Part 77

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

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.

The lobes of the vermis are readily distinguished on a median section. Enumerated from the anterior to the posterior extremity they are: (1) lingula, (2) lobus centralis, (3) lobus ascendens, (4) lobus culminis, (5) lobus clivi, (6) tuber vermis, (7) pyramis, (8) uvula, (9) nodulus. Each hemisphere is cut into laterally by two sulci which mark off two sagittal discoid masses, termed by Ziehen tabulations. The external tabulation consists of four or five lobules, the lowest of which is regarded as the flocculus. The inner part of the hemisphere is divided into three or four lobes. In the absence of a satisfactory morphological basis it seems undesirable to deal with the lobation of the cerebellum in further detail.

The =cerebellar peduncles=, three on each side, join the central white matter of the cerebellum at the base. The =posterior peduncle= is the restiform body of the medulla, a large rounded tract derived from the lateral and ventral columns of the cord. Near the middle of the medulla it inclines outward, forms the lateral wall of the fourth ventricle, and ends by entering the central white matter of the cerebellum. The =middle peduncle= is formed, as previously seen, by the brachium pontis. The =anterior peduncles= (Brachia conjunctiva) pass forward on either side on the dorsal surface of the pons, forming the lateral boundary of the fore part of the fourth ventricle. They disappear under the corpora quadrigemina into the substance of the mid-brain. At the point of disappearance the =trochlear= (fourth) =nerve= emerges from the mid-brain. In some cases two or three bundles of fibers (Fila lateralia pontis) arise in the angle between the middle and anterior peduncle, curve obliquely forward and downward over the outer aspect of the latter, and spread out on the ventral face of the cerebral peduncle just in front of the pons.

On sagittal section the cerebellum is seen to consist of a layer of =cortical gray matter= (Substantia corticalis) and the =medullary white matter=. The white matter consists of a large =basal mass= (Corpus medullare), which is joined by the peduncles, and gives off primary laminæ to the lobules; from these secondary and tertiary laminæ arise, the latter entering the gyri. The arrangement on sagittal section is tree-like, hence the classical term “arbor vitæ” which is applied to it. The central gray matter consists of groups of cells which form small =nuclei= embedded in the central white substance.

As noted above the central gray matter does not form a large nucleus, the corpus dentatum, which is so conspicuous an object on sagittal sections of the cerebellar hemisphere in man.

The principal connections established by the peduncular fibers of the cerebellum are as follows: The posterior peduncle (Corpus restiforme) is composed of afferent and efferent fibers which connect the cerebellum with the medulla and spinal cord. The =cerebellospinal fasciculus= or =direct cerebellar tract=, which arises from the cells of the nucleus dorsalis (Clarke’s column) of the cord, ends in the cortex of the vermis; many of its fibers cross to the opposite side. Numerous =arcuate fibers= from the nucleus gracilis and nucleus cuneatus of the same and opposite sides establish connections with =cells= of the cerebellar cortex. =Olivocerebellar= fibers (chiefly afferent) connect with the olivary nucleus of the same and of the opposite side of the medulla oblongata. The =nucleo-cerebellar fasciculus= comprises fibers derived from the nuclei of the fifth, eighth, and tenth cranial nerves (Edinger). The =descending cerebellospinal fasciculus= consists of fibers which terminate in relation with cells of the ventral horns of the spinal cord. The chief facts concerning the middle peduncle have been mentioned in the description of the pons. The anterior peduncle is essentially an efferent tract, the fibers of which pass forward to the tegmentum of the cerebral peduncle, the subthalamic region, and the thalamus. After the peduncles disappear under the corpora quadrigemina, they converge and many of their fibers intercross, forming the =decussation of the superior peduncle=. A considerable number of fibers end in the nucleus ruber. Thence impulses are transmitted in two directions: first, by =thalamo-cortical fibers= to the cerebral cortex; second, by the =rubrospinal tract= through the brain stem and lateral columns of the cord to the ventral horn cells. The ventro-lateral cerebellospinal fasciculus (Gowers’ tract) is an ill-defined tract which connects the spinal cord with the cerebellum. Its fibers appear to be axones of cells of the posterior horns of the cord; they pass in the lateral column of the cord, become scattered in passing through the reticular formation of the medulla and pons, and enter the cerebellum by way of the anterior medullary velum.

THE FOURTH VENTRICLE

The =fourth ventricle= (Ventriculus quartus) is the cavity of the rhombencephalon; it communicates with the central canal of the spinal cord behind, and through the aqueduct with the third ventricle in front. It is somewhat rhomboid in outline, elongated from before backward and narrowest behind. It is lined completely by an epithelium (Ependyma) and contains a small amount of fluid.

Its =floor= (Fossa rhomboidea) is formed by the =medulla= and =pons= and is marked by three longitudinal furrows which converge behind. It is widest and deepest a little in front of its middle. The posterior part narrows to a point at the opening of the central canal, and on account of its appearance in man it has been termed the calamus scriptorius. The =median sulcus= (Sulcus medianus) extends the entire length of the floor and is deepest toward the ends. The =limiting sulci= (Sulci limitantes) begin on either side of the opening of the central canal and extend forward as the lateral limits of the rhomboid fossa. Just beyond the middle of the fossa they expand into a shallow depression, the =anterior fovea= (Fovea nasalis). On either side of the median sulcus and margined by the limiting sulcus is a slightly rounded column, the =eminentia medialis=. Opposite the fovea this presents an elongated prominence, the =colliculus facialis=, so named because it overlies the bend formed by the fibers of origin of the facial nerve. External to the limiting sulcus is a long fusiform elevation, the =area acustica=, from which a band of fibers (Striæ acusticæ) winds over the anterior end of the restiform body to the superficial origin of the cochlear nerve.

The =lateral wall= is formed by the restiform body and the anterior peduncle of the cerebellum.

The =roof= is formed in its middle part by the vermis of the cerebellum, covered by the epithelium before mentioned. There is commonly a dorsal recess (Recessus tecti s. fastigium) between the extremities of the vermis. The anterior part of the roof is formed by a thin lamina of white substance, the =anterior medullary velum= (or valve of Vieussens), which extends backward from the corpora quadrigemina, and is attached on either side to the anterior peduncles of the cerebellum. Its anterior part is relatively thick and contains the decussation of the fibers of the trochlear nerves. Posteriorly it blends with the white matter of the cerebellum. A thin lamina of white matter, the =posterior medullary velum=, backed by pia mater, completes the roof posteriorly. After removal of the cerebellum the line of attachment (Tænia ventriculi quarti) to the medulla is seen; it begins centrally over the opening of the central canal, runs forward on the inner face of the restiform body, and turns outward behind the brachium pontis. The thick part which stretches over the posterior angle of the ventricle is termed the =obex=. The posterior part of the ventricle forms three recesses, of which two are lateral and the third median and posterior. The =lateral recesses= communicate with the subarachnoid space by distinct openings (Aperturæ laterales). The layer of pia which strengthens the roof here is named the =tela chorioidea= of the fourth ventricle. It is triangular in outline and closely adherent to the velum. It forms three fringed masses which contain vascular convolutions and are designated the =median= and =lateral chorioid plexuses= of the fourth ventricle. They appear to lie within the ventricle, but are really excluded from the cavity by the epithelial lining, which they invaginate.

THE MESENCEPHALON

The =mesencephalon= or =mid-brain= connects the rhombencephalon with the fore-brain. In the undissected brain it is covered dorsally by the cerebral hemispheres. It consists of a dorsal part, the corpora quadrigemina, and a larger ventral part, the cerebral peduncles, which are visible on the base of the brain. It is traversed longitudinally by a narrow canal, the cerebral aqueduct, which connects the fourth ventricle with the third.

The =corpora quadrigemina=[191] are four rounded eminences which lie under the posterior part of the cerebral hemispheres. They consist of two pairs, separated by a transverse groove. The =anterior pair= (Colliculi nasales) are larger and much higher than the posterior pair. They are gray in color, almost hemispherical, and are separated by a narrow furrow which leads forward to the subpineal fovea. A wide groove intervenes between them and the optic thalami. The =posterior pair= (Colliculi caudales) are relatively small and are paler than the anterior pair. They are marked by a wide median depression, and are limited below by a transverse furrow (Sulcus postquadrigeminus), at either side of which the =trochlear= (fourth) =nerve= emerges. Laterally each is prolonged to the inner geniculate body by a band of white matter termed the inferior brachium.[192]

The =cerebral peduncles= or crura cerebri (Pedunculi cerebri) appear on the base of the brain as two large rope-like stalks which emerge from the pons close together and diverge as they extend forward to enter the cerebrum. At the point of disappearance the =optic tract= winds obliquely across the peduncle. About half an inch further back a small tract (Tractus peduncularis transversus) curves across the peduncles, and behind this near the median line is the superficial origin of the =oculomotor= (third) =nerve=. The triangular depression between the diverging peduncles is the =interpeduncular space= (Fossa interpeduncularis). It is covered to a large extent by the =pituitary body=, a discoid brown mass which is connected with the base of the brain by a hollow stalk, the =infundibulum=. The posterior part of the space is pierced by numerous minute openings which transmit blood-vessels, and is therefore termed the =locus perforatus posticus=. The objects in the space belong to the diencephalon, and will be described later. The lateral aspect of the peduncle is marked by a groove (Sulcus lateralis mesencephali) which indicates the division into a dorsal part, the =tegmentum=, and a ventral part the =basis pedunculi=; these are separated by a layer of dark gray matter, the =substantia nigra=. The triangular area (Trigonum lemnisci) above the lateral groove is faintly marked by fibers passing obliquely upward and backward to the anterior cerebellar peduncle; these belong to the fillet or lemniscus, an important tract that connects the thalamus and corpora quadrigemina with the sensory reception nuclei of the opposite side of the medulla.

The =aqueduct of the cerebrum= or of Sylvius (Aquæductus cerebri) is the canal which extends through the mid-brain from the fourth to the third ventricle. It is surrounded by a layer of gray matter (Stratum griseum centrale), in the ventral part of which are the nuclei of origin of the oculomotor and trochlear nerves, and laterally nuclei of the mesencephalic roots of the trigeminal nerves.

THE DIENCEPHALON

The =diencephalon= or =inter-brain= comprises the thalamus and a number of other structures grouped about the third ventricle, the cavity of this division of the brain.[193] To expose its dorsal aspect, the greater part of the cerebral hemispheres, the corpus callosum, the fornix, the hippocampus, and the tela chorioidea of the third ventricle must be removed.

The =thalamus= (or optic thalamus) is the principal body in this part of the brain. It is a large ovoid gray mass placed obliquely across the dorsal face of each cerebral peduncle, so that the long axes of the two thalami would meet in front about at a right angle. Medially they are fused to a large extent, and around the area of adhesion they are separated by a sagittal circular space, the third ventricle. The dorsal surface is convex in both directions, and is separated from the overlying hippocampus by the tela chorioidea (or velum interpositum). Laterally it is separated from the nucleus caudatus by an oblique groove in which there is a band of white matter termed the =stria terminalis= or tænia semicircularis. Internally it is bounded by a narrow band, the =stria medullaris=, on which is a delicate ridge of ependyma termed the =tænia thalami=. The striæ unite posteriorly and blend with the stalk of the pituitary body. Near this point they present a small enlargement caused by the nucleus habenulæ. Anteriorly there is a small eminence, the =anterior tubercle=. The posterior part of the thalamus has the form of a rounded ridge which is continuous laterally with the optic tract.[194] Behind the point of origin of the tract, in the angle between the thalamus and the cerebral peduncle, is the =internal geniculate body= (Corpus geniculatum mediale), a well defined oval prominence.