Part 219

9. The =cruciate fissure= is the deepest and most characteristic. It cuts deeply into the anterior third of the dorso-medial border and runs almost straight outward. It is continuous on the inner surface of the hemisphere with the calloso-marginal or splenial fissure.

10. The =sulcus rhinalis= is continued on the tentorial surface of the hemisphere by the occipito-temporal fissure. Anteriorly it is prolonged as the olfactory fissure, which is concealed by the olfactory bulb and tract.

FIG. 545.—DEEP DISSECTION OF HEAD OF DOG, SHOWING ESPECIALLY TRIGEMINAL AND HYPOGLOSSAL NERVES.

a, Hypoglossal nerve; b, cervical branch of a; c, mandibular division of trigeminus; d, lingual nerve; e, nerve to submaxillary gland; f, deep temporal nerve; g, pterygoid nerve; h, buccinator nerve (cut); i, inferior alveolar or dental nerve; k, staphyline branch of lingual nerve; l, chorda tympani; m, mylo-hyoid nerve; n, sphenopalatine nerve; o, lesser palatine nerve; p, great palatine nerve; q, infraorbital nerves; r, subcutaneus malæ; s, branch of oculomotor nerve to inferior oblique muscle; t, lacrimal nerve; u, frontal nerve; v, trochlear nerve; w, abducens; 1, carotid artery; 2, lingual artery; 3, internal maxillary artery; 4, thyro-pharyngeus muscle; 5, hyo-pharyngeus muscle; 6, thyro-hyoideus; 7, sterno-hyoideus; 8, baseo-glossus. 9, genio-hyoideus; 10, genio-glossus; 11, stylo-glossus; 12, pterygoideus internus; 13, outline of submaxillary gland (dotted); 14, atlas; 15, bulba ossea; 16, zygomatic arch (dotted); 17, rectus oculi inferior; 18, obliquus oculi inferior. (Ellenberger-Baum, Anat. d. Hundes.) ]

11. The =calloso-marginal= or =splenial fissure= is parallel with the splenium and middle part of the corpus callosum on the inner surface of the hemisphere. It is continuous in front with the cruciate fissure and runs downward and outward across the tentorial surface to join the occipito-temporal fissure and sulcus rhinalis. It gives off from its highest part the short lesser or accessory cruciate fissure.

12. The =suprasplenial fissure= runs almost parallel with the posterior part of the calloso-marginal fissure on the inner and tentorial surfaces of the hemisphere.

The =olfactory bulb= is large and is strongly compressed laterally; its anterior end projects beyond the frontal pole of the hemisphere. The tract is short and is also flattened laterally. Both contain a narrow cavity which communicates with the lateral ventricle. The =pyriform lobe= is large and rounded.

The =cranial nerves= present the following special features which are worthy of notice:

The =ophthalmic nerve= gives off lacrimal, frontal, long ciliary, ethmoidal, and infratrochlear branches. The =lacrimal nerve= arises from the ophthalmic at its origin; its recurrent branch emerges at the orbital ligament and concurs with the zygomatic and frontal nerves in the formation of the anterior auricular plexus. The =frontal nerve= emerges from the orbit in front of the upper end of the orbital ligament, ramifies in the upper lid and the adjacent skin of the forehead, and sends branches backward to the anterior auricular plexus. The =long ciliary nerve= accompanies the optic nerve and divides into several branches which pierce the posterior part of the sclera. The =ethmoidal nerve= gives off internal nasal branches and ends in the muzzle. The =infratrochlear nerve= runs forward between the internal straight and the superior oblique muscles of the eye and ramifies on the face in the vicinity of the inner canthus.

FIG. 546.—NERVES OF FACE OF DOG. PAROTID GLAND IS REMOVED.

a, Facial nerve; b, posterior auricular nerve; c, internal auricular nerve; d, digastric nerve; e, inferior buccal nerve; f, cervical branch of facial; g, auriculo-palpebral; h, superior buccal nerve; i, temporal branch; k, k′, k″, zygomatic branch; l, auriculo-temporal; m, malar branch of l; n, buccinator nerve; o, mylo-hyoid branch; p, subcutaneus malæ; q, lacrimal nerve; r, frontal nerve; s, infratrochlear nerve; t, infraorbital nerve; 1, paramastoid process; 2, digastricus muscle; 3, base of concha; 4, masseter; 5, zygomaticus; 6, scutularis; 7, zygomatic arch; 8, maxilla. (Ellenberger-Baum, Anat. d. Hundes.) ]

The =maxillary nerve=, after giving off the zygomatic, divides into infraorbital and sphenopalatine branches. The =zygomatic nerve= emerges through an opening in the upper part of the orbital ligament and ramifies in the lower lid and on the adjacent surface of the face. The =infraorbital nerves=, two in number, divide within the infraorbital canal and after their emergence upon the face, thus forming seven or eight =external nasal= and =superior labial branches=. The =sphenopalatine nerve= gives off =lesser= and =greater palatine= and =posterior nasal nerves=.

The =mandibular nerve= passes out through the foramen ovale. The =superficial temporal nerve= divides into auricular, temporal, and malar branches; the last-named crosses the masseter and ramifies with the buccal nerves. The =mylo-hyoid nerve= is given off from the mandibular nerve almost immediately after the latter emerges from the cranium; it innervates the mylo-hyoideus and digastricus and gives off a branch which turns around the lower jaw, joins the inferior buccal nerve, and ramifies on the lateral surface of the face. The =inferior alveolar nerve= arises by a common trunk with the mylo-hyoid; it gives off dental branches and terminates in mental and inferior labial branches. The =lingual nerve= supplies vasodilator and excito-secretory filaments to the submaxillary and sublingual salivary glands; these fibers are derived from the chorda tympani.

The =facial nerve= divides near the posterior border of the jaw into four branches. The upper branch is the =auriculo-palpebral nerve=, which divides after a very short course into anterior auricular and zygomatic branches. The latter curves upward and forward across the zygomatic arch toward the eye, and divides into branches which supply the eyelids and nasal region and concur with the frontal and lacrimal nerves in forming the anterior auricular plexus. The =superior buccal nerve= accompanies the parotid duct across the masseter. The =inferior buccal nerve= runs forward along the lower border of the masseter and the mandible. The two nerves ramify on the cheek and anastomose with each other and the infraorbital nerves to form a plexus from which branches go to the muscles of the lips and nostrils. The =cervical branch= runs downward and backward over the submaxillary gland and ramifies in the cervical panniculus; it communicates with the inferior buccal nerve and sends twigs to the parotido-auricularis muscle and the submaxillary space.

The =vagus= bears a =jugular ganglion= and a =ganglion nodosum=. The latter is situated near the superior cervical ganglion on the rectus capitis anterior major and dorsal to the carotid artery; it is fusiform and may be about half an inch (ca. 1 to 1.5 cm.) long in a large dog. In its course in the neck the nerve is inclosed with the sympathetic trunk in a common sheath and is related ventrally to the common carotid artery. The formation of dorsal and ventral =œsophageal trunks= by the union of the dorsal and ventral divisions of the two nerves takes place at the hiatus œsophageus. The =dorsal trunk= concurs with branches of the solar plexus in forming the =posterior gastric plexus= on the visceral surface of the stomach. The =ventral trunk= ramifies on the parietal surface of the stomach, forming the =anterior gastric plexus=, from which a considerable branch passes along the lesser curvature to the pylorus. Some special features of the collateral branches are: Two pharyngeal branches are present. The =superior pharyngeal branch= arises from the vagus above the ganglion nodosum. It passes under the deep face of the carotid artery and descends on the lateral surface of the pharynx to end in the crico-thyroid muscle. It furnishes a root of the inferior pharyngeal branch, and communicates with the pharyngeal branch of the ninth and with the superior laryngeal and hypoglossal nerves. The =inferior pharyngeal branch= is formed by the union of roots derived from the superior pharyngeal nerve and the ganglion nodosum, together with filaments from the superior cervical ganglion. It crosses the side of the pharynx behind the superior pharyngeal and ramifies on the posterior constrictor of the pharynx and the origin of the œsophagus. It contributes filaments to the pharyngeal plexus, communicates with the recurrent nerve, and supplies twigs to the thyroid gland. The =pharyngeal plexus= is formed on the lateral surface of the pharynx by branches of the pharyngeal nerves and the communications above described; branches from it innervate the muscles and mucous membrane of the pharynx. The =superior laryngeal nerve= arises from the ganglion nodosum and descends over the side of the pharynx, crossing beneath the carotid artery and the superior laryngeal nerve. It passes through the thyroid notch and ramifies in the mucous membrane of the larynx. It communicates with the superior cervical ganglion and the superior pharyngeal branch of the vagus, and gives twigs to the hyo-pharyngeus muscle. Immediately after its entrance into the larynx it gives off a large branch which, instead of uniting with the recurrent as in the other animals, has a peculiar arrangement. It runs back near the dorsal border of the thyroid cartilage, gives a branch to the crico-arytenoideus posterior as it passes over that muscle, and continues along the trachea internal to the recurrent nerve. At the thoracic inlet it communicates with the inferior cervical ganglion and continues backward to unite with the vagus at or near the point of origin of the recurrent nerve. Its collateral filaments supply the trachea and concur with the inferior pharyngeal branch in forming a plexus on the cervical part of the œsophagus which innervates that tube. The =depressor nerve= is a very delicate filament which arises usually from the superior laryngeal nerve and accompanies the vago-sympathetic trunk to the thorax; its fibers reach the heart through the cardiac nerves. The =pulmonary= and =posterior œsophageal plexuses= are highly developed.

The =hypoglossal nerve= gives off a long branch (R. descendens) which runs downward and backward across the pharynx and larynx, communicates with the ventral branch of the first cervical nerve, and supplies the thyro-hyoid, sterno-hyoid, and sterno-thyroid muscles.

The =spinal nerves= number thirty-six or thirty-seven on either side, and comprise eight cervical, thirteen thoracic, seven lumbar, and five or six coccygeal.

The =brachial plexus= (Fig. 487) is derived from the ventral branches of the last four cervical and first thoracic nerves; the root supplied by the fifth cervical nerve is very small. The roots unite at the lower border of the scalenus. The more important special features in the arrangement of the nerves which emanate from the plexus are as follows:

The =musculo-cutaneous nerve= passes between the coraco-brachialis and the brachial artery and descends in the arm in front of the artery. At the shoulder joint it gives off branches to the biceps and coraco-brachialis, and in the lower third of the arm is connected with the median nerve by an oblique branch. It terminates near the elbow by dividing into a branch for the brachialis and a small cutaneous nerve which passes down over the inner face of the elbow and, inclining a little forward, descends over the deep fascia of the forearm to the carpus.

The =radial nerve= descends behind the ulnar nerve, gives branches to the extensors of the elbow, dips in between the internal head of the triceps and the accessory head of the anconeus, winds around the arm, and divides between the brachialis and the external head of the triceps into two branches. The =deep branch= (R. profundus) supplies the extensor and supinator muscles on the forearm. The =superficial branch= (R. superficialis) emerges upon the flexor surface of the elbow and divides into two branches which terminate by supplying two =dorsal digital nerves= to each digit, except the fifth, which receives its outer dorsal nerve from the ulnar. The inner branch descends along the inner side of the cephalic vein to the carpus, where it divides into dorsal digital nerves for the first digit and the inner side of the second. The outer branch is much larger. It runs downward on the middle of the front of the forearm and supplies the remaining dorsal digital nerves except that to the outer side of the fifth digit.

The =ulnar nerve= is as large as or larger than the median, with which it is united for some distance. At the distal third of the arm it separates from the median and passes over the internal epicondyle of the humerus. At the proximal part of the forearm it gives off the =dorsal branch= (R. dorsalis), which supplies cutaneous twigs to the dorso-lateral surface of the lower part of the forearm and carpus and terminates as the =external dorsal digital nerve= of the fifth digit. Descending under cover of the flexor carpi ulnaris, the ulnar nerve inclines inward under the tendon of insertion of that muscle and divides into superficial and deep branches. The =superficial branch= (R. superficialis) runs downward along the outer border of the flexor tendons, gives off the =external volar digital nerve of the fifth digit= (N. dig. vol. lat. dig. V), and a branch (N. met. vol. IV) which descends in the space between the fourth and fifth metacarpal bones and unites with the deep branch. The =deep branch= (R. profundus) descends in the carpal canal and divides under the deep flexor tendon into its terminal branches. The smaller of these supply the volar metacarpal muscles. The larger terminals are the three =volar common digital nerves= (Nn. dig. vol. comm. II, III, IV), which descend along the second, third, and fourth intermetacarpal spaces, subdivide, and concur with the volar metacarpal branches of the median nerve in forming the =volar proper digital nerves= (Nn. dig. vol. proprii).

The =median nerve= descends behind the brachial artery, passes over the internal epicondyle of the humerus, then under the pronator teres, and continues in the forearm under cover of the flexor carpi radialis. It gives branches below the elbow to the flexor and pronator muscles, and lower down a palmar branch to the skin on the inner and posterior aspect of the carpus, and terminates between the superficial and deep flexor tendons by dividing into three =volar metacarpal nerves= (N. met. vol. I, II, III). These descend in the first, second, and third intermetacarpal spaces and unite with the volar common digital nerves in forming volar proper digital nerves. The arrangement of the digital nerves is indicated in the annexed schematic figures.

FIG. 547.—NERVES OF DISTAL PART OF RIGHT FORE LIMB OF DOG, ANTERIOR VIEW (SCHEMATIC).

p, Dorsal proper digital nerves. ]

FIG. 548.—NERVES OF DISTAL PART OF RIGHT FORE LIMB OF DOG, POSTERIOR VIEW (SCHEMATIC).

p, Volar proper digital nerves. ]

The =lumbo-sacral plexus= is formed from the ventral branches of the last five lumbar and first sacral nerves. The more important special facts in regard to the nerves of the pelvic limb are as follows:

The =saphenous nerve= (Fig. 492) is relatively large, and, in fact, might be considered as the continuation of the femoral nerve. It descends at first in front of the femoral artery, then passes over the lower part of the inner surface of the thigh with the saphenous artery, continues down the leg with the dorsal branch of that vessel, and anastomoses with branches of the superficial peroneal nerve. It supplies cutaneous twigs from the stifle to the metatarsus.