674 
wards, and a little backwards. As the brain 
rests on the upper surface of its hemispheres 
with its base upwards, the medulla oblongata 
is seen to occupy a notch or depression be- 
tween the hemispheres of the cerebellum. The 
fibres of the pons Varolii are seen passing out- 
wards and backwards into each hemisphere of 
the cerebellum, forming the inferior layer of 
each crus cerebelli. On each side of the me- 
dulla oblongata is the inferior convex surface 
of each hemisphere of the cerebellum marked 
by its fissures and lamine. The basilar artery 
in a groove along the middle of the pons 
m before backwards. The fifth nerve emerges 
’ from the crus cerebelli, the sixth immediately 
below the posterior margin of the pons, and 
the seventh, eighth, and ninth nerves are seen 
springing from each side of the medulla by a 
series of fascicles similar to those which form 
the roots of the spinal nerves. 
Of the dissection of the brain from above 
downwards.—It will facilitate our subsequent 
descriptions, if, previous to examining the se- 
veral segments of the encephalon in detail, 
I give a rapid sketch of the dissection of the 
brain according to the topographical method, 
proceeding from above downwards. 
This dissection is commenced by making a 
horizontal section of one hemisphere, a little 
above the level of the corpus callosum. The 
surface, which is thus ex , has in shape 
the charaeter of a demi-oval. Itis chiefly com- 
posed of white substance, which occupies the 
centre of the space, bounded by a wavy border 
of grey matter. Anatomists designate it cen- 
trum ovale minus. We find this is a convenient 
section on which to study the anatomy of the 
convolutions, and to give some idea of the 
composition of that portion of the hemispheres 
of the brain which is situate above the ventricles. 
On making a similar section of the other hemi- 
sphere at the same level, a similar surface 
is exposed, and the conjunction of both con- 
stitutes what Vieussens denominated the cen- 
trum ovale majus. 
By separating the hemispheres slightly, after 
this section, the horizontal portion of the cor- 
pus callosum is well displayed. The con- 
tinuity of its transverse fibres with the white 
substance of the hemispheres may be traced ; 
and by following its anterior and posterior 
reflections they will be found to connect the 
hemispheres at their inferior as well as their 
superior parts. The corpus callosum, wher 
examined in its full extent, exhibits somewhat 
of a vaulted shape, and is found to enter 
largely into the formation of the roof of the 
lateral ventricles. 
We notice some remarkable longitudinal 
fibres, passing along the middle of the corpus 
callosum, varying greatly in developement in 
different brains. These consist of two bundles 
laced in juxta-position, but easily separable. 
We may trace them throughout the whole length 
of the corpus callosum. They cut the trans- 
verse fibres at right angles, and may be readily 
dissected up from them. They seem to tie the 
pene l rk together, and are probably com- 
missural. They form what has been improperly 
” 
NERVOUS SYSTEM. (Nervous Centres. Tue Excernatoy.) ; 
called the raphé of the callosum, m 
correctly the longitudinal tracts (Vieq d’Az 
By scraping away the white substar x 
each side of the corpus callosum, the latera 
ventricles may be opened. If this be dor 
with great care, a considerable portion of the 
membrane that lines the interior of each ve 
tricle may be exposed, but such is its gi 
delicacy that a very slight force ruptures 
When there is fluid in the ventricles, 
membrane may be more easily demonstr 
from its floating upon the fluid. The place 
which the ventricles may be most cer 
opened without the risk of injuring any of 
carts contained within them, is about the i 
of an inch external to the blending of the fi 
of the corpus callosum with the white subst 
of the centrum ovale. With the handle ofa k 
the fibrous matter which forms the roof of 
ventricle may be torn through in the ante O-p 
terior direction, and the cavity 2X pos 
Each lateral ventricle consists of a horizo 
and a descending portion. The former 
sembles in shape an inverted italic S. Its 
terior extremity, or cornu, is directed outwa 
the posterior turns inwards towards that of 
opposite side. The descending cornu pas 
downwards, forwards, and inwards in a curt 
course with the concavity forwards and inw 
and terminates at the fissure of Sylvius. 
first has been appropriately designated 
JSrontal ventricle, the second the occipital, | 
the third the sphenoidal, from their relatior 
the bones after which they have been r 
tively named. The posterior cornu is also ni 
the digital, or ancyroid cavity. a 
The anterior cornua of the lateral 
are separated from each other by a vertical : 
tum situated on the median ry 
and transparent, the septum lucidum. 
may be easily demonstrated on a a 
tion of the heatti made a little to one. 
of the mesial plane, or if both lateral 
tricles have been o , by supportin 
corpus callosum on each side with the ha 
of a knife, by which means the eptul 
stretched, and its extent and connections 
be more readily determined. The sept 
of a triangular form with curvilinear base, } 
is directed forwards, and fits into the at 
reflection of the corpus callosum. Post 
it tits in between the corpus callosum 
the anterior extremity of the horizont 
of the fornix. “ 
The septum lucidum, although so ext 
delicate and transparent, is very obvious! 
posed of two layers, which enclose a sj 
cavity called the fifth ventricle. This 
shewn by dividing the septum horizont | 
J 
rd 
? 
behind forwards. Each of these lamin 
sists, as may be easily observed by ex 
the margin of the section, of four laye 
outer one is derived from the rr r 
of the ventricles ; i within 
layer of a pale ish matter continuous 
a similar 5 ae hiss covers the of ic t ; 
and the internal surface of the third ve 
consisting of clear nucleus-like particles: 
geneous in texture; a third layer is com 
