Part 200

The parts that are visible when the brain is viewed from above are the cerebral hemispheres, the cerebellum, and part of the medulla oblongata. The =cerebral hemispheres= form an ovoid mass, and are separated from each other by the median =longitudinal fissure=, in which the falx cerebri is situated. Their surfaces are marked by thick folds, the =gyri cerebri=, separated by =sulci=. The upturned ends of the =olfactory bulbs= are seen in front of the frontal poles of the hemispheres. The occipital poles of the hemispheres overlie the anterior part of the cerebellum, from which they are separated by the =transverse fissure= and the tentorium cerebelli contained in it. The =cerebellum= is a much smaller rounded mass which conceals the greater part of the medulla oblongata. Its surface is divided into a middle lobe, the =vermis=, and two lateral =hemispheres=. It is marked by numerous gyri and narrow sulci which have in general a transverse direction. The posterior third of the =medulla oblongata= is not covered by the cerebellum.

The brain is developed from the expanded cephalic part of the neural tube of the embryo. The process comprises a series of thickenings, flexures, and unequal growth and expansion of various parts of the tube. In the higher animals the result is that the tubular character of the brain is not very evident, since the lumen comes to consist of four irregular cavities, the ventricles, which are connected by narrow passages. The tube is first subdivided by two constrictions into three brain vesicles, termed respectively the hind-brain or rhombencephalon, the mid-brain or mesencephalon, and the fore-brain or prosencephalon. The hind-brain gives rise to three secondary segments and the fore-brain to two. The annexed table indicates the origin of the principal structures of the fully developed brain from the primitive vesicles. It has become customary to describe the brain with reference to its embryological relations.

TABLE INDICATING THE DERIVATION OF THE PRINCIPAL PARTS OF THE BRAIN PRIMARY SEGMENTS.│ Secondary │ DERIVATIVES. │ CAVITIES. │ SEGMENTS. │ │ Rhombencephalon │ │ │ (posterior │Myelencephalon │Medulla oblongata│Fourth ventricle vesicle) │ │ │ „ │Metencephalon │Pons │ „ „ │ „ │Cerebellum │ „ │Isthmus │Anterior │ „ │ rhombencephali │ cerebellar │ „ │ │ peduncles │ „ │ „ │Anterior │ „ │ │ medullary velum│ Mesencephalon │ │Corpora │ (middle │Mesencephalon │ quadrigemina │Cerebral aqueduct vesicle) │ │ │ „ │ „ │Cerebral │ „ │ │ peduncles │ Prosencephalon │ │{Optic thalami │Posterior part of (anterior │Diencephalon │ } │ third ventricle vesicle) │ │ │ „ │ „ │{Subthalamic │ „ │ │ tegmenta │ „ │ „ │{Pineal body │ „ │ │ │Anterior part of │ │ │ third │ │{Pituitary body │ ventricle. „ │Telencephalon │ } │ Lateral │ │ │ ventricles and │ │ │ olfactory │ │ │ continuations. „ │ „ │{Optic nerves and│ „ │ │ retinæ │ „ │ „ │{Cerebral │ „ │ │ hemispheres │ „ │ „ │{Olfactory tracts│ „ │ │ and bulb │

THE RHOMBENCEPHALON

THE MEDULLA OBLONGATA

The =medulla oblongata= lies on the basilar part of the occipital bone. It is quadrilateral in outline, but much wider in front than behind, and compressed dorso-ventrally. Its length, measured from the root of the first cervical nerve to the pons, is about two inches (ca. 5 cm.).

Its =ventral surface= is convex in the transverse direction. It is marked by a =median fissure= (Fissura mediana ventralis) which is continuous behind with the similar fissure of the spinal cord. The posterior part of the fissure is faintly marked, but in front it becomes deeper and ends in a small depression (Foramen cæcum) behind the central part of a transverse band, the =corpus trapezoideum=. On either side of the fissure is a rounded tract, the =pyramid= (Pyramis), which is bounded externally by a faint lateral groove (Sulcus intermedius ventralis). The pyramids join the pons in front; behind they become narrower and disappear into the substance of the medulla, in which their fibers intercross, forming the =decussation of the pyramids= (Decussatio pyramidum).

The superficial origin of the =sixth cranial nerve= (N. oculomotorius) is just lateral to the anterior end of the pyramid. The =corpus trapezoideum= is a transverse band which extends across the surface immediately behind the pons. It is crossed by the pyramids, which cut off a small central part. The lateral part extends out to the roots of the seventh and eighth nerves on either side. Behind the outer part of the corpus trapezoideum there is a variably developed rounded eminence, the =tuberculum faciale=. The root-fibers of the twelfth or hypoglossal nerve form an oblique linear series lateral to the posterior part of the pyramid.

FIG. 499.—BASE OF BRAIN OF HORSE, NATURAL SIZE. ]

The =dorsal surface= is largely concealed by the cerebellum and forms the greater part of the floor of the fourth ventricle. The =dorsal median fissure= (Fissura mediana dorsalis), the direct continuation of the corresponding groove of the spinal cord, extends forward to about the middle of the surface. Here the =restiform bodies=, which constitute the lips of the fissure, diverge to form the lateral boundaries of a triangular depression; this is the posterior part of the rhomboid fossa or floor of the fourth ventricle of the brain. The dorso-lateral fissure winds outward and forward to the lateral aspect of the medulla, where it presents the roots of the =ninth=, =tenth=, and =eleventh cranial nerves=. External to it is a distinct oval eminence on the anterior part of the lateral column, termed the =tuberculum cinereum=. The central canal of the cord is continued in the posterior part of the medulla, inclines dorsally, and opens in the posterior angle of the fourth ventricle. Hence it is customary to distinguish a closed and an open part of the medulla. The dorsal aspect of the latter, which is concealed at present, will be considered later in the description of the fourth ventricle.

FIG. 500.—LINE DRAWING OF BASE OF BRAIN OF HORSE. (KEY TO FIG. 499.) ]

The lateral surface is narrow behind, wider and rounded in front. From it the root-fibers of the ninth, tenth, and eleventh cranial nerves arise in a linear series, and alongside of it the spinal part of the eleventh nerve passes forward to join the medullary root. Close inspection reveals the presence of striæ which curve obliquely downward and backward from the surface of the restiform body toward the hypoglossal root-fibers; these are the =external arcuate fibers= (Fibræ arcuatæ externæ). The recess between the lateral aspect of the medulla and the cerebellum is occupied by an irregular mass of villous projections of the pia mater, containing tufts of vessels; this is the =chorioid plexus of the fourth ventricle=, and is the lateral edge of the =tela chorioidea= of the ventricle. On raising the chorioid plexus it is seen that the tela chorioidea is attached to the dorsal aspect of the medulla, and reinforces here the wall of the fourth ventricle; also that the restiform body terminates in front by entering the base of the cerebellum, forming its posterior peduncle.

In the medulla the fiber tracts of the spinal cord either terminate in the nuclei of the gray matter or undergo changes in their relative position, and new tracts appear. The gray matter is highly modified and forms masses which have no homologues in the cord. The central canal of the closed part of the medulla is surrounded by a thicker layer of gray matter than is the case in the spinal cord. As the medulla opens out this gray matter is naturally spread in the floor of the fourth ventricle. The dorsal horns of gray matter become wide, spread apart, and are broken up to a great extent in the formatio reticularis. Two elongated masses of cells appear above the central gray matter; these are the =nucleus gracilis= and =nucleus cuneatus=, and in them the fasciculi of like name gradually end. In front of these are the =terminal nuclei= of the afferent or sensory cranial nerves and of the sensory portions of the mixed nerves. The ventral horns are succeeded by the nuclei of =origin= of the efferent or motor cranial nerves and the motor root-fibers of the mixed nerves. Of the twelve pairs of cranial nerves, the last eight are connected with nuclei in the medulla and pons. The =posterior olivary nucleus= (Nucleus olivaris caudalis) is a conspicuous gray mass which lies above the pyramid on each side. On cross-sections it appears as a wavy layer of gray matter which is folded on itself so as to inclose a mass of white matter. It is about 2 cm. long in the horse, but is smaller in circumference than in man and hence does not cause any very distinct external enlargement (olivary eminence), as in the latter. It is connected with the opposite side of the cerebellum by the cerebello-olivary fibers. At its inner side are two small accessory olivary nuclei. The =pyramidal tracts=, which are small in ungulates, send most of their fibers across to the opposite side in the posterior part of the medulla, forming the =pyramidal decussation=. Some fibers, however, continue in the ventral column of the same side of the cord, and others are connected with the nuclei of the motor nerve-roots. From the nucleus gracilis and nucleus cuneatus fibers arise which are traceable forward to the thalamus. These are the =internal arcuate fibers= (Fibræ arcuatæ internæ), which curve across the median plane below the central gray matter and form with those of the opposite side the =decussation of the fillet= (Decussatio lemniscorum). Beyond the decussation the fibers form an important longitudinal tract which extends forward in the mid-brain. This is the =fillet= or =lemniscus=, the chief continuation of the sensory conducting path from the dorsal roots of the spinal nerves. The =external arcuate fibers=, some of which were seen on the lateral aspect of the medulla, have a similar origin. Part of them (Fibræ dorsales) pass directly to the dorsal aspect of the restiform body of the same side; others (Fibræ ventrales) cross to the opposite side, descend close to the ventral fissure, and then curve upward and forward to the restiform body. The decussation of the arcuate fibers forms the distinct median =raphé= seen on cross-sections of the medulla anterior to the pyramidal decussation. The restiform body, situated dorso-laterally, contains, in addition to the arcuate fibers, the =cerebello-olivary fasciculus= before mentioned, and the =cerebellospinal fasciculus= or direct cerebellar tract. The =dorsal longitudinal fasciculus= corresponds to the ventral ground-bundle of the spinal cord, displaced dorsally by the decussation of the pyramids and fillet. In the posterior part of the medulla it is not marked off from the fillet, along the dorsal edge of which it lies. From the level of the hypoglossal nucleus forward it is distinct and can be traced as a conspicuous tract in the ventral margin of the gray matter of the floor of the fourth ventricle and of the central gray matter of the mid-brain. Below the restiform body and related externally to the external arcuate fibers, there is a considerable bundle of longitudinal fibers, the =spinal root= (Tractus spinalis) =of the fifth nerve=; internal to it is the terminal nucleus of the sensory root of the nerve.

FIG. 501.—BRAIN OF HORSE, LEFT LATERAL VIEW.

The larger arteries are shown. The nerve-roots are indicated by Roman numerals. P.c., Chorioid plexus of fourth ventricle; B.p., middle peduncle of cerebellum; P, pons; H, pituitary body; L.p., pyriform lobe; A.p., trigonum olfactorium; B.o., olfactory bulb; S.r.a., sulcus rhinalis anterior; S.r.p., sulcus rhinalis posterior; F.l., lateral fissure (of Sylvius); F.l.a., anterior branch of lateral fissure; S.p., presylvian fissure; S.p.n., anterior presylvian fissure; S.s., suprasylvian fissure; S.e., ectomarginal fissure. ]

THE PONS

FIG. 502.—CROSS-SECTION OF MEDULLA OBLONGATA OF HORSE, PASSING THROUGH FACIAL NUCLEUS.

Cr, Corpus restiforme; D, dorsal longitudinal fasciculus; Fa, ascending part of facial nerve; L, fillet; N7, nucleus of facial nerve; N8, triangular nucleus of vestibular root of eighth nerve; N8′, spinal root of eighth nerve; Py, pyramid; Ra, raphé; R7, radicular part of facial nerve; R8, vestibular root of eighth nerve; Sg, substantia gelatinosa; Ta, posterior end of tuberculum acusticum; V, spinal root of trigeminus. (Ellenberger-Baum, Anat. d. Haustiere.) ]

FIG. 503.—CROSS-SECTION OF MEDULLA OBLONGATA OF HORSE; SECTION PASSES THROUGH MIDDLE OF CORPUS TRAPEZOIDEUM.

Cr, Corpus restiforme; D, dorsal longitudinal bundle; Fa, ascending part of facial nerve; Fa′, emergent or descending part of facial nerve; R7, root of facial nerve; L, fillet; M, central white matter of cerebellum; N6, abducens nucleus; N8, triangular nucleus; N8′, nucleus of Deiters; N8″, tuberculum acusticum; Oo, anterior olive; Py, pyramid; R6, root of abducens nerve; R7, root of facial nerve; Rc, cochlear nerve; Rv, vestibular nerve; Sg, substantia gelatinosa; T, corpus trapezoideum; V, spinal root of trigeminus. (Ellenberger-Baum, Anat. d. Haustiere.) ]

The =pons= (Varolii) is that part of the brain stem which lies between the medulla and the cerebral peduncles; it is marked off from these ventrally by anterior and posterior grooves. Viewed from below it is elongated transversely, convex in both directions, and presents a wide shallow median groove (Sulcus basilaris), which lodges the basilar artery. Laterally a large part of its mass curves upward and backward into the base of the cerebellum, forming the =middle cerebellar peduncle= (Brachium pontis). The superficial origin of the trigeminal (fifth) nerve is at the lateral limit of the ventral surface. Transverse striations indicate the course of its superficial (ventral) fibers which connect the two sides of the cerebellum. The dorsal surface is blended on either side with the overlying anterior peduncles of the cerebellum; the central free portion forms the anterior part of the floor of the fourth ventricle, and will be considered in the account of that cavity.

On cross-section the pons is seen to be composed of dorsal and ventral parts. The =dorsal part= (Pars dorsalis pontis) consists superficially of a layer of gray matter covered by the ependyma of the fourth ventricle. Beneath this the median raphé of the medulla is continued into the pons, dividing it into similar halves. In the anterior part of the pons the fillet divides into an inner and outer part, the =medial= and =lateral fillets= (Lemniscus medialis, lateralis); the latter arches outward to reach the outer side of the anterior cerebellar peduncle. The =dorsal longitudinal fasciculus= becomes sharply defined into a round bundle which lies close to the raphé under the gray matter of the floor of the fourth ventricle. In cross-section the =formatio reticularis= forms a large area below the superficial gray matter and the longitudinal bundles. Dorso-laterally is the large rounded section of the =anterior cerebellar peduncle=. Lower down is a large bundle, the =sensory root of the fifth nerve=. In front of this is the =motor nucleus= of the same nerve, external to which is its =motor root=. The =ventral part= of the pons (Pars basilaris pontis) is composed of transverse and longitudinal fibers, and a large amount of gray matter which is broken up into small masses (Nuclei pontis) by the intersection of the fibers. The =transverse fibers= are gathered laterally into a compact mass which turns upward and backward and enters the central white matter of the cerebellum, forming the =middle cerebellar peduncle=. Centrally the fibers are arranged in bundles which intercross. The transverse fibers are chiefly of two kinds. Some arise from the Purkinje cells of the cerebellar cortex and pass either to the opposite side of the cerebellum or turn at the raphé and run forward and backward in the brain stem. Others are axones of cells of the nuclei pontis, and pass to the hemispheres of the cerebellum. The corpus trapezoideum is mainly the central continuation of the cochlear division of the auditory nerve. Above it is the small =anterior olivary nucleus= (Nucleus olivaris nasalis). The =longitudinal fibers= of the ventral part of the pons consist chiefly of the =cerebrospinal= or =pyramidal fasciculi=. These come from the ventral part (pes) of the cerebral peduncles and are situated laterally, interspersed among the deep transverse fibers in the anterior part of the pons. Toward the posterior part the bundles incline toward the median plane and become collected into a compact mass which appears superficially at the posterior border. Many fibers come from the cerebral cortex and terminate in the nuclei of the gray matter of the pons; they may be designated =corticopontile fibers=.

THE CEREBELLUM

The =cerebellum= is situated in the posterior fossa of the cranium, and is separated from the cerebral hemispheres by the transverse fissure and the tentorium cerebelli which occupies it. It overlies the pons and the greater part of the medulla, from which it is separated by the fourth ventricle. Its average weight is about two ounces (ca. 60 gm.) or about 9 per cent. of the weight of the entire brain. Its shape is approximately globular but very irregular. It is somewhat compressed dorso-ventrally and its transverse diameter is the greatest.

The =anterior surface= faces upward and forward and is covered partially by the tentorium cerebelli. The =posterior surface= is almost vertical. The =ventral surface= or =base= lies over the fourth ventricle, and is connected by three pairs of peduncles with the medulla, pons, and mid-brain.

It is customary to recognize three gross divisions of the cerebellum, viz., the median vermis and two lateral hemispheres. The =vermis= (cerebelli) is curved in a circular manner so that its two extremities are close together or even in contact on the ventral surface. The anterior extremity is termed the =lingula=; it lies between the cerebellar peduncles and gives attachment to the =anterior medullary velum=, a thin lamina which forms the anterior part of the roof of the fourth ventricle. The posterior extremity, the =nodulus=, gives attachment to the =posterior medullary velum= which covers the posterior recess of the fourth ventricle. The =hemispheres= (Hemisphæria cerebelli) are clearly separated from the vermis by two deep paramedian fissures. They lie in the lateral depressions of the cerebellar compartment of the cranium.

In tracing the fissures from behind forward it will be noticed that they are nearly sagittal as far as the anterior surface, where they diverge widely, so that the vermis forms all of the fore part of the cerebellum.

The surface of the cerebellum is further cut up into numerous =gyri= or =folia= by narrow and relatively deep =sulci=, many of which approach a transverse direction. Certain of the sulci are more pronounced than the others, and by means of them it is possible to define groups of gyri. Such groups are termed =lobes=, and have received specific names, derived chiefly from the systematic descriptions of the human cerebellum.