlear nerve (IV) is attached to the dorsal surface between the optic lobes and the cerebellum. The trigeminal nerve (V) arises from the side and anterior part of the medulla oblongata. The abducens (VI) arises behind the pituitary body close to the median line from the ventral surface of the medulla halfway between the sulcus and the origin of the vagus. The facial and auditory (VII and VIII) nerves arise from the medulla oblongata behind the trigeminal nerve, the facial being in front of the auditory nerve. The glossopharyngeal (IX) nerve arises, in common with the pneumogastric nerve (X), behind the auditory nerve.

*2.* *The several parts of the brain.*

*a.* The medulla oblongata (Figs. 98, 102, and 103 Mo)--α. *External form.* The medulla is limited behind by the origin of the first pair of spinal nerves, at which point a very faint constriction is sometimes found: it extends forwards as far as the cerebellum. It gradually widens as it passes forwards until just before it reaches its anterior limit, where it presents a shallow but sharp constriction. The *dorsal surface* is characterised by the presence of a deep, triangular fossa, the *fourth ventricle* (Fig. 98 S.r), (ventriculus quartus, Stieda; sinus rhomboideus s. sinus triangularis, Reissner; fossa rhomboidalis); the sides of the triangle are, however, not quite straight, but are slightly bent outwards just before they converge towards the posteriorly-directed apex; the base of the triangle is formed by the cerebellum. By careful examination, the ventricle is seen to be continued for a short distance under the cerebellum, where it opens into the Sylvian aqueduct. In the floor of the fourth ventricle is a well-marked median longitudinal fissure (sulcus centralis), (Fig. 98 S). Into the posterior part of the ventricle opens the central canal of the spinal cord. As the fourth ventricle is formed by the white matter passing to either side, and the simultaneous flattening of the grey matter, the floor of the fourth ventricle is composed of grey matter.

The fourth ventricle is closed in by a highly vascular membrane, the *choroid plexus* of the fourth ventricle (plexus choroideus ventriculi quarti, Reissner; velum medullare posterius). The blood-vessels of the plexus will be described together with the other vessels of the brain (p. 162). They are supported by a connective-tissue matrix, and the whole covered with flattened epithelium, which in the fourth ventricle is ciliated and often pigmented.

The *ventral surface* of the medulla oblongata (Fig. 102 Mo) has a median ventral longitudinal fissure, a direct continuation of that of the cord; in the anterior part of the medulla oblongata there is also to either side of this a lateral fissure, continued on to the crura cerebri; these fissures correspond to the positions of the two rami posteriores of the internal carotid arteries; they are always well seen in microscopical sections. The medulla oblongata is so intimately connected with the pars commissuralis (pp. 149, 150) that the minute anatomy of the two is best described at the same time.

β. *Internal structure.* Examined by means of serial sections, the medulla oblongata is seen to have, in comparison with the cord, an increased amount of grey matter; this is especially the case in its anterior part. The floor and inner parts of the walls of the ventricle are formed of grey matter, in which the largest-sized cells have disappeared, to be replaced by medium-sized cells. Traced from behind, the ventral horns of the cord are seen to increase in size and to be more widely separated until they form two isolated masses, while the dorsal horns gradually diminish; at the same time they are forced outwards and upwards, until they lie under the floor of the ventricle, and so extend to the pars peduncularis.

Dorsal view of brain of Rana esculenta.

Ad Choroid plexus. C Cerebellum. f Groove between cerebral hemispheres and olfactory lobes. G Opening in the roof of the third ventricle. Gp Pineal body. Hc Cerebral hemispheres. I Olfactory nerve. Li Wall of fourth ventricle. L.ol Olfactory lobe. L.op Optic lobe. Mo Medulla oblongata. S Longitudinal fissure of the fourth ventricle. S.r Fourth ventricle. Tho Thalamencephalon. ]

*Grey matter.* The substantia reticularis is not present, but the septum medium extends forwards as far as the pars peduncularis. The central canal extends upwards at the expense of the tissue above it, and is here pear-shaped; at the same time the dorsal longitudinal fissure deepens until the two meet in the fourth ventricle; beyond this point one cannot speak of dorsal and ventral horns.

Transverse section through hinder end of Medulla oblongata (magnified 30–80), from Stieda.

b Inferior commissure. f Dorsal horns. g Ventral horns. h Fourth ventricle. i Nucleus centralis. k Isolated mass of grey matter in which longitudinal fibres of the pneumogastric nerve course. ]

Small nerve cells are irregularly distributed throughout the whole of the grey matter and cannot be grouped; the larger cells, on the other hand, are arranged in distinct groups which have special relations with the nerves arising from the part. Occasionally these groups are not so isolated as usual; in this case processes of one group can be traced into another group (Reissner). Of these groups the chief are:

1. The nucleus centralis (upper inner group, Reissner), nucleus medullae oblongatae, Stieda (Fig. 99 i), is a group of cells found towards the hinder end of the medulla oblongata, on either side of and below the central canal; the group can be traced under the floor of the fourth ventricle to about its middle. The cells are rounded or spindle-shaped, the processes directed upwards, downwards, or outwards; their average size is 0·040–0·048 mm. long by 0·020 mm. broad.

2. The *auditory nucleus* (nucleus acusticus, Reissner, Stieda), (Fig. 100 n) is a large group of cells found in the wall of the fourth ventricle opposite the point of origin of the auditory nerve. The cells are rounded, pear-shaped, or of spindle form, and interspersed between the nerve fibres; these cells have an average length of 0·040 mm., and are about half as broad. The fibres of the auditory nerve radiate from their superficial origin in all directions through the grey matter towards these cells, and evidently communicate with them (Fig. 101 p). One small group (Fig. 101 r) passes to a lower level than the rest, and is regarded by Stieda as the true auditory centre. Köppen considers that the auditory nerve has a threefold origin: (1) from small cells on the median surface of the auditory area; (2) from the large cells between the above; (3) from a group of free nuclei on the dorsal surface of the auditory area.

Transverse section through the Medulla oblongata at the point of origin of the abducens nerve, from Stieda. (Magnified 30 x 80.)

h Fourth ventricle. m Abducens nerve. n Auditory nucleus. o Abducens nucleus. ]

3. The *trigeminal nucleus* (nucleus trigeminus), (Fig. 101 q) lies in part beneath the auditory nucleus but extends further forwards. It forms a rounded group of cells placed under the outer angle of the grey matter. The cells are somewhat crowded together, and are chiefly of an elongated spindle-form, with their processes directed obliquely downwards and outwards. The fibres of the trigeminal nerve separate into two groups; the upper group is best traced in a horizontal section, the fibres curving round to join the longitudinal fibres continued from the dorsal columns of the cord. The fibres of the lower, smaller group pass transversely inwards to the trigeminal nucleus. According to Reissner the latter fibres are motor, the former sensory. Probably other nerves are connected with the hinder part of this group.

4. The *abducens nucleus* (Fig. 100 o). From its superficial origin, the fibres of the abducens nerve may be traced vertically upwards to a small, rounded, grey mass; at this point the mass is somewhat isolated, but further forwards it may be traced as belonging to the central grey matter; it contains small spindle cells.

Transverse section of the Medulla oblongata, at the point of origin of the auditory nerve, from Stieda. (Magnified 30 x 80.)

h Fourth ventricle. n Auditory nucleus. o Abducens nucleus. p Auditory nerve. p′ Ganglion of auditory nerve. q Hinder portion of trigeminal nerve. r Bundle of fibres arising from trigeminal nucleus and joining the auditory nerve. ]

5. The *pneumogastric nucleus*. The pneumogastric, with its numerous irregular roots, arises from the side of the medulla oblongata. The hindermost fibres can be traced as a small bundle, passing almost transversely through the white matter to the outer margin of the grey matter. The larger portion of the fibres is placed in front of these; part of this seems to be directly continuous with the longitudinal fibres of the white matter; a second part, however, can be traced from the surface transversely through the white matter to the grey matter. These latter fibres, together with those of the group first described, do not arise from the grey matter in this part of the medulla oblongata, but curve round and run backwards longitudinally through the grey matter, thus forming a rounded bundle of fibres (Fig. 99 k). Between these fibres are interspersed small nerve-cells and nuclei which disappear as the fibres approach the white matter. The vagus undoubtedly receives fibres from the grey matter throughout a long course, and again receives a large bundle just before leaving the grey matter. The more exact origin of the various fibres has not been traced.

6. The nucleus magnus (Reissner and Stieda) is a very peculiar group of cells placed on either side, in the most anterior portion of the pars commissuralis, that is, immediately underneath the valvula cerebelli. The large cells are arranged in a transverse section in a single row so as to enclose a pear-shaped space on either side, which has its long axis directed from above, downwards and outwards, the narrower end being above. In longitudinal section the line of cells is seen to be open in front. The space enclosed by these cells is occupied by a granular ground-substance which contains only few nuclei. Bellonci is of opinion that these nuclei represent the corpora quadrigemina posteriora of higher animals.

*White matter.* In the hinder part of the medulla oblongata the arrangement of the white matter resembles that of the white matter of the spinal cord; further forwards the white matter of the dorsal surface commences to pass to either side, and ultimately it forms the outer part of the walls of the fourth ventricle. The fibres of the white matter of the ventral surface are unchanged in direction as they proceed forwards. The fibres of the anterior part of the medulla are thinner than those of the posterior portion (Stieda), according to Reissner they gradually thin as they pass forwards. The fibres are nearly all longitudinal, such transverse and oblique fibres as are present being chiefly in connection with the various nerve-roots and the commissures.

The commissura superior is naturally lost in consequence of the opening of the central canal into the fourth ventricle; the commissura inferior is increased in the anterior half and decreased in the posterior half of the medulla oblongata; in the latter the fibres become more and more oblique, and decussate very freely; ultimately they seem to be either continued as longitudinal fibres or to join the ganglia.

Near the pars commissuralis is a transverse arched band of fibres, passing from the under surface of one half of the cord over the ventral longitudinal fissure through the septum medium to the under surface of the opposite half; part of the fibres are continued upwards along the periphery to the cerebellum, part to the nucleus magnus. Vertical, straight, or slightly arched fibres are found in the walls of the fourth ventricle.

A section from the medulla oblongata has a larger amount of *pigment* than a section from the spinal cord, and the anterior portion of the medulla oblongata contains more than the posterior portion. The pigment is chiefly found in a curved line, placed in the lower and outer parts of the grey matter; the amount present varies in different specimens.

*b.* The Cerebellum and Valvula cerebelli.

α. *External form.* The cerebellum is a thin, semilunar plate, which projects between the optic lobes and the fourth ventricle, its base covering the most anterior part of the ventricle; the posterior surface possesses a very faint median fissure (Reissner). The valvula cerebelli (Velum medullare anterius, Reissner) is the thin lamella which connects the anterior surface of the cerebellum with the optic lobes.

β. *Minute structure.* By means of longitudinal, vertical sections, the posterior surface of the cerebellum is seen to be covered with epithelium; in the lower part of the surface this is columnar or conical, above it is flattened: immediately beneath, that is in front of this is a finely granular layer, with very closely packed and granular nuclei. In front of these is a stratum of nerve-fibres forming the second layer of the cerebellum.

Still more anteriorly is the third layer of the cerebellum, an irregular double layer of large cells (Purkinje’s cells, Denissenko); the cells have an average length and breadth of 0.040 mm. and 0.015 mm respectively; they are pear-shaped or of spindle-form, and possess usually two well-marked processes, one passing into the layer behind, the other forwards into the anterior layer to be immediately described, while other less distinct processes radiate irregularly in all directions. The fourth and most anterior layer of the cerebellum is a thick stratum of nerve fibres with numerous nuclei (0.006 to 0.008 mm. diameter). The fibres are for the most part arranged transversely, but some course in various directions. These fibres underlie the flattened epithelium which covers the anterior surface of the cerebellum.

The fibres of the second layer course, for the most part, in an almost vertical plane; they connect the cerebellum with the optic lobes (processus cerebelli ad corpora bigemina) and with other parts of the brain.

The fibres of the fourth layer receive numerous long processes from the large cells of Purkinje; they form a large commissural system, which can be followed ventrally on each side into the pars commissuralis. A part of the fibres ends here in the grey matter, a second portion enters the auditory area and forms a descending auditory root, a third part joins the lateral columns (in the medulla oblongata), and more anteriorly some join the ventral columns. The descending fibres from the cerebellum, together with the fibrae arcuatae found in the ventral columns, indicate the presence of a pons Varolii. The fibres of this ventral commissure decussate only on its dorsal surface (Köppen).

The Valvula cerebelli contains a few medullated fibres and the roots of the trochlear nerves; these pass from the medulla oblongata into the valvula cerebelli, cross in the median line, and then proceed forwards as the trochlear nerves.

The *pigment* in the pars commissuralis is arranged in a curved line similar to that found in the medulla oblongata, but the line is shortened at either extremity, and consequently does not extend into the cerebellum.

*c.* The *optic lobes* and Crura cerebri. (Corpora geminata and Pars peduncularis, Reissner; Lobus opticus, Stieda; Vierhügel, Tiedemann; Vierhügel (Zweihügel) and Pedunculi cerebri, Schiess; Mesencephalon, Huxley.)

Ventral view of brain of Rana esculenta.

Cho Optic chiasma. Hc Cerebral hemispheres. Hy Pituitary body. L.ol Olfactory lobe. L.ol^1 Origin of olfactory nerve from the cerebral hemisphere. Lt Lamina terminalis. Mo Medulla oblongata. To Optic tract. Tu.c Tuber cinereum. I 1st } I^1 2nd } root of the olfactory nerve. II Optic nerve. III Oculo-motor nerve. IV Trochlear nerve. VI Abducens nerve. V, VII, VIII Trigeminal, facial, and auditory nerves. IX, X, XI Glossopharyngeal, pneumogastric, and accessory nerves. ]

α. *External form* (Figs. 98, 102, 103 L.op). The optic lobes are two prominent ovoid bodies placed immediately in front of the cerebellum, and connected with it by the valvula cerebelli; posteriorly they touch each other in the median plane, while anteriorly they diverge and thus constitute the widest part of the brain; in the angle thus formed is the thalamencephalon. The optic lobes are always more darkly pigmented than any other part of the central nervous system.

Lateral view of brain of Rana esculenta.

Ad Choroid plexus. C Cerebellum. Gp Pineal body. Hc Cerebral hemisphere. Hy Pituitary body. L.ol Olfactory lobe. L.ol^1 Disc at origin of second root. L.op Optic lobe. Mo Medulla oblongata. Tho Thalamencephalon. To Optic tract. Tu.c Tuber cinereum. I 1st } I^1 2nd } root of olfactory nerve. II Optic nerve. IV Trochlear nerve. V Trigeminal nerve. VII Facial nerve. VIII Auditory nerve. IX, X, XI Glossopharyngeal, pneumogastric, and accessory nerves. ]

The Crura cerebri are two columns of white matter, placed beneath the optic lobes, and partly hidden by the pituitary gland. At their junction with the medulla oblongata, or rather with the pars commissuralis, is a very slight transverse fissure; at the same point the ventral longitudinal fissure is interrupted by an extremely small grey tubercle (Stieda).

β. *Internal structure.* From the anterior extremity of the fourth ventricle a canal, the Sylvian aqueduct (Aqueductus Sylvii, iter a tertio ad quartum ventriculum), may be traced forwards under the cerebellum, in the median line of this section of the brain. At about opposite the middle of the length of the optic lobes the canal is dilated and communicates with the cavities or ventricles (Ventriculi lobi optici, Stieda) enclosed by these; a general cavity is formed, which in transverse section has something of the form of the letter T. The roof of the cavity is thinner than the floor; this is especially the case in the median plane opposite the superior longitudinal fissure between the optic lobes; the floor is thinned in the middle line by the descending portion of the cavity. The cavity of each optic lobe extends both forwards and backwards beyond its point of communication with the dilated Sylvian aqueduct, hence in a transverse section taken in front of this point (Fig. 104 h′) the cavity of either side appears to be isolated; in a horizontal and longitudinal section (Fig. 105 Aq) the general arrangement of the parts may be well seen.

The grey matter is chiefly arranged in a layer so as to surround the cavity (Fig. 104), this layer being deeper on either side of the descending portion of the cavity than elsewhere. For the rest the grey matter is much interspersed among the white matter, except at the circumference of the section, which is entirely formed of white matter. It contains a large number of small cells, of which the nuclei are alone visible; in the parts mentioned where the layer is most marked these cells are arranged in oblique rows, between which pass fine bundles of medullated fibres (Fig. 104, between u and h′). A group of large cells (Fig. 104 u) is found on either side of the middle line and under the floor of the cavity; the cells are about 0.032 mm. long, and 0.016 mm. broad; the oculo-motor nerve may be traced to this group, which is the *oculo-motor nucleus*. A small commissure of decussating fibres connects the nuclei of opposite sides (Köppen).

Transverse section through the anterior portion of the optic lobes opposite the origin of the motor-oculi nerve; from Stieda (magnified 30–80).

h Lower portion of cavity. h′ Lateral portion of cavity. u Ganglia of oculi-motor nerve. v Oculo-motor nerve. w Large cells of the optic lobe. x Roof of optic lobe. y Posterior commissure. z Anterior diverticulum of the cavity. ]

The substance of the roof of the optic lobes (Fig. 104 x) is arranged in very distinct layers: above is a layer free from cells; the fibres of this layer are extremely fine and delicate, and have not been accurately traced; under this is a nuclear layer; a second layer of fine fibres follows, which is succeeded by a second layer of nuclei in a granular matrix, and lastly epithelium (Stieda). (Reissner describes three nuclear layers in Bufo variabilis, and this is also the case in R. temporaria, G. H.; according to Köppen, the number is variable.) In the anterior portion of the roof a distinct bundle of fibres may be made out; externally they bend downwards, and can be traced as far as the crura cerebri.

Fibres corresponding with the commissural and arched fibres of the medulla oblongata are continued into the hinder portion of the crura cerebri, the change from pars commissuralis to crura cerebri being very gradual. The longitudinal white fibres are much increased in number in the crura cerebri, and a portion of them can be traced to the nucleus magnus.

A peculiar irregular group of large cells (Fig. 104 w) is found where the roof meets the crura cerebri; these vary in diameter from 0.024 to O.040 mm., and their processes are very indistinct.

The fibres of the optic tracts arise, according to Köppen, from two different origins: the one lies on the hindermost part of the optic lobe; from this point the fibres curve downwards and forwards to form longitudinal fibres; this root Köppen names the ventral ascending root, it can be traced through the entire length of the organ. The second root arises in the tectum opticum near the longitudinal fissure; it is smaller than the foregoing, and has been named the dorsal ascending optic root. The fibres of these two roots unite anteriorly near the posterior commissure, at which point they receive additional fibres (Köppen). Bellonci traces a large proportion of the fibres of the optic tract to the nucleus magnus, which pair of nuclei, as already stated, he regards as the posterior pair of corpora quadrigemina of higher animals.

The pars peduncularis is the continuation of the pars commissuralis underneath the optic lobes; a gelatinous mass lying in the median plane and containing numerous isolated nuclei (Ganglion interpedunculare) divides it into two lateral halves. The longitudinal fibres are ungrouped posteriorly, but arranged in rounded strands in the middle, especially dorsally; anteriorly the grouped arrangement is lost and the number of fibres diminished.

The *pigment* of this region has, in a transverse section, an outline which has something the form of a lyre; commencing on either side of the median line, and underneath the deepest portion of the cavity, the pigment line passes, first, directly outwards; then suddenly turns upwards and slightly outwards parallel with the wall of the deeper part of the cavity; it then curves outwards to pass below the cavity of the optic lobe, where it divides, one portion passing outwards, the other between the Sylvian aqueduct and the ventricle of the optic lobe.

*d.* The *Thalamencephalon* (Huxley), (Lobus ventriculi tertii, Stieda; Thalami optici, Reissner; Thalamus opticus s. Lobus ventriculi tertii, Stannius; Ganglien der Haemisphaeren, Carus).

α. *External form.* From above (Fig. 98 Tho) the thalamencephalon is seen as a lozenge-shaped mass lying in front of the optic lobes, and behind and between the diverging posterior ends of the cerebral hemispheres; it is covered by a thick vascular membrane, the *choroid plexus*, through which passes the pedicle of the *pineal body* (Glandula pinealis). On removing the choroid plexus a small aperture is seen in the roof of the thalamencephalon, connecting the hollow pedicle of the pineal gland with the *third ventricle*. The ventricle appears as a narrow slit in the median line, its walls being formed by the optic thalami. By pressing aside the cerebral hemispheres the *posterior commissure* (Commissura posterior) may be seen lying quite in front and deep in the cleft of the ventricle. Immediately behind the pedicle of the pineal body is a slight but well-marked depression (Fig 98 G), the origin of which has not been investigated (Wiedersheim).

The choroid plexus is continued forwards between the cerebral hemispheres (Figs. 98, 103 Ad) for some distance, and terminates in a fine thread of connective-tissue.

The under surface of the thalamencephalon (Fig. 102) is divided into two parts by the *optic chiasma* (Cho): the anterior portion (Lt) is the lamina terminalis (Substantia cinerea anterior); the posterior (Tu.c) the tuber cinereum. The lamina terminalis is bounded on either side by the cerebral hemispheres. The tuber cinereum (Figs. 102, 103 Tu.c) is a small median swelling immediately behind the optic chiasma, and caused by the depression of the floor of the third ventricle to form the infundibulum (Diverticulum infundibuli, Reissner).

Horizontal section through the brain to show the ventricles.

Aq Ventricles of optic lobes and the aqueduct of Sylvius. Dv Third ventricle. MF Foramen of Monroe. Sv Lateral ventricle. Vv Fourth ventricle. ]

The *pituitary body* (Hypophysis cerebri) is a flattened sac, placed behind the tuber cinereum and continuous with it by means of the infundibulum.

β. *Internal structure.* The aqueduct of Sylvius, after communicating with the ventricles of the optic lobes, again contracts (Fig. 105), but still remains somewhat larger than before. In the thalamencephalon the Sylvian aqueduct opens into the third ventricle, which gradually assumes the form of a vertical slit with the walls bulging slightly outwards in their upper parts. The thin roof of this ventricle, where complete, contains a band of transverse fibres. The floor is depressed both before and behind the part immediately above the optic chiasma, the posterior depression lying above the tuber cinereum, which here descends towards the infundibulum: a transverse section through this portion of the third ventricle has the form of a square standing on one angle, the superior angle being produced into the vertical slit of the general cavity. The anterior depression is formed by the general cavity being prolonged downwards and forwards to the lamina terminalis in the form of a narrow and shallow slit.

The white and grey matter of the thalamencephalon are only so far distinct in that the portion immediately surrounding the cavity is darker than the rest of the section. In the immediate neighbourhood of the cavity are many small cells and nuclei, which become scarcer further from the ventricle; they are arranged in rows, separated by a fibrillated matrix. On either side is a distinct bundle of longitudinal fibres, the ‘round bundle’ of Köppen, which come from the optic lobes but appear to arise from either the pars commissuralis or the medulla oblongata, and to receive additional fibres from the optic thalami; they pass forwards to the base of the cerebral hemispheres (Stieda). A second set of longitudinal fibres arises in the substance of the tuber cinereum and passes forwards to the hemispheres (strand of the Tuber-cinereum and Thalamus, Köppen); this band, together with the ‘round bundle,’ form a crus cerebri (Köppen).

The commissural fibres of the thalamencephalon are: (1) a commissura transversa Halleri in the posterior portion of the organ; (2) an optic commissure, consisting of fibres arising from the thalamencephalon (thalamencephalic root) and crossing the median line to join the optic tract of the opposite side; (3) a probable commissure between the optic nerves just in front of the chiasma opticorum; the existence of the latter is not yet proved beyond doubt; (4) the large commissure of transverse fibres found in the roof of the third ventricle; whether the fibres decussate or not is uncertain (Köppen).

The fibres of the large commissure of the roof are, in part, continued into the strand of the Tuber-cinereum above mentioned, and thus conducted to the posterior parts of the hemispheres (Köppen). A group of fibres (Meynert’s band, Köppen) is found in each lateral wall of the third ventricle; they pass from the region of the nucleus parvus downwards in a curved course almost parallel with the external border of the thalamencephalon.

A distinct group of cells may be noted in this section of the brain, an arciform or circular group (Nucleus parvus, Reissner; ganglion of the habenula, Köppen) of large spindle cells (average diameter 0.016 mm.), placed under the upper border close to the third ventricle; the group extends alongside the whole length of the ventricle. A second group lying in the middle and posterior parts beneath the ventricle, Köppen names the ‘ventral nucleus’ of the thalamencephalon.

The *pigment* in the posterior part of the thalamencephalon is arranged in a manner similar to that in the optic lobes and crura cerebri; anteriorly where the third ventricle is prolonged forwards and downwards the arrangement is different; the pigment lies in a curved line above the process of the ventricle, with its concavity directed downwards, each end bifurcating, in order that one branch may pass upwards, the other downwards.

The *pineal body* is a small vesicle placed underneath the skin above the fronto-parietal bones; in the embryo it is connected with the third ventricle by means of the pedicle already mentioned; the skin covering the body is always paler than the surrounding skin, and the usual cutaneous glands are absent in this part; the paler spot on the head may always be found, but is more distinct in Rana temporaria than in Rana esculenta. The structure on the roof of the third ventricle, which is usually known as the pineal body, is nothing more than a thickened portion of the choroid plexus, and consists of a group of convoluted vessels surrounded by pia mater, which is described by Wyman as being covered with ciliated epithelium (R. pipiens). The true pineal body is a small body with an outer connective-tissue capsule, derived from the pia mater; this encloses an irregular mass of epithelial cells; according to de Graaf a twig of the ramus supramaxillaris reaches it subcutaneously, and a blood-vessel accompanies the pedicle through the foramen parietale. According to Darkschewitsch, the pedicle contains medullated nerve-fibres derived directly from the brain.

The *pituitary body* (Figs. 102, 103 Hy) when examined with a lens is seen to consist of two portions: an anterior, superior, and smaller white portion, and a larger, inferior, posterior, and reddish portion. The anterior portion has the form of a very small, flat disk, and is enclosed in a connective-tissue capsule which sends in larger and smaller processes. In either transverse or longitudinal section it is seen to be formed of two horizontal layers separated by a line of blood-vessels and connective-tissue. The upper layer consists of a granular and reticular matrix, containing many nuclei (averaging 0.006 to 0.010 mm. diameter), and divided into irregular rounded or polyhedral spaces by bands of tissue derived from the capsule. This layer is more vascular than the lower. The lower layer consists of a mass of clear, nucleated rounded or polyhedral cells (0.016 to 0.024 mm. in diameter; nuclei from 0.008 to O.012 mm. in diameter, Reissner), pierced by very fine connective-tissue septa derived from the capsule. The septa are, for the most part, vertical and longitudinal (Reissner), the blood-vessels are very few.

The posterior larger portion of the pituitary body (Fig. 106) is also compressed from above downwards, and in transverse section as an oval outline. It possesses an external thin connective-tissue capsule, which sends in fine processes to support a mass of convoluted tubes, between which course a few blood-vessels; these tubes possess an outer nucleated basement-membrane, and are lined with a single layer of more or less cylindrical epithelium, which entirely fills the tube; hence the tubes possess no lumen. The tubes are from 0.04 to 0.08 mm. in diameter; the cells are clear or granular, and possess distinct, rounded nuclei.

*e.* The *Cerebral Hemispheres* and *Olfactory Lobes*. The cerebral hemispheres (Lobi hemisphaerici, Stieda; Lobi cerebrales, Reissner; Centralmasse des Geruchssinns, Carus; Hemisphaeren des grossen Hirns, Tiedemann; Grosse Hemisphaeren, Schiess; Prosencephalon, Huxley). The olfactory lobes (Tubercula olfactoria, Stieda; Lobi olfactorii, Reissner; Riechkolben, Schiess; Rhinencephalon, Huxley).

α. *External form* (Figs. 98, 102, 103 Hc and L.ol). The two cerebral hemispheres form together the largest section of the brain; from above they are seen to be separated by a dorsal longitudinal fissure, which is here well marked: each hemisphere is an ovoid body with the smaller end directed forwards and continuous with the corresponding olfactory lobe; the posterior end forms one half of the anterior boundary of the thalamencephalon. The olfactory lobes are two elongated, rounded bodies directly continuous with the corresponding cerebral hemispheres, and likewise partially separated in the median line by a dorsal longitudinal fissure: at the point of union of the cerebral hemispheres and olfactory lobes is a faintly marked transverse depression.

Section through the lower division of the pituitary body (magnified 360 times); from Stieda.

a Tubes lined with epithelium. b Blood-vessels. ]

On the ventral surface the parts are again marked off from one another by a corresponding ventral longitudinal fissure and a transverse groove; the two longitudinal fissures being continuous, anteriorly, between the olfactory lobes. The cerebral hemispheres appear to be more widely separated behind (Fig. 102) than is the case on the dorsal surface, and in the space so formed is the lamina terminalis (Lt). Seen from the side, the slight depression of the upper surface, between the cerebral hemispheres and the olfactory lobes, is seen to be continued downwards and slightly backwards to join the corresponding groove on the inferior surface.

The longitudinal fissures are shallow and do not meet, except at one point, at about the middle of the cerebral hemispheres (Fig. 105). The olfactory bulbs arise superficially (Figs. 102, 103 I′ and L.ol) from the whole length of the olfactory lobe, between the anterior extremity (I′) and the posterior (L.ol), where they are also attached to the cerebral hemispheres.

β. *Internal structure.* The cerebral hemispheres and olfactory lobes are hollow, the common cavity of each side being known as the lateral ventricle (Ventriculus lateralis); these ventricles communicate with each other, and with the third ventricle (Fig. 105). The narrow aperture by which the lateral ventricles communicate is known as the *Foramen of Monro* (MF); it communicates with the third ventricle posteriorly, and with the space between the cerebral hemispheres anteriorly, and thus forms a common cavity (Ventriculus communis loborum hemisphaericorum, Stieda).

In general terms, each ventricle may be said to be a semilunar cavity, prolonged backwards and forwards (Fig. 107 c); the outer wall is always more or less concave, while the inner varies according to the part of the hemisphere examined. At the anterior and posterior extremities the inner wall is convex and bulges into the cavity (Fig. 107); in the middle portion of the cavity the inner wall presents a longitudinal groove (Ventriculi lateralis cornu internum, Reissner), (Fig. 108 d), and consequently the inner wall has here two rounded masses, an upper and a lower (Corpus striatum, Wiedersheim), projecting into it. By tracing them backwards and forwards, the lower swelling is seen to increase at the expense of the upper, while at the same time the lateral grooves disappear; the outline of the cavity shown in Fig. 107 is then obtained. The roof of the ventricle is arched and broader than the floor, which, in the middle part, exists only as a vertical slit (Fig. 108), (Ventriculi lateralis cornu inferius, Reissner): towards the anterior and posterior extremities it widens and becomes shallower (Fig. 107).

Transverse section through the hinder portion of the cerebral hemispheres; from Stieda.

c Lateral ventricle. c′ Common ventricle of Stieda. d Longitudinal fibres. f Anterior prolongation of the third ventricle. ]

The cerebral hemispheres and olfactory lobes are composed of a fine granular matrix, enclosing spindle-shaped, rounded, or pear-shaped nerve cells and nuclei, and containing very fine fibres. The cells (Fig. 109 b) are more numerous towards the ventricle, and somewhat sparse towards the superficial surface. The cells are of two chief sizes, the smaller and more numerous average 0.004 mm. to 0.008 mm. in diameter; they are found chiefly in the deeper portions of the section, but also form a very thin irregular layer beneath the pia mater: the larger cells have an average diameter of 0.010 mm. to 0.012 mm., and are placed towards the periphery, especially in the dorsal part of the inner wall.

Transverse section near the middle of the cerebral hemispheres.

c Lateral ventricle. d Groove on the inner wall. ]

In this irregular collection of cells the following centres have been described: (1) The nucleus, through which the corpus callosum passes (Köppen); (2) the lower internal or median cell-area (Osborne), situated above the foregoing nucleus in the posterior and middle portions of the hemispheres; (3) the upper internal cell-area (Osborne) is the area of large cells in the dorsal part of the inner wall; (4) the Corpus striatum (Osborne) is a mass of cells between the corpus callosum and the commissura anterior; Köppen doubts the correctness of Osborne’s opinion, and suggests that a group of cells found in the wall of the third ventricle in front of ‘Meynert’s band’ may perhaps be a corpus striatum.

From a transverse section through one of the cerebral hemispheres; from Stieda. (Magnified 360 times.)

a Epithelium of lateral ventricle. b Nerve cells. c Connective-tissue processes from the Pia mater. ]

A bundle of longitudinal medullated nerve-fibres, the ‘round bundle,’ is found on either side of the median line (Fig. 107 d), and near the lower border in the posterior portion of this region; these fibres can be traced from the posterior section of the thalamencephalon; they diminish in number as they course through the base of the cerebral hemispheres, and are ultimately lost in the lower anterior part of the outer walls. A second strand of longitudinal fibres is the continuation of that of the Tuber-cinereum above described, which can be followed to the outer wall of the posterior part of the ventricles, and to the anterior commissure.

The commissures are: (1) The corpus callosum, a large bundle of transverse fibres, seen best in a transverse section, at the point of junction between the lamina terminalis and the cerebral hemispheres, forming an arch over the roof of the anterior prolongation of the third ventricle. The fibres course to the inner and anterior parts of the hemispheres, and are situated chiefly behind the foramen Monroi. To this commissure must be added those fibres which unite the two olfactory lobes, and possibly the fibres (Commissura posterior) found in the roof of the third ventricle (Köppen). (2) The Commissura anterior (Stieda), a smaller set, found immediately under the floor of the common ventricle, forming in their course outwards a curve, with the concavity directed downwards. This commissure connects the ‘round bundles’ of opposite sides, and those fibres coursing with the ‘round bundles’ to the olfactory lobes constitute the pars olfactoria of the commissura anterior. To this commissure must also be added some fibres found on the ventral surface of the commissura anterior and connecting the two strands of the Tuber-cinereum; an unusually coarse strand of these fibres can be traced to the inner wall of the ventricle, and is termed the pars olfactoria interna by Osborne.

The general structure of the olfactory lobes resembles that of the hemispheres; the olfactory nerves arise each by two roots, an outer and inner. The outer root arises from the outer wall near the groove between the corresponding hemisphere and olfactory lobe; the inner or anterior root arises from the anterior surface of the olfactory lobe. Both roots have a peculiar method of origin from the extremely fine fibrillar network of the matrix (Nerve-fibre-conglomerate, Köppen), in which are rounded dark bodies known as ‘glomeruli;’ in the ‘glomeruli’ dark points and nuclei are seen, between larger and smaller bands of nerve-fibres. Köppen holds that all the sensory nerves of the brain arise in a similar manner.

A decussation takes place between the two inner roots of the olfactory nerves; possibly the external roots are connected by means of the commissura anterior.

Very little *pigment* exists in the cerebral hemispheres or olfactory lobes, the greater portion is found in the upper part of the inner walls of the cerebral hemispheres.

The *epithelium* of the *ventricles of the brain*, like that of the central canal of the *spinal cord*, consists of conical cells with their bases directed towards the cavity, and their apices directed peripherally and prolonged into distinct processes (Figs. 104, 109 a). In such situations, as the choroid plexuses, where nervous tissue is absent and the cavity is completed by pia mater alone, the epithelial cells are flattened. Everywhere else it is ciliated and possesses distinct round nuclei which are as broad as the cells themselves. The epithelium is somewhat irregularly and sparsely pigmented; the ventral parts of the central canal of the spinal cord, of the fourth ventricle, and of the Sylvian aqueduct are always more pigmented than the dorsal parts.

PLAN OF THE ARRANGEMENT OF THE NERVE-FIBRES OF THE CENTRAL NERVOUS SYSTEM, ACCORDING TO KÖPPEN.

NERVE FIBRE NERVE-FIBRE COMMISSURES. NERVE-FIBRES STRANDS IN STRANDS IN WHICH CONNECT THE WHITE THE GREY THE STRANDS OF MATTER. MATTER. WHITE FIBRES.

{1. The ventral 1. Longitudinal 1. The ventral { columns, fibres in commissure. { including the dorsal 2. The dorsal { the strand portion of commissure { of coarse the grey (rudimentary). The { fibres. matter. spinal {2. The lateral cord. { columns. {3. The dorsal, { columns { including { Goll’s { column.

{1. The ventral 1. Longitudinal 1. The ventral 1. The fibrae { columns, fibres of the commissure: arcuatae. { including Substantia a. Dorsal { the strand gelatinosa portion. { of coarse Rolandi, and b. Ventral { fibres which give portion. { dorsal off ascending 2. (The dorsal { longitudinal vagal and commissure { strand. trigeminal is wanting.) {2. The lateral roots. { columns, The { which give medulla { off an as- oblongata.{ cending { (recurrent) { vagus-root. {3. The dorsal { columns. { a. Ascending { trigeminal { root. { b. Auditory { root. { c. Descending { auditory { root.

{1. The ventral 1. Longitudinal 1. The ventral 1. The fibrae { columns, fibres of the commissure. arcuatae. { including grey matter. 2. The dorsal { the dorsal Small ascend- commissure, { longitudinal ing trigeminal cerebellar { strand (the root. commissure { strand of and trochlear { coarse fibres decussation. The { has dis- cere- { appeared). bellum {2. The lateral and the { columns (each pars { receives a commis- { strand from suralis. { the cere- { bellum). {3. The dorsal { columns. { a. Large as- { cending tri- { geminal root. { b. Descending { auditory { root.

{1. The ventral (Wanting.) 1. The ventral 1. The fibrae { columns, commissure. arcuatae. { including the 2. The dorsal { dorsal longi- commissure. { tudinal a. Commissura { strand. posterior. {2. (The lateral b. Marginal The { columns can- commissure. optic { not now be lobes. { distinguished { from the ven- { tral columns.) {3. The dorsal { columns: two { ascending { opticus-roots.

{1. The ventral (Wanting.) 1. The ventral 1. The fibrae { and dorsal commissure: arcuatae. The { columns. a. Commissura thalam- { a. Fibres from transversa enceph- { the optic Halleri. alon and { lobes. b. Commissura the cere- { b. The ‘round anterior. bral hemi-{ bundle.’ 2. The dorsal spheres. { c. The ‘Thal- commissure, { amus-Tuber- the corpus { cinereum callosum. { strand.’

C. THE COVERINGS OF THE BRAIN AND SPINAL CORD.

1. The Pia mater closely invests the whole of the brain and spinal cord; it may, in fact, be regarded as the flattened, outermost layer of the connective-tissue material which supports the nervous elements; fine processes (Fig. 109 c) pass radially from its inner surface to join the general connective-tissue matrix or neuroglia. It is usually pigmented, those portions covering the optic lobes and spinal cord being especially deeply pigmented: on the cerebral hemispheres it contains very little pigment; at times, indeed, in R. temporaria, pigment is absent from this part.

The pia mater is continued on to the choroid plexuses and pituitary body, and on to the pineal body by means of the pedicle; the membrane is very vascular, and forms, especially for the cerebral hemispheres and the olfactory lobes, a very important source for the direct supply of blood-vessels.

2. The Dura mater, or lining membrane of the cranial cavity and vertebral canal, is a connective-tissue membrane containing many very much branched, pigmented cells. This membrane is not so deeply pigmented as the pia mater, except in that portion covering the cerebral hemispheres and the olfactory lobes, which is much darker than the corresponding portion of the pia mater.

3. The *arachnoid membrane* is the layer of endothelial cells covering the inner surface of the dura mater: by means of the blood-vessels and nerves, etc., it is continued on to the pia mater of the spinal cord and brain, which it in like manner closely invests. Masses of *calcareous crystals* are found between the epineurium and the arachnoid (Wiedersheim) on each spinal nerve at its exit from the intervertebral foramen, also on the trigeminal nerves in the cranium. Additional smaller and more irregular masses are found on the dorsal part of the dura mater of the spinal cord.

D. THE VESSELS OF THE BRAIN AND SPINAL CORD. (Fig. 110.)

The arteries of these organs are derived from the internal carotids and the arteriae vertebrales. As soon as the internal carotid arteries reach the cranial cavity each divides into two branches, an anterior (Ramus anterior, Schöbl) and a posterior (Ramus posterior, Schöbl); the anterior branches course forwards on the lower part of the surface of the brain as far as the anterior portion of the thalamencephalon, where they again divide; one division, arteria lobi hemisphaerici inferior externa (Schöbl), continues forwards along the outer surface of the cerebral hemisphere and of the olfactory lobe as far as the olfactory bulb, where it may still be distinctly seen; it gives off branches to the adjacent parts in its course forwards. The vessels of the two sides communicate with each other by means of delicate transverse vessels (Arteriae communicantes anteriores). The second division, arteria lobi hemisphaerici superior interna (Schöbl), of the ramus anterior courses on the thalamencephalon to the dorsal surface, gives a large branch to the choroid plexus of the fourth ventricle, and runs forwards in the dorsal longitudinal fissure; it supplies vessels to all the neighbouring parts.

The rami posteriores converge as they course backwards, and ultimately unite to form an arteria basilaris, which is continued in the median line of the under surface of the spinal cord as the arteria spinalis anterior. The ramus posterior gives off, in its course, many small vessels to the neighbouring parts, and two larger vessels on either side, one of which, the arteria lobi optici (Schöbl), is distributed to the optic lobes, while the other passes to the pituitary body.

The branches of these vessels form a network in the pia mater, from which the nervous tissue is supplied; they also send numerous branches directly into the brain and cord, and these have a similar arrangement for all parts of the brain with the exception of the cerebral hemispheres and olfactory lobes; more or less vertical branches arise from the posterior parts of the upper borders of the rami anteriores, from the whole of the rami posteriores, and from the arteria basilaris; these course upwards from their place of origin into the corresponding part of the brain, give off a few branches in the white matter, and then branch freely, and at somewhat sharp angles, in the grey matter. In the pars commissuralis a large branch may be traced from the ramus posterior on either side upwards into the cerebellum almost to its upper border. The vessels of the cerebral hemispheres and olfactory lobes seem to possess no other definite arrangement than that described above. The grey matter seems, on the whole, to be more vascular than the white; the vessels of the latter are chiefly arranged radially to the surface, and run in courses which are more or less straight; the vessels of the grey matter are more irregular and sinuous.

The Arteria spinalis anterior courses along the whole length of the spinal cord, giving off lateral branches and communicating with rami spinales (branches of the Arteria vertebralis). These branches form a plexus in the pia mater, from which vessels pass at irregular points into the cord; one set of small vessels, described by Reissner, pass in a straight course from the superior longitudinal sinus towards the substantia reticularis, where they divide. Other branches pass directly from the arteria spinalis anterior, through the ventral longitudinal fissure, and there divide; the twigs as a rule avoiding the septum medium and passing in greater part towards the ventral horns. The vessels in the white matter are for the most part radial and straight, while in the more vascular grey matter they are irregular and more sinuous in their course.

Diagram to show the Vena spinalis posterior, the Venae spinales superiores, and the origin of the Vena jugularis interna.

H Hemispheres. ji Internal jugular. op Optic lobes. mo Medulla oblongata. spp Vena spinalis posterior. ]

The choroid plexus of the third ventricle (Plexus choroideus ventriculi tertii, Reissner) lies, as already described, on the roof of the third ventricle; it is somewhat triangular in form (Plexus venosus triangularis, Schöbl), and is evidently a prolongation of the pia mater, with an increased supply of vessels. It receives, at its anterior angle, veins from the cerebral hemispheres and the adjacent parts. The under surface is covered with a layer of ciliated pavement epithelium. At the posterior angles of the plexus the veins communicate on either side (Fig. 110) with the anterior lateral prolongation of the vena spinalis superior, and with the internal jugular vein. The small body usually named the pineal body, and placed on the roof of the ventricle, is only a small thickened portion of the plexus, and consists of a group of convoluted blood-vessels. This plexus sends a considerable bunch of vessels into the upper part of the third ventricle. The choroid plexus of the fourth ventricle (Plexus choroideus ventriculi quarti, Reissner) is a triangular membrane, slightly attached to the borders of the fourth ventricle. Its upper surface is flat and bounded at either side by a large vein, the vena spinalis superior; anteriorly it is bounded by the cerebellum. The under surface is not flat; in the middle line is a slight furrow corresponding to the position of a median vessel, which may, when injected, be seen from the dorsal surface; from it a number of short vessels pass outwards and slightly backwards to the outer border. The connective-tissue between these vessels is pushed down into the cavity; in this manner one obtains a double row of flattened villous-looking bodies, which frequently have their tips pigmented. The under surface of the plexus is clothed with flattened, ciliated epithelium.

On either side of the pituitary body is another small venous plexus (Plexus lateralis, Schöbl); it communicates above with the posterior angle of the choroid plexus of the third ventricle; externally, with the internal jugular vein; and internally with its fellow of the opposite side by more or less irregular transverse vessels, which, together with a median vein from the fissure between the cerebral hemispheres, form an irregular circulus venosus around the pituitary body. These plexuses receive blood from the lower part of the cerebral hemispheres, the thalamencephalon, the optic lobes, and from the pituitary body. The veins of the spinal cord open into the dorsal, vena spinalis posterior (Fig. 110 spp), which bifurcates at the posterior angle of the fourth ventricle, one division passing to either side and forming, as already described, the outer border of the choroid plexus; anteriorly it opens into the internal jugular vein (Fig. 110 ji).

The blood-vessels, both arteries and veins, are irregularly pigmented, both on the surface and in the interior of the central nervous system; those of the cerebral hemispheres and olfactory lobes have very little or no pigment.

DESCRIPTION OF THE FIGURES ON PLATE I.

Fig. 111. Dorsal view of the orbit, etc.; deep dissection.

c Course of the palatine nerve. db Fibrous plate. dm M. depressor maxillae. ef Terminal branches of the ophthalmic nerve. F Facial with R. anterior of the glossopharyngeal. g,g,g Terminal twigs of the ophthalmic nerve. h Muscular twigs of the Ram. maxillaris. ics M. intertransversar. capitis superior. ii Twigs of upper eyelid. K Terminal twigs of the Ram. maxillaris. la M. levator anguli scapulae. pt M. pterygoideus. sc M. sterno-cleido-mastoideus. Sp Sympathetic nerve. t M. temporalis. tr Nasal branch.

Union of facial nerve with Ram. anterior of the glossopharyngeal nerve.

I Olfactory nerve. II Optic nerve. III Motor oculi nerve. IV Trochlear nerve. Vb R. palatinus } Vc R. maxillo-mandibularis } of the trigeminal Vc′ R. maxillaris } nerve. Vd R. mandibularis } Ve(VII) Facial nerve. Vg Gasserian ganglion. VN Trigeminus. VS Sympathetic nerve. VI Abducens nerve. VII′ Facial nerve. VIII Auditory nerve. X1 Ram. anterior of the glossopharyngeal. X2 Glossopharyngeal nerve. X3 Pneumogastric nerve. XG Ganglion nervi vagi. XI Accessory nerve.

Fig. 112. Dorsal view of the orbit, etc.; superficial dissection.

e External branch of the ophthalmic nerve. f Internal branch of the ophthalmic nerve. g,g,g Terminal twigs of the ophthalmic nerve. II Optic nerve. III Motor oculi nerve. IV Trochlear nerve. Va Ophthalmic nerve. Vc R. maxillo-mandibularis. Vc′ R. maxillaris of the trigeminal nerve. Vd R. mandibularis of the trigeminal nerve. VI Abducens nerve. VI′ Ciliary nerves.

Fig. 113. View of roof of mouth; mucous membrane, etc., removed.

a Branch of the Ram. palatinus to the Harderian gland. b Branch of the Ram. palatinus to the intermaxillary gland. c Inosculating-branch of the Ram. palatinus. Ch Internal naris. d Cutaneous branch of the Ram. palatinus. db Fibrous plate. F Ramus mandibularis of the facial nerve. Gl Intermaxillary gland. HD Harderian gland. l Eyeball. m M. masseter. Ms.ob.i. M. obliquus inferior. OK Upper jaw. r M. retractor bulbi. ri M. rectus internus. ri′ M. rectus inferior. sc M. sterno-cleido-mastoideus. uk Mandible. † Terminal branch of Ram. palatinus. * Union of facial nerve with Ram. anterior of the glossopharyngeal. III Motor oculi nerve. Vb Ramus palatinus of trigeminal nerve. Vc Ramus maxillo-mandibularis of the trigeminal nerve. Vc′ Ramus maxillaris of the trigeminal nerve. Vd R. mandibularis. Ve(VII) Facial nerve. X Pneumogastric nerve. X1 Glossopharyngeal nerve.

Fig. 114. Lateral dissection of head, etc., to show the cranial nerves, etc.

c Cardiac nerve. F Union of facial and Ram. anterior of the glossopharyngeal nerve. h Ram. hyoideus of the glossopharyngeal nerve. la M. levator angi scapulae. lgd M. longissimus dorsi. m M. masseter. mt Ram. mandibularis of the glossopharyngeal nerve. M1 Hypoglossal nerve. M2 Brachial nerve. Oh Anterior cornu of the hyoid bone. oi M. obliquus internus. ph1 M. petrohyoideus I. ph2 M. petrohyoideus II. ph3 M. petrohyoideus III. ph4 M. petrohyoideus IV. pp Pharyngeal branches of the pneumogastric nerve. tt′ M. temporalis. us Muscular twigs of the Ram. mandibularis of the trigeminal nerve. Z Tongue. Va Terminal twigs of the ophthalmic nerve. Vc Ram. maxillaris of the trigeminal nerve. Vd Ram. mandibularis of the trigeminal nerve. Ve(VII) Facial nerve. X1 Ram. anterior of the glossopharyngeal nerve. X2 Glossopharyngeal nerve. X3 Pneumogastric nerve. X3g Ram. gastrici of the pneumogastric nerve. X3l Ram. laryngeus. X3p Ram. pulmonales of the pneumogastric nerve. XI Accessory nerve of the pneumogastric nerve.

Fig. 115. Dissection of the floor of the mouth.

dm M. depressor maxillae. F United facial nerve and Ram. anterior of the glossopharyngeal nerve. gh } M. geniohyoideus. gh′ } h Ram. hyoideus of the glossopharyngeal nerve. H Heart. Lg Lung. m′ Ram. mandibularis of the glossopharyngeal nerve. M1 Hypoglossal nerve. M2 Brachial nerve. Oh Anterior cornu of the hyoid bone. oh Momohyoideus. ph1 M. petrohyoideus I. ph2 M. petrohyoideus II. ph3 M. petrohyoideus III. ph4 M. petrohyoideus IV. sc M. sterno-cleido-mastoideus. sh M. sterno-hyoideus. sm M. mylohyoideus (submaxillaris). smt M. submentalis. TT Thyroid. uk Mandible. us Muscular twigs of the Ram. mandibularis. X2 Glossopharyngeal nerve. X3 Pneumogastric nerve. X3c Rami cardiaci of pneumogastric nerve. X3l Ram. laryngeus of the pneumogastric nerve. X3p Ram. pulmonalis of the pneumogastric nerve.

II. THE PERIPHERAL NERVOUS SYSTEM.

E. THE CRANIAL NERVES.

(To facilitate reference the original arrangement of this part has been altered.)

*1.* The *olfactory nerve* (N. olfactorius), [(Figs. 97, 98, 102, 103, 111 I) runs a very short course only a few lines in length, and escapes from the cranium by an opening in the cartilage of the sphenethmoid into the nasal cavity, where it divides into two branches, each of which breaks up into a brush of filaments, to be distributed in the olfactory mucous membrane. The nerve contains no white fibres]. (See organ of smell, p. 385.)

*2.* The *optic nerve* (N. opticus), (Figs. 97, 102, 103, 111 II) [arises, as already described, by the optic tract, and joins with its fellow at the optic chiasma or commissure, where part of the fibres pass over to the opposite side (according to Michel all the fibres cross). Each optic nerve then courses outwards, piercing the cartilage of the cranium and so reaching the eyeball]. (See organ of sight, p. 408.)

*3.* The *motor oculi* (N. oculomotorius, Oculo-motor, Motor communis), (Figs. 102, 111, 112, 113 III). From its origin it courses outwards and forwards, perforating with a slight obliquity the cartilaginous wall of the cranium, just in front of the Gasserian ganglion (Fig. 116 III); on reaching the orbit it divides into two branches, between the Levator bulbi and the Rectus internus and inferior. One branch, Ramus superior, runs over and parallel to the Ramos ophthalmicus Trig. and enters the under surface of the Rectus superior; the second, lower branch, Ramus inferior, supplies filaments to the Rectus internus and inferior and to the Obliquus inferior. Apparently it exchanges fibres with the ophthalmic division of the trigeminal nerve.

*4.* The *pathetic* or *trochlear nerve* (N. trochlearis, patheticus), (Figs. 102, 103, and 112 IV). From its origin it courses forwards with and then crosses the motor-oculi; it perforates very obliquely the cartilaginous lateral wall of the cranium, in front of the motor oculi but above and very slightly behind the optic foramen (Fig. 116); it runs parallel to and in company with the Ramus opthalmicus trigemini, and appears to exchange a few fibres with it, which, however, according to de Watteville, Stannius, Cuvier, and Wyman, is not really the case. In this course it lies first to the inner, then to the outer side of the ophthalmic, over which it passes to supply the Rectus superior. [The pathetic and ophthalmic nerves are sometimes enclosed in a common sheath (de Watteville).]

*5.* The *trigeminal nerve* (N. trigeminus), (Figs. 97, 102, 103, 111, 112, 113, and 114 V) is the largest of the cranial nerves in the frog; from its origin it runs outwards and forwards to the skull wall, and just before reaching this enters the large Gasserian ganglion. It then passes through the cranial wall immediately in front of the auditory capsule, and divides at once into two main branches (Fig. 111 Vg, etc.), the Ramus ophthalmicus and the Ramus maxillo-mandibularis.

The Gasserian ganglion is a large, yellow, oval ganglion placed in a depression in the outer wall of the cranium; it is covered by a ‘periganglionic gland,’ which is similar in structure to that of a spinal ‘periganglionic’ gland (see p. 180), the fibrous capsule being stronger. This ganglion receives three other nerves besides the trigeminal, viz. the sixth and seventh nerves, and branches of the sympathetic.

According to de Watteville, these nerves are arranged on the lower surface of the ganglion, so that the sympathetic is below, the sixth above it, then the seventh, while the fifth is uppermost; the sympathetic splits into several bundles which join the various branches of the ganglion; the sixth divides into two bundles, one of which usually joins the ophthalmic, while the other makes its exit independently; the seventh splits into two bundles, one of which leaves the ganglion as the hyomandibular branch, the other gives some fibres to the palatine branch of the trigeminal nerve, and is then continued as the palatine branch of the facial.]

*a.* The Ramus ophthalmicus (Ramus nasalis, Fischer; Ophthalmic or Orbital Branch, Wyman; Orbito-nasal or Ophthalmic Nerve, de Watteville), (Figs. 111 and 112 Va). After leaving the Gasserian ganglion the nerve is directed forwards parallel to the side of the cranium, between this and the eyeball. It lies beneath the Rectus superior, but above all the other muscles of the eyeball and the optic nerve. At the anterior end of the orbit it divides into two branches (e and f) which pass through apertures (Foramen pro ramo nasali) in the cartilage of the sphenethmoid, to reach the nasal cavity, where they lie between the cartilage and mucous membrane. In this position the nerves supply branches to the mucous membrane and then pierce the skull to appear on the surface (Figs. 111, 112 g,g,g, 114 Va), the skin of which they supply. The branches inosculate with each other and communicate with the anterior twigs of the Ramus maxillaris (Fig. 114 Vc) and with those of the Ramus palatinus (Fig. 113 Vb).

In its course through the orbit the Ramus ophthalmicus gives off:--

(1) Near the Gasserian ganglion one, two, or three small branches, which communicate with the trochlear nerve, and may then be traced to the sclerotic of the eyeball; some of the twigs enter near the optic nerve, others nearer the cornea. They are regarded as Ciliary nerves (Nervi ciliares).

(2) A large branch, the *palatine nerve* (Ramus palatinus), (Figs. 111 and 113 Vb, b). This nerve, after receiving the palatine branch of the facial, descends on the inner border of the Levator bulbi to the mucous membrane of the mouth, to which it gives numerous filaments; it then runs parallel and near to the median line, lying on the base of the skull; and near the front of the orbit it divides into three branches. The smallest (a) supplies the Harderian gland (HD); the second (b) continues in the course of the original nerve, pierces the vomer, and divides, giving filaments to the mucous membrane in its whole course, while the terminal twigs supply the intermaxillary gland (Gl) and the surrounding structures. One of these (†) ascends on the anterior border of the sphenethmoid to the intermaxillary (internasal, Born) space to supply the numerous glands and to inosculate by one or two twigs with the nasal branch (tr) of the trigeminal. The third branch of the palatine nerve (c) curves directly outwards, just behind the palatine bone, to the inner surface of the maxillary bone; it then courses backwards, pierces the fibrous plate (db) between the eyeball and the pterygoid bone, and ultimately unites with superior maxillary division of the trigeminal nerve (Vc′). Just before piercing the fibrous plate it gives off a tolerably large nerve (d) to the mucous membrane, and in its whole course it gives off twigs to the mucous membrane and surrounding structures; many of these twigs inosculate with other terminal twigs of the trigeminal.

*b.* The Ramus maxillo-mandibularis (Ramus maxillaris, Ecker and Hoffmann; Supra-maxillary, Humphry; Upper Maxillary Branch, Wyman), (Figs. 111, 112, and 113 Vc). This nerve is the largest division of the trigeminal; it runs directly outwards behind the eyeball, in front of the auditory capsule, and between the temporal and pterygoid muscles. After a very short course it divides into the maxillary and mandibular nerves. The nerve gives off before its division a few fine filaments (i,i) to the hinder half of the upper eyelid and to the surrounding skin, also a branch (h) [not correctly drawn in Fig. 111] which divides to supply twigs to the temporal and pterygoid muscles.

(1) The Ramus maxillaris (Ramus supramaxillaris, Ecker; Ramus maxillaris superior, Hoffmann; Upper Maxillary Branch, Wyman; Supramaxillary, Humphry and de Watteville), (Figs. 111, 112, 113, and 114 Vc) runs outwards and then forwards, between the eyeball and the outer wall of the orbit, to the margin of the upper jaw (K); a large portion terminates in a number of small branches for the supply of the skin of the lower eyelid, the upper lip, and of the parts between the tympanic membrane and the external nares. Some of these twigs inosculate with twigs of the palatine and ophthalmic nerves. The second portion of the nerve unites with the palatine nerve, as already described.

(2) The Ramus mandibularis (Ramus maxillaris inferior, Hoffmann; Mandibular or Lower Jaw Branch, Wyman; Inframaxillary, Humphry and de Watteville), (Figs. 111, 112, 113, 114, and 115 Vd, us). As far as the outer margin of the eyeball, this nerve runs parallel to and behind the Ramus maxillaris, in which course it supplies branches to the temporal and pterygoid muscles; it then curves backwards, outwards, and downwards to the under surface of the squamosal bone, where it supplies a twig to the Depressor maxillae, and then perforates the Masseter: in this manner it reaches the outer surface of the mandible, just behind the insertion of the temporal muscle; it then courses forwards, under the skin, to the Symphysis menti. In this course the nerve gives off numerous branches to the skin and surrounding parts, it also supplies the mylo-hyoid and submental muscles; one branch (Figs. 114 and 115 us) is larger than the rest, and supplies the under surface of the floor of the mouth and lower lip.

*6.* The *abducens nerve* (Nervus abducens, Ecker and Hoffmann; included in the trigeminal, Wyman), (Figs. 111 and 112 VI). This very slender nerve courses along the inner wall of the cranium from its origin to the Gasserian ganglion (q.v.) which it joins, and leaves this in contact with the ophthalmic division of the trigeminal nerve: the nerve then bifurcates in the orbit; the outer branch supplies the Rectus externus, the inner inosculates with small twigs of the ophthalmic division of the trigeminal, and then gives off a number of small ciliary nerves, already described, and one special branch to the Retractor bulbi.

*7.* The *facial nerve* (N. facialis, Ecker and Hoffmann; Facialis (Portio dura), Wyman; Facial or Jugular Nerve, Stannius), (Figs. 111, 113, and 114 Ve VII) arises immediately behind the trigeminal, and runs forwards, first in company with the auditory nerve, then alone, to the Gasserian ganglion (q.v.), with which it unites; it again appears at the posterior angle of the ganglion, and escapes from the cranium in company with, and immediately behind, the Ramus mandibularis of the trigeminal, and divides at once into two branches.

*a.* The Ramus palatinus at once unites with the palatine branch of the ophthalmic division of the trigeminal nerve. Wiedersheim doubts this arrangement, and holds that the two portions arise in common from the Gasserian ganglion; neither has Wyman (Rana pipiens) described this branch of the facial. [De Watteville states that the Ramus palatinus and the palatine branch of the ophthalmic nerve are separated by the carotid artery. In urodeles the two nerves are distinct.]

*b.* The Ramus hyomandibularis (Ramus jugularis, Hoffmann; Facial Nerve, Wyman), (Figs. 113 and 114, Ve VII). The nerve is directed outwards and backwards so as to pass around the bony wall of the auditory capsule, it then crosses over the inner end of the columella, with which it is in close contact, and is then joined, under cover of the sterno-cleido-mastoideus (sc), by a branch (Ramus communicans) of the glossopharyngeal (see Fig. 111 *). The single trunk (F) so formed is directed downwards in the posterior wall of the Eustachian tube to just above the angle of the mandible, where it divides into three branches, or sometimes into two, in which case a third nerve is supplied by one of the others, usually by the Ramus hyoideus.

(1) The Ramus mandibularis (Ramus mentalis, Hoffmann; Third Trunk of the Facial, Wyman), (Figs. 114 mt and 115 m′) passes inside the angle of the jaw and courses forwards, between the skin and mylo-hyoid muscle, parallel to the Ramus mandibularis of the trigeminal nerve, as far as the symphysis. It supplies small twigs to the neighbouring parts of the mucous membrane of the mouth.

(2) The Ramus hyoideus (Ramus jugularis, Hoffmann; Second Trunk of the Facial, Wyman), (Figs. 114, 115 h) is the largest division of the facial; it courses forwards subcutaneously over the deltoideus and the hinder fibres of the mylo-hyoideus to the anterior cornu of the hyoid bone, supplying its muscles and the skin of the throat and sternal region.

(3) The Ramus auricularis (First Trunk of the Facial, Wyman); is a small branch, directed outwards; it supplies a branch to the walls of the tympanic cavity, and is finally distributed in the skin under the tympanic membrane and behind the angle of the mouth.

*8.* The *auditory nerve* (N. acusticus, Ecker, Hoffmann; Auditory Nerve, Wyman). This nerve has a very short course in the cranium; it reaches the auditory organ by a foramen (Fig. 116 VIII) in the auditory capsule, and is then distributed in the ear, with which it will be described.

*9.* The *glossopharyngeal nerve* (N. glossopharyngeus, Ecker; Ramus glosso-pharyngeus, Hoffmann; Glossopharyngeal, Wyman; Glossopharyngeal of the Vagus, Müller), (Figs. 102 and 103 X, 111, 113, 114, and 115 X^2) arises in common with the pneumogastric nerve, and quits the skull with it through an opening (Canalis nervi vagi) immediately behind the auditory capsule, and at once divides into two branches; both of which lie under the MM. intertransversarii capitis and are covered by the Depressor mandibulae.

*a.* The Ramus anterior (Verbindungsast des Glossopharyngeus, Wiedersheim; Ramus communicans, Hoffmann; Uniting Branch, Wyman; Laryngeal Branch of the Vagus, Volkmann; Communicans ad facialem, de Watteville), (Figs. 111, 113, and 114 X^1). This nerve curves downwards and forwards around the auditory capsule and beneath the Depressor mandibuli to join the facial nerve, as already described.

*b.* The Ramus posterior (N. glossopharyngeus, Wiedersheim; Ramus lingualis, Hoffmann), (Figs. 111, 113, and 114 X^2). This nerve runs downwards and forwards to the ventral surface of the pharynx, dips underneath the Petrohyoideus IV, to appear again, after a short interval, by piercing the Petrohyoideus II or III, courses parallel to and behind the anterior cornu of the hyoid bone, and thus reaches the floor of the mouth. The nerve then runs forwards in a very sinuous course, close to the median line, and between the Geniohyoideus and Hyoglossus; in its course on the floor of the mouth it crosses the hypoglossal nerve. It supplies the petrohyoid muscles, and gives numerous small branches to the mucous membrane of the pharynx (Ramus pharyngeus, Hoffmann).

*10.* The *pneumogastric* or *vagus nerve* (Vagus; Ramus intestinalis nervi vagi, Fischer, Hoffmann; Vagal Trunk, Wyman; Vaso-sympathetic, Gaskell), (Figs. 111, 113, 114, and 115 X^3). This nerve arises in common with the glossopharyngeal; the two nerves leave the skull together by an opening in the exoccipital bone (Canalis nervi vagi); immediately outside the skull they acquire a ganglionic enlargement (Ganglion condyloideum, Ganglion nervi vagi), (Fig. 111 XG); in this course the glossopharyngeal lies in front of the pneumogastric, which it then leaves. The pneumogastric lies first upon the M. levator anguli scapulae, then running backwards and downwards along the hinder border of the Petrohyoideus IV it comes to the side of the pharynx; it is covered by the trapezius, and passes between the hypoglossal nerve and the Aorta ascendens; arriving at the Arteria pulmonalis, it gives off its terminal branches. [Gaskell has proved that this nerve contains both sympathetic and vagal elements, and that it is therefore really a vago-sympathetic.]

Right half of skull of Rana esculenta, seen from within.

II Optic nerve. III Motor oculi nerve. IV Trochlear nerve. VN Root of the trigeminal nerve. Vg Gasserian ganglion. VS Sympathetic nerve. VI Abducens nerve. VII^1 Facial nerve. VIII Auditory nerve. X^{1–3} Pneumogastric and glossopharyngeal nerves. ]

The branches and communications of the pneumogastric nerve are:--

*a.* Communications between the ganglion of the pneumogastric and the sympathetic system (Fig. 111 Sp), which again connect it with the Gasserian ganglion of the trigeminal nerve (Fig. 111 VS, Vg); (see sympathetic system).

*b.* [The Ramus cutaneus dorsalis (Fischer) is considered by Stannius and Fürbringer to be the homologue of the Ramus auricularis; it passes outwards between the digastricus and temporalis to be distributed in the skin of the suprascapular region. This nerve is the persistent portion of the N. lateralis nervi vagi of the tadpole.]

*c.* During its course over the petrohyoidei the vagus gives off a few twigs (Fig. 114 pp), which form a fine plexus to supply these muscles and the pharynx [also a twig to the trachea, according to Hoffmann].

*d.* The Ramus accessorius (Fürbringer, Hoffmann, Ecker), (Fig. 114 XI) is usually a single small nerve which runs between the intertransversarii muscles and the trapezius: it supplies the under surface of the latter muscle.

*e.* The Ramus scapularis (Hoffmann) is a very slender nerve arising near the Ramus accessorius; it passes along the under surface of the trapezius to the inner surface of the interscapularis, which it supplies.

*f.* The Ramus laryngeus (Recurrens vagi), (Figs. 114 and 115 X3l) runs for some distance parallel to the pneumogastric, separated from it by the petrohyoideus IV; arriving at the hinder cornu of the hyoid bone the nerve loops round the Arteria pulmonalis, and divides into two branches for the supply of the larynx.

*g.* The Rami gastrici (Fig. 114 X3g) are usually two in number: they pierce the partial diaphragm formed by the anterior fibres of the M. obliquus internus and terminate in the walls of the stomach.

*h.* The Rami pulmonales (Figs. 114 and 115 X3p) also perforate the partial diaphragm, and then course along the pulmonary arteries to the lungs.

*i.* The Ramus cardiacus (Figs. 114 and 115 X3c). This nerve is usually smaller on the right side than on the left; it passes along the dorsal surface of the pulmonary artery and Vena cava superior to the Sinus venosus; in this course it gives off two or three twigs to the roots of the lungs: the two nerves communicate just before reaching the heart and pass on to the auricular septum, whence they are distributed to the heart.

*j.* A slender *Laryngeal nerve* arises from the vagus alongside the Ramus cardiacus; it courses along the hinder end of the greater cornu of the hyoid to the outer side of the pharynx, which it pierces to pass to the larynx.

F. THE SPINAL NERVES.

I. *General description.* Ten pairs of nerves arise, as already described (p. 135), from the spinal cord; each nerve has two roots, a ventral or anterior, and a dorsal or posterior, which unite at their points of exit from the intervertebral foramen: just before, and for a short distance beyond this union, each dorsal root bears a ganglionic enlargement.

*a.* The length and direction of the various *Nerve-roots* vary greatly; the roots of the anterior spinal nerves run a very short course, almost transversely outwards, from their points of origin to the intervertebral foramina: the roots of the middle and posterior nerves, in consequence of the vertebral column being considerably longer than that part of the cord belonging to it, pass obliquely backwards to the foramina, the hinder nerves of the Cauda equina running for a considerable distance in the vertebral canal (Fig. 116). The relations of these roots are as follows:--

(1) The roots of the first nerve (N. hypoglossus) arise from the spinal cord at a point between the first and second vertebrae; they run transversely outwards to escape between the first and second vertebrae. The dorsal root is extremely slender.

The nervous system of Rana esculenta, from the ventral surface. (From Icones physiologicae by A. Ecker. Pl. XXIV, Fig. 1.)

F Facial nerve. G Ganglion of pneumogastric nerve. He Cerebral hemisphere. Lc Optic tract. Lop Optic lobe. M Boundary between medulla oblongata and spinal cord. M1–10 Spinal nerves. MS Connection between fourth spinal nerve and sympathetic chain. N Nasal sac. Ni Sciatic nerve. No Crural nerve. o Eyeball. S Trunk of sympathetic. S1–10 Sympathetic ganglia. SM Rami communicantes of the same. Sp continuation of sympathetic into head. I Olfactory nerve. II Optic nerve. III Motor oculi nerve. IV Trochlear nerve. V Trigeminal and facial nerves. Va Ramus ophthalmicus. Vc Ramus maxillaris. Vd Mandibular branch of trigeminal. Ve Hyomandibular branch of facial. Vg Gasserian ganglion. Vs Upper end of sympathetic trunk in connection with Gasserian ganglion. VI Abducens nerve. VII Facial nerve. VIII Auditory nerve. X Glossopharyngeal and pneumogastric nerves. X1 Ramus anterior of glossopharyngeal. X2 Ramus posterior of glossopharyngeal. X3, 4 Branches of pneumogastric. ]

(2) The roots of the second nerve (N. brachialis) arise at the level of the second vertebra and leave the vertebral canal between the second and third vertebrae.

(3) The roots of the third nerve arise from the cord between the second and third vertebrae, and pass out between the third and fourth vertebrae.

(4) The fourth nerve arises by its two roots at the level of the fourth vertebra, and quits the vertebral canal between the fourth and fifth vertebrae.

(5) The fifth nerve arises opposite the fourth vertebra, and passes outwards and slightly backwards to the foramen between the fifth and sixth vertebrae.

(6) The roots of the sixth nerve are attached to the cord opposite the fourth vertebra, and leave the vertebral canal between the sixth and seventh vertebrae.

(7) The seventh nerve arises from the cord at a point between the fourth and fifth vertebrae, and leaves the canal between the seventh and eighth vertebrae.

(8) The eighth nerve quits the cord at a point opposite the articulation between the fifth and sixth vertebrae, then runs backwards to the eighth vertebra, and escapes between this and the ninth vertebra.

(9) The ninth nerve arises at the level of the sixth vertebra, and passes out between the sacrum (ninth vertebra) and the urostyle.

(10) The tenth nerve (N. coccygeus) arises from the cord immediately behind the ninth nerve and opposite the sixth vertebra, and courses alongside the terminal filament to pass out through a foramen in the urostyle (canalis coccygeus).

*b.* [The *Spinal ganglia* (Ganglia intervertebralia).

At their exit from the intervertebral foramen the two roots of each nerve unite and bear a ganglionic enlargement, which is in intimate connection with the dorsal root, but is merely in contact with the ventral root. The ganglia lie in the large intervertebral depressions, upon the under surface of the Proc. obliqui, above and behind, being in relation with the bodies of the vertebrae internally, and the Musculi intertransversarii externally (Fig. 119); ventrally the ganglia are, more or less, covered by the ‘periganglionic glands.’ The ganglia vary much in size; that of the first spinal nerve is the smallest. The ganglion is not attached to the dorsal root alone but is prolonged a short distance beyond the point of union of the two roots; this is best seen in the nerves of the lumbar plexus. Each ganglion is a yellowish-white rounded or oval body, with an outer thick (sometimes 0.15 mm. thick) sheath of connective-tissue, connected by the epineurium of the nerve roots with the Dura mater. This sheath is composed of bundles of parallel, wavy, white, connective-tissue fibres, which enclose a large number of connective-tissue corpuscles, and here and there contains brown pigment.

Ventral view of the brain and spinal cord, to show the points of exit of the spinal nerves.

M1–10 Spinal nerves. VG Trigeminal ganglion. W1–10 Vertebrae. XG Ganglion of vagus. ]

By means of longitudinal and transverse sections it may be seen that the ganglion surrounds the ventral root, but that it forms a thinner layer on that side, where the dorsal root joins the ventral; although at this point the two roots and the ganglion are firmly united together, the thick fibrous sheath of the ganglion prevents any communication between the nervous elements of the ventral root and those of the ganglion.

The sheath contains nerve-cells and fibres, and sends in a few fine processes to support these structures. The fibres of the posterior root enter and pass out of the ganglion without suffering any changes, except that the nerve fibres are slightly separated by the presence of a few nerve-cells, and that a distinct increase in the number of nerve fibres takes place. The nerve-cells, of which the ganglion is chiefly composed, and upon the number of which its size directly depends, are arranged chiefly around the fibres of the dorsal root; such few as lie between the fibres are smaller than the rest and vary more in their relative numbers; at times one or two isolated cells are found in the capsule or even in the adjoining ‘periganglionic gland.’ The cells near the capsule are somewhat smaller than the deeper cells. Each of these cells, which are usually pear-shaped, possesses only one process; the cell membrane is thick, resistant, and possessed of an external nucleated, endothelial covering, the space around being probably a pericellular lymph-space; it often contains one to three small fat-globules; the protoplasm of these has, according to v. Lenhossék, a concentric fibrillation; the nucleus is round, clear, and distinct, and relatively larger the smaller the nerve-cell; it is usually placed in the centre of the cell.

Ventral view of the spinal ganglia; on the right side they are still hidden by the ‘periganglionic glands.’ ]

Schema of spinal ganglion.

A Ventral root. C Ramus communicans. G Ganglion O Dorsal division. P Dorsal root. R Ventral division. ]

In that portion of the cell towards the process is a portion brighter and less easily stained than the rest of the cell (Polarkernen, Courvoisier); this apparently possesses one or two nuclei, and is regarded as a cell by Lenhossék (Polarzellen); the process of each cell soon acquires a medullated sheath, and after a short course, 0.09 mm. to 0.35 mm., divides usually about the third constriction (Lenhossék). These processes all pass peripherally, even those which appear at first to pass in the opposite direction curve round again; it is chiefly due to these fibres that the spinal nerve is larger than its two roots.

In addition to the above, smaller, irregular cells from 5 µ to 7 µ in length are found; Rawitz regards them as young cells, Lenhossék as cells capable of (entwicklungsfähige) developing into nerve-cells. The ganglia contain only few blood-vessels.]

The above description is founded upon that of Lenhossék, and has been confirmed by the translator in every particular, except that only very indistinct and unsatisfactory indications of the concentric fibrillation so clearly delineated by Lenhossék were obtained; the structure of the Gasserian ganglion was found to correspond exactly with the description here given of the structure of the spinal ganglia. The nucleus was oftener at the side than at the centre of the cell.

*c.* The ‘*Periganglionic Glands*’ (Periganglionäre Kalkdrüsen, v. Lenhossék; Kalksäckchen, Ecker, and others; Crystal capsules, Wyman; Calcareous Sacs or Masses, of other writers), (Fig. 119). These bodies are found on the ganglia of all the spinal nerves and on the Gasserian ganglion of the trigeminal nerve; they have recently been carefully investigated by v. Lenhossék. According to his description there are usually two to each ganglion, lying on its sides and ventral surface; each consists of a connective-tissue capsule which sends in a few fine trabeculae to support a system of glandular tubes: these usually run parallel to the long axis of the gland, and are about 14 µ to 15 µ in diameter; each tube is lined with a single layer of somewhat columnar epithelium, the cells of which have sharp, distinct outlines and oval nuclei.

A membrana propria was not made out with certainty; the lumen of the tubes is wide and contains a milky fluid, which gives the whole structure its characteristic appearance; it has long been known (Blasius, 1681, mentions the fact, l.c., p. 291) that this fluid contains calcareous matter, which effervesces and dissolves in the presence of hydrochloric acid; the crystals vary in form but are chiefly oval (Wyman).

These glands bear no definite proportion to the size of the nerves to which they are attached (Fig. 118); they already contain calcareous matter during the tadpole stage before the limbs are developed (Stannius, Wyman). No ducts have been traced to these glands. This description, founded upon that of Lenhossék, can be confirmed by the translator in every particular; his sections, however, would lead him to believe that the glands are far more vascular than the description and the drawings of Lenhossék imply.]

*d.* The *Branches* of the complete spinal nerves formed by the union of the dorsal and ventral roots with the ganglion (Fig. 120) are usually described as two, a dorsal and a ventral branch, which latter at once gives off a Ramus communicans to the sympathetic system; these two branches are given off almost immediately beyond the ganglion.

II. The Individual Nerves.

A. The Dorsal Branches (Fig. 121).

Dorsal branches of the spinal nerves. The Fascia dorsalis is dissected, removed from the left side.

rc Rami cutanei. rm Rami musculares. ]

The dorsal branches are smaller than the ventral; they pass upwards between the inner borders of the Musc. intertransversarii and the articular processes of the vertebrae to reach the under surface of the M. longissimus dorsi; the general course of these nerves is very similar for all; each nerve gives off--

(a) A twig to the Musc. intertransversarius and then divides into two branches.

(b) A Ramus muscularis (Fig. 121 rm), which passes outwards to supply the M. longissimus dorsi.

(c) A Ramus cutaneus (Fig. 121 rc); this nerve continues under the extensor muscle of the back, running on the arch of the vertebra towards the middle line; it then pierces the muscle and Fascia dorsalis to reach the large dorsal lymph-sac (Sacc. cranio-dorsalis), through which it courses to end in the skin. In its course through the sac the nerve is connected with a small artery and vein by a small amount of connective-tissue, the whole being enclosed in a sheath of endothelium continuous with that of the general cavity.

The three anterior cutaneous nerves pierce the Fascia dorsalis near the spinous processes, the fourth a little to the side, the fifth perforates the M. coccygeo-iliacus at about the junction of its anterior and middle thirds; the sixth and seventh perforate the same muscle, but more posteriorly. There are seven of these Rami cutanei, the first coming from the second spinal nerve, the rest from the succeeding six spinal nerves.

B. The Ventral Branches.

The ventral branches of the spinal nerves are larger than their corresponding dorsal branches: each nerve gives off immediately a Ramus communicans to the sympathetic system (Figs. 117 SM, 120 C); these Rami communicantes will be described with the sympathetic system; the remaining portions of the ventral divisions of the original spinal nerves are usually known as the spinal nerves; they will be described as such in detail.

*1.* The *hypoglossal nerve* (N. hypoglossus, N. spinalis I), (Figs. 114 115, and 117 M1), or first spinal nerve, is of small size; it leaves the vertebral canal between the first and second vertebrae to lie between the Musculi intertransversarii and the pharynx, where it has in front the Arteria vertebralis, behind the Arteria and Nervus brachialis; passing under the Levator anguli scapulae, it touches the Aorta ascendens and crosses the Vagus and the Carotid-gland, and thus reaches the space between the Musculi sterno-hyoidei and the Petro-hyoideus; then curving forwards sharply it turns under the Mylo-hyoideus to course between the fibres of the Genio-hyoideus to the root of the tongue, where it ends. It first lies to the inner and then to the outer side of the glossopharyngeal nerve. In its course the nerve gives off the following branches:--

(a) One or two communicating twigs (Figs. 114 and 122) to the second spinal nerve.

(b) Muscular twigs to the Longissimus dorsi, Intertransversarii capitis, Levator anguli scapulae, and the Retrahens scapulae.

(c) When near the glossopharyngeal it supplies twigs to the Geniohyoideus, Sternohyoideus, and Omohyoideus, where it bifurcates.

(d) One of the branches passes inwards to supply the Hyoglossus.

(e) The other terminal branch passes forwards with the Ramus lingualis and the glossopharyngeal, between the fibres of the Geniohyoideus, which it partially supplies, and sends twigs to the neighbouring parts.

(f) [Hoffmann describes a communicating branch to the pneumogastric nerve, which the translator has not been able to discover, and which no other observer has mentioned.]

*2.* The *second spinal* or *brachial nerve* (N. brachialis, N. spinalis II), (Fig. 122) is a large nerve, leaving the vertebral canal between the second and third vertebrae; it then accompanies the Art. axillaris along the anterior border of the transverse process of the third vertebra, over the hinder end of the Musc. levator scapulae and the anterior end of the m. transverso-scapularis major, to the inner border of the Musc. subscapularis, under which it passes into the arm. In this course it gives off or receives the following branches:--

(a) It first receives a branch from the second spinal nerve.

(b) It gives off a large branch, the N. coraco-clavicularis (Fig. 122 Cc), which first accompanies a branch of the Art. axillaris on the Musc. levat. scapulae, then courses forwards and outwards over the Musc. subscapularis to pass from above into the Foramen ovale between the clavicle and coracoid bones. It gives off two branches:

(1) One immediately beyond its origin, which runs backwards over the Musc. subscapularis to supply the Musc. obliq. abdominis internus (Fig. 122 Cc′).

(2) While in the Foramen ovale, the N. coraco-clavicularis bifurcates; the anterior twig runs forwards and outwards to the deltoideus and also supplies a recurrent filament to the Musc. sterno-radialis; the posterior twig enters the upper surface of the Musc. stero-radialis.

(c) At the outer extremity of the Musc. transverso-scapularis the brachial nerve gives off a posterior branch, which at once bifurcates:

(1) The first branch passes into the under surface of the Musc. latissimus dorsi (ld), giving a twig to the Infraspinatus.

(2) The second is the Ramus cutaneus axillaris (IIc); it passes on the under surface and posterior border of the Musc. latissimus dorsi to the skin of axilla and dorsal surface of the upper arm.

(d) Several small branches are given off from the upper and anterior surfaces of the brachial nerve near the Foramen ovale; they accompany an artery to pass forwards and upwards between the Musc. subscapularis and the M. transverso-scapularis on the one side, and the long head of the M. triceps brachialis on the other, and are lost in the M. deltoideus and the M. infraspinatus.

The brachial plexus.

Cc N. coraco-clavicularis. Cc′ Nerve to musc. obliq. abdom internus. d Deltoid muscle. la Musc. lev. ang. scap. ld Musc. lat. dorsi. oi Musc. obliq. abdom. intern. sc Musc. sterno-mastoid. ss Musc. subscapularis. SI First spinal nerve. SII Second spinal nerve. SIII Third spinal nerve. IIc Ramus cutaneus axillaris. IIIc Ramus cutaneus abdominalis. ]

The N. brachialis continues its course by passing between the long and inner heads of the M. triceps, reaches the arm and splits into two nerves, the N. ulnaris and the N. radialis.

α. The Ramus subscapularis passes dorsally to the M. subscapularis.

β. A Ramus pectoralis (Fig. 123 up), passes forwards to the M. abdomino-pectoralis and to the skin of the pectoral region (R. cutaneus pectoralis).

γ. The N. cutaneus antibrachii superior s. medialis (uc) passes to the skin of the inner side of the forearm; this and the two following nerves are given off near the Plica cubiti.

Nerves of the central surface of the arm. The hand pronated.

rc Upper cutaneous branch of the N. radialis. rc′ Lower cutaneous branch 4-,f the N radialis U N. ulnaris. uc R. cutan. sup. of the N. ulnaris. uc′ R. cutan. inf. of the N ulnaris. up R. pectoralis of the N. ulnaris. ]

δ. The N. cutaneus antibrachii inferior s. dorsalis s. musculo-cutaneus (uc′) at once supplies branches to the M. flexor carpi radialis, and then runs downwards upon the muscle and bifurcates:--

The R. lateralis is distributed, by two twigs, in the M. flexor carpi ulnaris and in the skin of the second finger, to which it supplies the Rami digitales volaris and dorsalis.

The R. dorsalis, after giving a cutaneous twig to the second finger, is distributed by numerous twigs to the skin of the dorsal surface of the hand.

The N. ulnaris then passes between the M. flexor carpi radialis and the tendon of the M. sterno-radialis, sinks deeply into the Plica cubiti, where it lies between the M. flexor carpi radialis and the Flexor carpi ulnaris, then between the latter and the Flexor anti-brachii medialis on the one side and the Flexor digitorum communis on the other: it supplies branches to all these muscles, and while still in the forearm divides into two terminal branches:--

α. The R. ulnaris medialis is the smaller; it runs inwards, passes under the tendon of the M. extensor carpi ulnaris into the palm of the hand and ends as the N. volaris digiti V medialis.

β. The R. ulnaris lateralis passes near the thumb-rudiment, covered by the M. abductor pollicis, deeply into the palm of the hand, and supplies by bifurcating branches the adjacent sides of the second, third, fourth, and fifth fingers (R. volares); it also gives twigs to the muscles of the palm.

Nerves of the ventral surface of the arm. The hand supinated, and the superficial layer (Flex. carpi rad. and uln., Flex. dig. commun.) has been removed.

R N. radialis. u N. ulnaris. uc R. cutan. inf. of the N. ulnaris. um Bifurcation of the N. ulnaris. ]

α. Rami musculares to the separate parts of the M. triceps.

β. The Ramus cutaneus superior (Fig. 125 rc) to the skin of the outer surface of the upper arm and forearm.

The nerve then sinks deeply under the origins of the M. extensor carpi ulnaris and the M. extensor digitorum communis, where it bifurcates after giving off some small twigs:--

γ. Small muscular twigs to the extensor muscles of the hand and fingers.

The N. radialis.

ec M. extensor carpi ulnaris. ed M. extensor digit. comm. R N. radialis. R1 R. lateralis. R2 R. medialis. rc R. cutaneus superior. rc′ R. cutaneus inferior. ]

δ. The R. radialis medialis (R2), the smaller terminal branch, supplies a branch to the M. extensor digit. comm. longus and the skin over the carpus (Ramus cutaneus inferior, rc′), and ends on the muscles of the little finger as the R. dorsalis digiti V medialis.

ε. The R. radialis lateralis (R), the larger of the two branches, supplies the extensor muscles of the fingers and gives bifurcating branches, Rami digitales dorsales, to the adjacent sides of the second, third, fourth, and fifth fingers (the Ramus dorsalis lateralis of the second finger is supplied by the N. ulnaris).

*3.* The *third spinal nerve* (N. spinalis III) (Fig. 122 SIII) leaves the vertebral canal by the foramen between the third and fourth vertebrae, and runs outwards and forwards upon the large transverse process of the third vertebra to the brachial nerve (N. spinalis II). Its behaviour at this point is subject to considerable variation; at times it joins the brachial nerve completely, at other times the two nerves are simply in contact; or lastly, it may send a small twig to the brachial nerve. Externally to this point the two nerves supply a number of small branches, which may belong almost entirely to either one nerve or the other; these nerves again inosculate in a variable manner, to form an axillary plexus. The more constant branches are:--

(1) One or more twigs to the Musc. transverso-scapularis major.

(2) Several branches, usually two larger and one smaller; they course in the M. obliq. abdom. internus (Fig. 122 oi) to the M. rectus, where they give off branches, then pierce the muscle to reach the skin (R. cutaneus abdominalis), (Fig. 122 IIIc).

*4.* The *fourth spinal nerve* (N. spinalis IV) (Fig. 126 M4) appears between the fourth and fifth vertebrae; it runs on the ventral surface of the transverse process of the fifth vertebra and upon the Musc. intertransversarius obliquely outwards and downwards, and reaches the deeper surface of the Musc. obliquus internus, into which it descends, about opposite the articulation between the eighth and ninth vertebrae; it then bifurcates into:--

(1) A Ramus cutaneus abdominalis, which pierces the muscle and supplies the skin.

(2) A Ramus muscularis, which supplies the broad abdominal muscle and the M. rectus abdominis.

*5.* The *fifth spinal nerve* (N. spinalis V) (Fig. 126 M5) emerges through the foramen between the fifth and sixth vertebrae, runs obliquely downwards and outwards over the transverse processes of the sixth and seventh vertebrae and the corresponding M. intertransversarii, and at a point nearly opposite the middle of the urostyle pierces the M. obliquus internus.

The rest of its course corresponds exactly with that of the fourth spinal nerve.

*6.* The *sixth spinal nerve* (N. spinalis VI) (Fig. 126 M6), after escaping from the vertebral canal between the sixth and seventh vertebrae, runs obliquely downwards and outwards on the under surfaces of the transverse processes of the seventh, eighth, and ninth vertebrae, then under the iliac bone to a point opposite the hinder half of the urostyle, where it descends under cover of the M. obliquus abdom. internus. In the rest of its course it repeats the corresponding courses of the fourth and fifth spinal nerves.

Ventral view of the brain and spinal cord, to show the points of exit of the spinal nerves.

M1–10 Spinal nerves. VG Trigeminal ganglion. W1–10 Vertebrae. XG Ganglion of the vagus. ]

*7*, *8*, *9*, and *10*. The *seventh*, *eighth*, *ninth*, and *tenth spinal nerves* (Nervi spinales VII, VIII, IX, and N. spinalis X s. N. coccygeus) (Figs. 126 and 127 M7, M8, M9, and M10) are best described together, as they are intimately associated to form the sciatic plexus (Plexus ischiadicus, Plexus cruralis).

Within the vertebral canal the roots of these nerves form the Cauda equina; the seventh nerve leaves the canal between the seventh and eighth vertebrae, the eighth nerve between the eighth and ninth vertebrae, the ninth between the sacrum or ninth vertebra and the urostyle, and the tenth by the Canalis coccygeus in the urostyle; from these points the four nerves run obliquely on the ventral surface of the M. ilio-coccygeus to the pelvis, where they form the sciatic plexus. Although subject to some variation, the usual arrangement is that the seventh and eighth nerves unite to form a trunk, which then receives the ninth nerve; the large nerve so formed is the sciatic nerve: a branch of the tenth usually then joins either the sciatic plexus or the sciatic nerve; the plexus gives off or receives the following branches:--

(a) Like other spinal nerves, these nerves give off Rami communicantes (see Sympathetic System).

(b) The M. ilio-hypogastricus (Fig. 127 M7a). This arises from the seventh spinal nerve, before its union with the eighth spinal nerve; it runs obliquely outwards and downwards on the Musc. ilio-coccygeus and under the border of the M. obliquus abdom. internus, where it bifurcates:--

(1) One branch, the Ramus cutaneus abdominalis, pierces the M. obliq. abdom. internus in the same fashion as the cutaneous branches of the fourth, fifth, and sixth spinal nerves; it supplies the skin of the abdomen.

(2) The second or Ramus muscularis supplies the flat abdominal muscles and the M. rectus abdominis.

(c) The M. cruralis (Fig. 127 M7b) also arises from the seventh spinal nerve at or just beyond its point of union with the eighth spinal nerve. It accompanies the A. cruralis on the pelvic wall to the thigh, where it lies upon the M. ilio-psoas in a triangle between the M. rectus fem. anterior and the M. adductor longus, and divides into two main branches:--

(1) Muscular twigs to the M. ilio-psoas, M. rectus femoris anticus, MM. adductores longus and brevis.

(2) The Ramus cutaneus femoris, which runs downwards in the hinder wall of the Lymph-sac (Saccus iliacus), and supplies the skin of the under and outer surfaces of the thigh.

The sciatic plexus, from a drawing by Wiedersheim.

a Communicating nerve from the ninth spinal nerve. a′ Nerve to musc. lev. ani. b Nerve to oviduct. c Urostyle. cc Communicating twig from the ninth spinal nerve. ci M. coccygeoiliacus. c.I M. coccygeosacralis. d Nerve to bladder. d′ Twig to the m. iliacus. e Twig from sciatic plexus to coccygeal plexus. i M. intertransversarius. il M. ilio-coccygeus. ip M. ilio-psoas. lev M. lev. ani. oi M. obliq. abdom. internus. t′ Transverse process of sacrum. Ao Aorta. Com Point of union of nerves a and cc. Is Ischium. M5 Fifth spinal nerve. M6 Sixth spinal nerve. M7 Seventh spinal nerve. M8 Eighth spinal nerve. M9 Ninth spinal nerve. M10 Tenth spinal nerve. M7a M. iliohypogastricus. M7b M. cruralis. Ni Sciatic nerve. Ovd Oviduct. Rect′ Rectum. S7, S8, S9, S10 Rami communicantes of seventh, eighth, ninth, and tenth spinal nerves. Ves Bladder. 8, 9 Eighth and ninth vertebrae. ]

(d) The Ramus dorsalis is a very small nerve; according to Waldeyer it possesses only twenty nerve-fibres. It arises from the coccygeal nerve immediately beyond the Canalis coccygeus, perforates the M. ilio-coccygeus, and runs on its dorsal surface obliquely over the lymph-heart, without supplying it; the nerve then pierces the fascia to supply the skin of the back and thigh.

(e) The Ramus abdominalis also arises from the coccygeal nerve; it is larger than the R. dorsalis, and arises at the same point; the nerve runs on the ventral surface of the M. ilio-coccygeus towards the lymph-heart, and inosculates with the sympathetic. It is from this nerve that the branch to the sciatic plexus is usually given off; it further supplies a varying number of branches, which with branches derived directly or indirectly from the sciatic nerve, form a plexus (Plexus coccygeus); the two plexuses may together be regarded as a Plexus ischio-coccygeus or a Plexus sacro-coccygeus.

(f) In addition to the Rami communicantes given off by the spinal nerves near the vertebral column, the sciatic plexus supplies a few twigs, usually two (Fig. 127).

(g) Other small twigs pass from the sympathetic system forwards and outwards to join the sciatic plexus or the sciatic nerve. According to Waldeyer two of these are very constant.

(h) Branches to the oviduct (Fig. 127 b).

(i) Branches to the rectum (Fig. 127 cc).

(k) Branches to the bladder (Fig. 127 d).

(l) Branches to the M. levator ani (Fig. 127 a′).

(m) A branch to the lymph-heart, which runs along the anterior border of the M. lev. ani, on to its dorsal surface, and then direct to the lymph-heart.

*Variations* in the *sciatic plexus*. The arrangement of the nerves in the sciatic plexus is subject to many variations; according to Wiedersheim most of these variations belong to two chief classes: either the N. coccygeus inosculates directly with the ninth spinal nerve by one or several branches, or it joins the ninth nerve indirectly by uniting with its branches. A case of the latter arrangement is seen in Fig. 127, and is thus described by Wiedersheim:--

‘After cutting through the pelvic symphysis and drawing to one side the contained viscera, namely, the hinder end of the oviduct, the rectum, and the bladder, one sees a row of small twigs (a, b, cc, d, and e) arising from the inner, hinder, and anterior surfaces of the ninth spinal nerve: the first (a) runs backwards parallel with the N. coccygeus over the M. ilio-coccygeus to join this nerve at the point marked Com. The twig cc behaves in like manner after receiving a twig from b. The twig b arises from the inner surface of the ninth nerve between the two foregoing and close to its union with the eighth spinal nerve; from this origin twig b passes almost transversely outwards to the hinder extremity of the oviduct (“uterus”), and partly to the rectum, crossing in its course twig a, the sciatic nerve, and the urostyle. Its branches form a net-like plexus with the terminal branches of the last sympathetic ganglion and with a branch cc from the point of union Com.

‘A second branch (a′), arising from the point of junction Com, is a continuation of the N. coccygeus (M10); it passes vertically downwards towards the hinder end of the cloaca and at the upper border of the Levator ani, divides into two branches, which are distributed to the inner and outer surfaces of this muscle and to the cloaca. Other branches pass dorsally to the lymph-hearts, while a third set pass to the hindermost part of the bladder.

‘These three sets of nerves, to the M. coccygeus, M. levator ani, and to the lymph-hearts, are not supplied entirely by the N. coccygeus, as this is reinforced by one or more branches (e) from the sciatic plexus: this branch (e) supplies twigs to the M. ilio-coccygeus, which is also supplied anteriorly from the trunk of the N. coccygeus.

‘The bladder receives a special branch (d), which arises from the sciatic plexus at the junction of the eighth and ninth spinal nerves; this nerve gives a twig (d′) to the M. iliacus.’

*I.* The *sciatic nerve* (N. ischiadicus) (Fig. 128 I) is the largest nerve of the body; it passes under the M. coccygeus, between the origins of the M. vastus externus and of the M. pyramidalis: lies then between the M. biceps and the M. pyramidalis, and later between the M. biceps and the M. semimembranosus; lastly, it bifurcates under the M. biceps to form the N. tibialis (II) and the N. peroneus (III). In this course it gives off:--

(a) A twig to the M. coccygeo-iliacus, while still in the pelvis.

(b) The N. cutaneus femoris posterior (Fig. 128, 2), which passes between the M. pyramidalis and the M. vastus externus, to appear behind and beneath the former; it accompanies an artery of like name to supply the skin of the hinder and inner surfaces of the thigh.

(c) A little below the foregoing it gives off a collateral branch (Fig. 128, 3), which passes under the M. pyramidalis and divides to form--

(1) A branch to the upper third of the M. semimembranosus.

(2) A branch to the Rectus internus minor. A twig of this branch (bb) pierces the muscle transversely in company with the Arteria cutanea, and passes to the skin of the middle of the inner surface of the thigh.

(3) Branches to both heads of the semitendinosus.

(4) Branches to the M. adductor magnus.

(d) Branches covered by the M. pyramidalis to the M. quadrat. femoris and M. obturator.

(e) A branch (5) forwards to the M. biceps.

(f) A branch (6) which accompanies the Art. circumflexa genu lateralis sup. forwards to the M. extensores cruris, the M. vastus externus, and the M. rectus anterior.

Distribution of the sciatic nerve.

I The sciatic nerve. II N. tibialis. III N. peroneus. 1 Branches to the M. pyramidalis. 2 N. cut. fem. posterior. 3 Large collateral branch. 5 Branch to the M. biceps. 6 Branch accompanying the art. circumfl. genu lateralis sup. 7 Branch of tibial nerve to the M. gastrocnemius. 8 N. suralis. ad‴ M. adductor magnus. b M. biceps. bb Twig accompanying the art. cutanea. ci M. coccygeo-iliacus. cp N. cut. cruris lateralis. ct R. cut. cruris posterior. g M. gastrocnemius. gl M. glutaeus. p M. pyramidalis. pe M. peroneus. ra M. rectus anterior. ri′ M. rectus internus minor. sm M. semimembranosus. st M. semitendinosus. ve M. vastus externus. ]

*II.* The Nervus tibialis (Figs. 128 II, 129 T) passes backwards and inwards to supply the extensors of the foot and the flexors of the toes. Its branches are:--

(a) The R. cutaneus cruris posterior (Figs. 128 and 129 ct), accompanied by an artery and vein, passes to the skin of the calf.

(b) A twig to the upper part of the gastrocnemius (Fig. 129 g′). The main nerve, after supplying this twig, bifurcates to form the next two nerves.

(c) The Nervus suralis (Figs. 128 8; 129 s) runs downwards on the inner border of the M. gastrocnemius and gives off:--

(1) A twig (g″) to the upper third of the muscle.

Nerves of the leg and sole of the foot.

ab5 Branch to the M. abduct. digit. V. ad5 Branch to the M. adduct. digit. V. ar M. rectus anterior. b M. biceps. cs Ramus cutaneus medius. ct Ramus cutaneus cruris posterior. dvI Ramus digitalis volaris I. dvII Branch supplying R. digit. volaris I and II. dvIII Second branch of the N. tibialis. F5 Branch to the M. flex. brev. digit. V. flp Branches to the M. plantaris and the M. flexor digitorum. g′ Branch to the M. gastrocnemius. g″ Branch to the M. gastrocnemius. pc N. peroneus. s N. suralis. sm M. semimembranosus. T, t Nervus tibialis. ]

(2) The Ramus cutaneus medius (cs) is given off below the middle of the M. gastrocnemius; it passes to the skin of the lower third of the leg. After giving off the latter branch, the N. suralis runs downwards along the inner aspect of the Tendo achillis to the Aponeurosis plantaris of the sole of the foot as far as the calcar, and gives off:--

(3) Branches (flp) to the Musc. plantaris and the M. flexor digitorum.

(4) A branch to the M. abductor hallucis.

(5) The R. digitalis volaris I (dvI) supplies the inner side of the first toe by its terminal twig.

(d) The Nervus tibialis (Fig. 129 t) passes downwards on the hinder surface of the tibio-fibula, sinks into the M. tibialis posticus to appear again at its hinder border; it then runs over the ankle-joint into the sole of the foot, where it lies midway in the space between the two Ossa tarsi and between the M. extensor tarsi and the M. abductor digiti I longus; the nerve then runs downwards in the groove of the small Os tarsi and divides to form three branches:--

(1) The first branch (dvII) runs to the space between the second and third fingers, where it bifurcates:--

(α) The Rami digital. volaris I run transversely over the muscles of the second toe, supply the muscles of the first toe, and bifurcate to form the R. digital. volaris I and II.

(β) The Ram. digit. volar. II divides in the space between the second and third fingers to supply the Flex. phalang. and the adjacent sides of these toes.

(2) The second branch (dvIII) runs over the M. flex. metatarsi of the third toe to the space between the third and fourth toes, and bifurcates to supply the adjacent sides of these toes and the web between them.

(3) The third branch at once divides to supply:--

(α) The M. transv. metatarsi.

(β) The MM. lumbricales of the fourth toe.

(γ) The M. abductor digit. V (ab5), the M. flexor brevis dig. V (F5), and the M. adductor dig. V (ad5); it then ends as--

(δ) A bifurcating branch forming the R. digit. volar. of the fourth and fifth toes.

*III.* The Nervus peroneus (N. peroneus communis superior, Ecker), (Fig. 130 pc) is the second division of the sciatic nerve; it passes between the outer head of origin of the M. gastrocnemius and the tendon of insertion of the M. biceps, it then lies on the tibio-fibula between the M. gastrocnemius and the M. peroneus, where it is accompanied by the Vena tibialis postica, then runs downwards upon the M. extensor cruris and the Flexor tarsi ant., and passes under the M. tibial. anticus and bifurcates; its branches are:--

*a.* The N. cutaneus cruris lateralis (Fig. 128 cp); which, like other cutaneous nerves, runs in a common sheath together with an artery and vein to the skin. It arises close to the sciatic nerve.

*b.* Muscular branches to the MM. peroneus, tibialis anticus, extensor cruris brevis, and the flexor tarsi anterior.

Distribution of the N. peroneus.

A Inner terminal twig of N. peron. comm. inf. B Outer terminal twig of N. peron. comm. inf. cpl N. cutaneus dorsi pedis lateralis. pc N. peroneus. pci N. peroneus communis inferior. pl N. peroneus lateralis. pm N. peroneus medialis. ]

*c.* The N. peroneus medialis (Fig. 130 pm) is the smaller of the two terminal branches of the N. peroneus; it courses with the Art. tibialis antica under the MM. flexores tarsi anterior and posterior and supplies the latter.

*d.* The N. peroneus lateralis (Fig. 130 pl) is the larger terminal branch of the N. peroneus; it passes downwards between the heads of the M. tibialis anticus to the tendon of origin of the Flexor tarsi posterior, where it gives off two branches (Nos. 1 and 2); the nerve then unites with the N. peroneus lateralis to form a common stem, the N. peroneus communis inferior (Ecker). The N. peroneus lateralis gives off:--

(1) The N. cutaneus dorsi pedis lateralis (cpl), which passes to the skin of the outer side of the dorsum of the foot.

(2) A second branch to the M. extensor of the fourth and fifth toes.

*e.* The N. peroneus communis inferior (Fig. 130 pci) runs on the dorsum of the foot in company with the Art. dorsalis pedis, underneath the MM. extensores digiti I and II; it supplies several branches and then bifurcates. It gives off:--

(1) Branches to the MM. extensor longus and brevis digiti I.

(2) Rami digitales dorsales to the adjacent sides of the first and second toes.

(3) Branches to the MM. extensores digiti II.

(4) The inner, terminal branch (Fig. 130 A) at once divides:--

(α) The outer branch runs between the third and fourth toes as far as the commencement of the web, where it bifurcates to form two Rami cutanei, which course along the adjacent sides of these toes as far as their apices.

(β) The inner branch passes to the extensor muscle, and, in part, to the adjacent sides of the third and fourth toes.

(5) The outer, terminal branch (Fig. 130 B) of the N. peroneus communis inferior passes to the muscles of the fourth and fifth toes, and supplies Rami cutanei dorsales to the outer side of the fourth and inner side of the fifth toes.

Cutaneous branches of the N. tibialis supply the outer side of the fifth and inner side of the first toe.

G. THE SYMPATHETIC SYSTEM (Sympathicus).

(Re-written by the translator.)

The *sympathetic cord* or *chain* is a row of nervous ganglia (vertebral or lateral ganglia), connected by nerve-fibres, and lying on either side of the vertebral column (Figs. 117 and 131); with the exception of the last spinal nerve there is usually one sympathetic ganglion associated with each spinal nerve; in the case of the tenth spinal nerve there may be only one ganglion or as many as twelve.

The first ganglion (Figs. 111, opposite ics; 117 S1) is placed on the hypoglossal nerve just as it emerges from the first intervertebral foramen; it is large, but smaller than the second; its Ramus communicans is represented by several fine and very short fibres, which connect the ganglion with the nerve. This ganglion is connected with the second by two or three nervous threads, between which passes the subclavian artery, a true Annulus Vieussenii being thus formed (de Watteville). The other ganglia are connected by single bands of fibres. The first ganglion supplies also branches to the axillary artery and to the cardiac plexus.

The second ganglion (Figs. 117 and 131) is the largest, and is closely applied to the brachial nerve; as in the case of the first ganglion and hypoglossal nerve, it is attached to the second spinal nerve without a distinctly marked Ramus communicans.

The third ganglion (Ganglion cardiacum basale, Gaskell and Gadow) is sometimes fused with the second, but is usually close to the third spinal nerve: it has a short but distinct Ramus communicans.

Sympathetic cord. From Ecker (Icones physiologicae, Pl. XXIV, Fig. 3).

The heart, lungs, and liver have been removed; the stomach, intestine, kidneys, and testes drawn to the right side; the left sympathetic cord is thus pulled to the right side to expose the Rami communicantes.

S Sympathetic cord attached to the ganglion of the vagus.

The numbers refer to the ganglia, which are enumerated from before backwards. ]

Behind the third ganglion the sympathetic cord is continued backwards along the corresponding aortic arch, then parallel with and close to the abdominal aorta (Figs. 117, 127, and 131), receiving Rami communicantes, which are long and well marked, from each of the spinal nerves; the fourth, fifth, and sixth nerves usually supply each one Ramus communicans, the seventh two, and the eighth and ninth each two or three: from the tenth nerve it receives three or more, as many as twelve having been noted. The ganglia are usually more or less spindle-shaped or flattened and triangular; the hinder part of the cord usually receives in addition two or three branches from the sciatic plexus and twigs from the R. abdominalis of the N. coccygeus.

The branches and communications of the sympathetic cords are as follows:--

*a.* Communicating branches between the two cords; these are extremely numerous and irregular, forming a net-like plexus, which surrounds the abdominal aorta and other adjacent structures, and gives off numerous small twigs to the neighbouring vessels and organs.

*b.* Communications with the cranial nerves; these are two branches (Wiedersheim), (Figs. 111 and 116) which pass from the first ganglion to the Ganglion nervi vagi, where one terminates, the other leaves the ganglion to pass on to the Gasserian ganglion (Figs. 111 and 116 VS): according to Gaskell a single nerve passes from the first ganglion to the Ganglion nervi vagi, whence one portion of its fibres is continued to the Gasserian ganglion, the remainder accompanying the pneumogastric nerve without any connection with the ganglion; he therefore names this nerve the vago-sympathetic. (See Gasserian ganglion, p. 168.)

*c.* Communications with the spinal nerves; these are:--

(1) The Rami communicantes.

(2) Communications between the sympathetic ganglia or their branches and the spinal nerves or their branches (Fig. 117); by means of these connections, fibres of the sympathetic system are conducted by the spinal nerves and their branches to all parts of the body.

*d.* Branches to the heart, which form the following ganglia:--

(1) A relatively large plexus lies on the auricles in the median plane immediately beneath the division of the M. hyoglossus. It supplies a network of fibres to the auricles and the adjacent large vessels. It is said to communicate at various points with the pneumogastric nerve.

(2) A smaller ganglion of oval form, supplies twigs to the neighbouring vessels and a communicating branch to the hypoglossal nerve (Wiedersheim).

*e.* Branches to the abdominal viscera; these form intricate plexuses by which the organs are supplied: the one best known is the solar plexus (Fig. 131); it is formed chiefly from branches derived from the third, fourth, and fifth ganglia, and supplies the stomach, etc.; other plexuses for the various viscera are known by corresponding names, such are the Plexus hepaticus, renalis, genitalis, haemorrhoidalis, and vesicalis.

The sympathetic system is characterised by the fact that the branches form intricate plexuses, which include numerous ganglia and which are very irregular; it is also characteristic that most of its fibres are non-medullated. According to the investigations of Gaskell, the fibres of the sympathetic system arise in mammalia as very fine medullated fibres from the posterior vesicular (Clarke’s) columns (Mason has recently described cells in the frog’s spinal cord, which he holds to be homologous with the cells of these columns); they leave the cord by both the ventral and dorsal roots of the spinal nerves, and are thus connected with two sets of ganglia, (1) with the ganglia of the dorsal roots, and (2) through the Rami communicantes with the sympathetic ganglia (vertebral or lateral ganglia); these two sets of ganglia Gaskell terms proximal. By means of the branches from the sympathetic ganglia (Rami efferentes) part of the fibres pass to another set of ganglia, the solar plexus, etc., which he terms prevertebral or collateral; from these the fibres pass to be distributed to the various viscera and blood-vessels, where a fourth set of very small ganglia (terminal ganglia) is found. The prevertebral and terminal ganglia are together classed as distal ganglia. Gaskell holds that the fine medullated fibres from the cord lose their medullary sheath in one or other of these ganglia according to the function they fulfil.

The inhibitory fibres of the heart and vaso-dilator fibres of the blood-vessels continue as white fibres along the vago-sympathetic and spinal nerves to the distal ganglia (Bidder’s ganglion, etc.), where the medullary sheath disappears: whereas the ‘augmentor’ fibres of the heart and vaso-constrictor fibres of the blood-vessels lose their medullary sheath in the proximal ganglia and pass on as non-medullated fibres. In the same way the nerve-fibres that bring about contraction of the circular muscle fibres of the hollow viscera lose their medullary sheaths in the proximal ganglia, while those fibres, the influence of which negatives the former, become pale fibres in the distal ganglia.

Waters has demonstrated that in the frog the various spinal nerves have each a localised physiological action upon the blood-vessels and muscular walls of various parts of the alimentary canal: he shows that

The third spinal nerve supplies the oesophagus.

The fourth spinal nerve supplies the stomach.

The fifth spinal nerve supplies the upper third of the small intestine.

The sixth spinal nerve supplies the lower two thirds of the small intestine.

The seventh spinal nerve supplies the large intestine.

The eighth spinal nerve supplies the bladder, this supply being, however, not so definite as the others given above.

It has long been known that the branches of the spinal ganglia (ganglia of the posterior roots) are together larger (one-third, Lenhossék), and contain more fibres than the ventral and dorsal roots together; this is supposed to be in part due to an acquisition of new fibres derived from the ganglia. The majority of recent observers hold that each ganglionic cell has only one process, which, however, soon bifurcates; whether any of the fibres so formed pass as far as the cord or beyond its blood-vessels is doubted by most observers, and denied by Gaskell. These remarks and the description of the cells of the spinal ganglia (p. 176) hold good for the lateral or vertebral ganglia and the prevertebral ganglia (solar ganglion, etc.) of the sympathetic system; the terminal ganglia will be described with the organs in which they are found.

H. HISTOLOGICAL NOTES ON THE NERVOUS SYSTEM.

(In order to render the foregoing description of the nervous system more complete, the following notes have been added by the translator.)

*1.* Medullated nerve-fibres or white fibres are found in all cranial and spinal nerves, with the exception of the olfactory nerves, and in many of the sympathetic nerves (see Sympathetic System); also in the white matter of the brain and spinal cord; examined microscopically the fibres are seen to consist of an external sheath or neurilemma, a medullary sheath, and an axis-cylinder:--

*a.* The neurilemma (Sheath of Schwann, Outer or Primitive Sheath) is a nucleated endothelial layer covering the nerve-fibre; it is continuous with the corresponding coat of the nerve-cells, and is uninterrupted throughout the length of the nerve; at the nodes, however, it dips down towards the axis-cylinder, the circular groove so formed being filled with cement substance.

*b.* The medullary sheath (White substance of Schwann); the presence of this sheath is the chief cause of the whiteness of these nerves; the thickness of the sheath varies considerably, and towards the ultimate distribution of the nerve it is entirely lost. At more or less regular intervals along the course of the nerve-fibre the continuity of the medullary sheath is broken, and gives the fibres the appearance of being constricted at these places; such constrictions are known as nodes of Ranvier; the portion between two such nodes being termed an internode. Each internode possesses an oval, flattened, granular nucleus at about its middle and placed between the neurilemma and the medullary sheath; the nucleus has a nucleolus. In the fresh state the medullary sheath seems to be fluid; it is of a fatty nature.

Medullary segments are caused by breaks in the continuity of the medullary sheath, which are seen only in nerves which are no longer in their normal condition, and are especially well marked after treatment with osmic acid. The breaks are oblique; hence the conical end of one segment fits into the funnel-shaped end of the next. How far they correspond to pre-existent structures is uncertain. The segments in the frog vary in length from 0.010 to 0.040 mm.

*c.* The axis-cylinder is the essential part of every nerve-fibre; it shows a longitudinal striation corresponding to the fine fibrils (primitive fibrillae) of which it consists; these fibrils often exhibit minute varicosities: at times it has the appearance of being invested with a very delicate structureless sheath. It is continuous through the nodes of Ranvier.

*2.* Non-medullated nerve-fibres (Grey or Varicose Fibres; Fibres of Remak); these occur chiefly in branches and plexuses of the sympathetic system; they consist of a neurilemma and an axis-cylinder, which agree exactly with the corresponding elements found in the medullated fibres.

The nerve-fibres, whether medullated or non-medullated, are bound together by connective-tissue to form nerves. A number of fibres bound together by connective-tissue to form a slender cord is known as a funiculus; a small nerve may consist of one such funiculus; the sheath surrounding it is known as the perineurium, and sends in supporting processes between the fibres (endoneurium): when several funiculi are bound together to form a large nerve the common sheath is known as the epineurium. These sheaths support nerves (nervi nervorum) and vessels (vasa vasorum) supplying the nervous elements, and their intercellular spaces form lymph-canals. The whole nerve is surrounded by an endothelial coat, which helps to form a lymph-space, which more or less completely surrounds the nerve.

The ultimate distribution of the nerve-fibres will be included in the description of the various organs in which they end.]