NERVOUS CENTRES. (Human Anatomy. Tur Encepnaton.) 
are the peduncles of the crus cerebelli. Through 
them the cerebellum forms a connection with 
other parts of the encephalon. The superior 
layer or peduncle is a bundle of fibres which 
extends to the corpora quadrigemina, and may 
be traced beneath them to the optic thalami. 
These are the processus cerebelli ad testes, but 
from their being obviously a medium of con- 
nection between the cerebellum and the cere- 
brum, they may be better named cerebro-cere- 
bellar commissures. It is worthy of remark, 
that these are the only fibres which appear to 
connect these two segments of the brain. The 
middle layer is continuous with the restiform 
ies, processus cerebelli ad medullam oblon- 
gatam. And the inferior layer is evidently de- 
rived from the transverse fibres of the pons 
Varolii, which thus pass from one hemisphere 
to the other, and constitute a great commissure 
to the cerebellar hemispheres. These fibres, 
_ moreover, connect each hemisphere to the me- 
_ socephale (fig. 380, ¢, r, v). 
_ From this triple constitution of the crus 
 ¢erebelli, it is plain that the cerebellum may 
exert an influence upon, or be affected by the 
optic thalami or quadrigeminal bodies, the 
_ restiform columns, or the mesocephale. 
e Of the fourth ventricle —This is a rhomboi- 
_ dal cavity, situated at the upper and posterior 
_ part of the medulla oblongata, and extending 
_ over i of the superior surface of the meso- 
f cephale. 
Itis limited superiorly by the poste- 
"rior margin of the testes, and inferiorly by the 
Superior blunt extremity of the posterior pyra- 
mids. Its two lateral angles correspond to the 
entrance of the restiform bodies into the crura 
_ cerebelli. In fact, it is formed by the diver- 
_ gence of the restiform columns in their ascent 
to the hemispheres of the cerebellum. The 
_ Median lobe of the cerebellum lies over the 
fourth ventricle, and conceals it from view. 
the anterior lobule of the inferior vermiform 
Process, the nodule, projects into it, and 
closes it below. On either side of this lobule 
@ process of pia mater, with small granulations 
upon it, is found. These processes are the 
_ choroid plexuses of the fourth ventricle. A- 
round these and thence on to the nodule, the 
and 
cavity of the fourth ventricle. 
Vertical section of the median lobe of cerebellum, mesocephale, 
lulla oblongata, to shew the fourth ventricle. 
0, corpus dentatum ; f, posterior surface of medulla oblon- 
gata; p, pons Varolii; a, processus cerebelli ad testes; v, 
693 
proper membrane of the ventricle is reflected, 
and thus its cavity is shut out from any com- 
munication with the subarachnoid cavity. A. 
vertical section in the median plane, or a little 
to one side of it, displays this arrangement 
well. ( Fig. 392.) 
Along the floor of the fourth ventricle we 
find the central or olivary columns of the me- 
dulla oblongata extending upwards to the optic 
thalami. A fissure, continuous with the pos- 
terior median fissure, separates these columns, 
and terminates above in a canal which pene- 
trates the mesocephale, to reach the third ven- 
tricle: iter a tertio ad quartum ventriculum or 
aqueduct of Sylvius. On either side of the 
fissure certain bundles of white fibres, conti- 
nuous with the auditory nerves, join it at right 
angles, crossing over the olivary columns. This 
fissure, with its white fibres on each side, has 
been compared to a pen with its barbs, and 
hence called calamus scriptorius. 
The fourth ventricle, although sometimes 
called the ventricle of the cerebellum, properly 
belongs to the medulla oblongata. Itis present 
in all the vertebrate classes, and in size bears 
a direct proportion to that of the medulla 
itself. 
OF THE HEMISPHERES OF THE BRAIN.— 
A mass of fibrous matter, covered on its ex- 
terior by a convoluted layer of vesicular matter, 
inflected towards the mesial plane above and 
below a pair of gangliform bodies, (optic tha- 
lami and corpora striata, ) which it thus encloses 
in a cavity or ventricle—this, with certain fibres 
connecting its anterior to its posterior parts, 
forms a cerebral hemisphere. The hemispheres 
of opposite sides are applied to one another 
along the mesial plane, leaving the fissure-like 
interval called the third ventricle; and they are 
united by a plane of transverse fibres, the 
greater part of which is placed above that ven- 
tricle, but which bends down anteriorly as well 
as posteriorly, closing the fissure at those 
situations. 
Of the convolutions.—That which first at- 
tracts attention in connection with the cerebral 
hemispheres, as affording the highest phy- - 
siological as well as anatomical interest, is 
their convoluted surface. This can 
only be well displayed by strip- 
ping off the pia mater. The ap- 
pearance which is then presented 
has been variously described by 
different writers. It has always 
seemed to metoresemblethe folded 
surface formed by the mucous mem- 
brane of the stomach when the mus- 
cular coat is very much contracted. 
The ruge of that membrane be- 
come enormously developed by the 
excessive contraction of the mus- 
cular coat: the mucous membrane 
not possessing any contractile power 
is thrown into thin folds to adapt 
it to the diminished capacity of the 
stomach. Its folded state indicates 
a great disproportion between the 
extent of the mucous surface and 
that of the muscular tunic. If both 
