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.