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.