668 
that great nervous mass which is situate above 
this plane, it will be obvious, even to the most 
Superficial observer, that it admits of a con- 
venient subdivision into certain great segments, 
each of which, although extensively connected 
with the neighbouring ones, may yet be capable 
of acting as an independent centre, and, in short, 
possesses the anatomical as wel! as physiolo- 
gical properties of a ganglion. And on a more 
minute investigation the number of gangliform 
segments will be found to be greater than the 
observation of the mere surface of the ence- 
phalon would lead us to conclude. The sub- 
division, huwever, which it is most conve- 
nient for the purpose of description to adopt, 
is that already stated at page 650, into, 
1, the medulla oblongata, which is immedi- 
ately continuous inferiorly with the spinal cord. 
This segment has certain characters of struc- 
ture which decidedly indicate its ganglionic 
nature; several nerves of considerable size and 
of great physiological importance are implanted 
in it, and its external anatomy very clearly in- 
dicates its distinctness from the spinal cord 
inferiorly and from the other encephalic seg- 
ments above, of which that next in order pro- 
ceeding from below upwards, is, 2, the meso- 
cephale. To this mass, so called because of its 
intermediate position between the other seg- 
ments, the term isthmus has been also very 
appropriately applied, as it is the connecting 
liuk between all the encephalic segments. In- 
ferior and posterior to it is placed, 3, the cere- 
bellum, which has very intimate relations to the 
medulla oblongata as well as to the segment 
last described, but much less extensive ones to 
that which forms by far the largest proportion 
of the encephalon, namely, 4, the cerebrum, 
which therefore occupies the principal portion 
of the cranial cavity. 
The distinction between these different seg- 
ments is very obvious on an examination of the 
surfaces of the brain, which indeed ought to 
be the first step to be taken by the anatomist. 
To discover how they are connected to each 
other and to the spinal cord, how the corres- 
ponding portions on opposite sides of the 
mesial plane are associated tozether, what fibres 
are common to all the segments, and what 
peculiar to some, and, lastly, how the grey 
matter is related to the white,—these are the 
chief objects to be attained in the dissection 
of the brain. No one method of dissection 
will suffice for this purpose. The anatomist 
should first make himself familiar with the 
simple topographical anatomy of the brain, 
that is, with all those parts in it which possess 
such characters of form or structure as may 
entitle them to be regarded as distinct and de- 
serving of separate description, and have ob- 
tained for them a special appellation. The 
form, size, general structure, and relations of 
these parts should be carefully noted. And 
this method of examination is equally applica- 
ble to the dissection of each segment of the 
encephalon. But the most convenient way in 
which it can be conducted for ordinary prac- 
tical purposes, is to commence with the cere- 
bral hemispheres, and having studied their 
NERVOUS SYSTEM. (Nervous Centres. Tae Excepnaton.) 
general structure as displayed on a horiz 
section, to examine the extent and connectio 
of the fibres which connect the right and 
hemispheres with each other (the corpus ¢ 
losum); then to open the ventricles, exan 
their shape and extent, and note the var 
particulars connected with the numerous ] 
which are brought into view by exposing | 
cavities. The dissector may next ot 
certain of the parts concealed by t 
ventricles are connected with the mesoe 
(the optic thalami for instance), and, 
been already acquainted with the vario 
minences which are seen upon the sur 
the latter, he may by vertical, or tran 
horizontal sections, investigate the m: 
which the white matter of this segme 
nects itself with that of the neighbourin 
In examining the cerebellum, the 
sures afford sufficient indication for 
venient subdivision of the organ, and by 
zontal or vertical sections at various pai 
the connexion of the grey and white 
may be displayed, and of the latter to 
socephale and medulla oblongata. T 
dulla oblongata has upon its surface” 
lines or fissures which denote the prop 
of its constituent columns, and which 
sufficient guide to the dissector in tr 
extent and connexions of each. 
and longitudinal sections also afford us 
formation respecting the structure of th 
ment of the encephalon and the relation 
parts. 
Such is the mode of dissection ’ 
downwards, against which it has been 
the fashion of late years to declaim wi 
vehemence. But, however the advo 
a particular theory may object, 
no dovbt that this method is b 
useful for all practical purposes. It ena 
anatomist, without difficulty, to study 
minent parts or landmarks (so to speak 
brain, without a knowledge of whicl 
vain to attempt any other mode of dis 
And for pathological investigations | 
only method which can be conveniently) | 
It is plain, therefore, that all who are | 
© 
of becoming acquainted with the an 
this organ should begin by making @ 
in this way. An additional advantage i 
in this mode of investigation, from its 
eae to the dissection of the br 
ower animals, of the Mammalia 
especially, for the purpose of compar 
with the brain of the human subject. 
The method of our celebrated ¢ 
Willis was very much the same as 1 
described. He removed the mem 
the posterior lobes of the hemisph 
thus separated the latter from the- 
parts, and by raising them as far 
as possible he was enabled to obser 
nections of the cerebral hemisphe 
mesocephale, and the attachments of 
behind. He also must have studi 
stance of the hemisphere by horizon 
By then dividing the or pi 
hemispheres horizontally along the p) 
