678 
The interval between the processus cerebelli 
ad testes is occupied by a horizontal stratum of 
nervous matter composed of a thin layer of grey 
and of white matter. This is called the valve of 
Vieussens, although there is evidently nothing 
valvular in its nature or office. Its surface is 
marked by slight transverse depressions and 
eminences. The median lobe of the cerebel- 
Jum overlaps and conceals it from view. 
The valve of Vieussens* must be regarded 
as a portion of the median lobe of the cerebel- 
lum, which is extended forwards between the 
processus cerebelli ad testes. Its constitu- 
tion is precisely the same as the lamine of that 
body, and the transverse markings upon its 
superior surface are indications of imperfectly 
developed fissures between the lamin. 
The corpora quadrigemina form the anterior 
superior part of the mesocephale. They lie above 
the crura cerebri, upon those columns of nervous 
matter by which the latter bodies are connected 
with the medulla oblongata. These columns 
are continuous above with the optic thalami, 
and below with the central portion of the me- 
dulla oblongata, the olivary tracts, or fasciculi 
innominati of Cruveilhier. They are distin- 
guished by their reddish grey colour and their 
close resemblance in point of structure to the 
optic thalami. In transverse section they appear 
as two columns, circular in outline, quite dis- 
tinct from the surrounding greyish matter in 
which they seem imbedded (fig. 388, i). 
The lower half of the thickness of the me- 
socephale is formed by transverse curved fibres 
with anterior convexity, which extend between 
the lateral lobes of the cerebellum, and of 
longitudinal fibres which interlace with the 
superior layers of those transverse fibresand cross 
them at right angles. The former constitute 
the om Varolii, a great commissure between 
the hemispheres of the cerebellum ; the latter 
are, in greater part at least, the fibres of the 
anterior pyramids of the medulla oblongata, 
which ascend through the pons, and enter into 
the formation of the inferior layer of each crus 
cerebri. 
In examining the inferior surface of the me- 
socephale, the pons Varolii, we observe that a 
longitudinal ve extends along its middle 
from above downwards. In this lies the ba- 
silar artery. Above the anterior edge of the 
pons, the crura cerebri are seen emerging, and 
diverging from each other as they pass, to enter, 
stalk-like, into the inferior surface of the cere- 
bral hemispheres. Beneath its posterior edge, 
the medulla oblongata is seen, its anterior and 
* Valvula cerebri major is the name which Vi- 
eussens applied to this process. He describes it as 
** membrana quam transversus medullaris tractus 
circa anteriora subit, processui vermiformi anteriori, 
rocessibus a cerebello ad testes et postice pontis 
arolii parti adhwret et unitur.” He further adds, 
** illam valvule vices gerere asserimus. Ex quo fit, 
ut habita officii et magnitudinis illins ratione, 
ipsam valvulam cerebri majorem nominemus, ut 
eam a membranaceis ligamentis distinguamus, que 
intra longitudinalis et lateralium sinuum cavitates 
valvularum minorum vices supplent et munia pre- 
stant.”—Neurographia Uni 
niversalis, p. 76, Ed. Lugd. 
NERVOUS SYSTEM. (Nervous Centres. Tue Encermaton.) 
middle columns ing through the mesoce-— 
phale to the crate caleli. “Om aa side the 
fibres of the pons off into each he 
sphere of the cerebellum and form the infer 
lamina of each crus of that organ. 
The cerebellum—Some account of the 
neral disposition of the cerebellum will 
to conclude this brief review of the topog 
of the brain. The superior surface of this 
organ is a little above the level of the qua- 
drigeminal bodies. It is smooth and slightly 
convex. The lamelle of the cerebellum are 
visible upon it, but cannot be se with- 
out removing the arachnoid pia mate 
A notch is seen, dividing the posterior ¢ 
into two equal portions, and a larger note 
exists in front, at which the cerebellum forms 
its connection with the mesocephale. These 
notches denote a subdivision of the organ into 
two lateral portions, or hemispheres, and @ me- 
dian portion. The superior surface of the 
median portion is called the superior 
miform process; its anterior terminal lamina 
form the valve of Vieussens. On the inferior 
surface the hemispheres of the cerebellum ar 
much more convex than on the superior. Th 
median portion too is somewhat different 
arranged on its inferior surface; it consists of ; 
series of lamine, following a transverse direc- 
tion ; those in its centre are of greater tran 
verse extent than those bs either extremity 
whence the a rance of a crucial figure re 
sults. This ie the inferior vermiform process 
The posterior margin of the cerebellum 1 
convex, and corresponds to the concave surfac 
of the occipital bone, the falx cerebelli oceup 
ing the notch in its middle. Along the line « 
this margin, the pia mater sinks into a de 
fissure, which takes a horizontal direction fr 
behind forwards, and divides the cerebellu 
into a superior and inferior portion. 
As the brain, removed from the craniu 
lies with its base upwards, the medulla of 
longata is seen between the lateral hemispher 
of the cerebellum occupying a portion of 
depression between them, in which is t 
ferior vermiform process (fig. 382). 
The fourth ventricle is a lozenge 
cavity situated in the upper and poste 
of the medulla oblongata, and formed by 
separation of its postero-lateral columns (¢ 
pora restiformia). The cerebellum contrib 
to inclose it above by means of the 
lamine of the superior vermiform process” 
the valve of Vieussens, and below and be 
by the inferior vermiform process (fig. 
We now proceed to the examination 0 
various segments of the encephalon, w 
more special reference to the structure 
physiological bearing of each, It may 
remarked that, while all the segments ai 
timately connected with each other 
therefore mutually dependent, there 
in their structure to justify the assumption 
each is capable of exercising an indep 
function, which is, however, liable to b 4 
dified by the influence which any one, or 2 
the other segments may have upon it. 
Or THE MEDULLA OBLONGATA. (Fr. mt 
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