= 
i 
_ the middle line. 
_ 382). 
NERVOUS CENTRES. (Human Anaromy. Tur Encepnaton.) 
of the parts. The whole of that portion of 
the cerebral hemisphere which is situate behind 
the Sylvian fissure should be called the pos- 
terior lobe. 
The hollow space between the middle lobes 
of the brain corresponds to the principal ante- 
rior reservoir of subarachnoid fluid. It is 
situate immediately above the Sella Turcica, 
and, indeed, the brain is, as it were, tied to the 
pituitary body, which is firmly lodged in this 
excavation of the sphenoid bone, by a funnel- 
shaped hollow process of nervous matter, called 
pituitary process or tube, (m, l, fig. 382), which, 
enveloped in a sheath of arachnoid membrane, 
is inserted into it by its small extremity. This 
Space communicates with the anterior fissure 
in the middle, and with the Sylvian fissure on 
either side. 
Commencing at the anterior fissure and pas- 
sing backwards, we notice the following parts, 
_ to see which clearly it is necessary that the 
_ adherent pia mater and the arachnoid should 
have been previously carefully dissected away. 
_ The anterior fissure is limited by the anterior 
fold or reflection of the corpus callosum : 
behind this we find a thin layer of a lightish 
grey matter, which, like a triangular plate, 
Seems to stop up the third ventricle at its in- 
ior surface. is, indeed, which is called 
tuber cinereum, constitutes a principal part of 
‘the floor of that ventricle. The pituitary pro- 
fess is continuous with and is probably an 
extension of it. A probe introduced into the cut 
extremity of this process will be found to pass 
readily into the third ventricle. 
Immediately in front of the pituitary process, 
the union of two white bands, which form la- 
teral boundaries to a large portion of the tuber 
Cinereum, the optic tracts, takes place along 
This forms the commissure 
of the optic nerves, from which these nerves 
diverge. Behind the pituitary process the tuber 
_ cinereum extends back to two small pisiform 
__ bodies of an extremely white colour on their sur- 
face, corpora mamillaria or albicantia (m, fig. 
ese, we shall see by-and-bye, are con- 
nected with one of the most important of the 
cerebral commissures, namely, the fornix. 
Behind the mamillary bodies we find a deep 
depression into which the pia mater sinks, car- 
tying with it very numerous bloodvessels., This 
depression lies between two thick processes of 
fibrous matter, which, traced from below, pass 
be. rc and outwards, expanding as they 
vauce, and upon which each hemisphere is 
placed (to use Reil’s simile) like a mushroom 
on its stalk. These are the crura cerebri, the 
peers of the cerebral hemispheres. The 
epression above described, which separates 
them, is the intercrural or interpeduncular 
space. When the pia mater has been removed 
from it, its surface appears cribriform from the 
perforations of the numerous minute vessels 
which penetrate it; it has been named by Vicq 
d’Azyrsubstantia perforata media. The nervous 
matter which forms the floor of this space has a 
greyish hue, and connects the crura to each other, 
like a bridge, whence the designation pons 
Tarini. At the interpeduncular space we see 
VOL. III, 
673 
the third pair of nerves emerging from their 
connexion with the crura cerebri. 
The inner margin of each middle lobe of the 
brain is separated from the corresponding crus 
cerebri by a fissure which passes from behind 
forwards, and terminates in the fissure of Syl- 
vius. If this fissure be followed backwards, it 
will be found to become continuous with a 
transverse fissure which separates the cerebrum 
from the cerebellum, and corresponds to the 
posterior edge of the corpus callosum. A con- 
tinuity is thus established between the lateral 
and the transverse fissures, whence results one 
great fissure of semicircular form, the concavity 
of which is directed forwards. This is the 
great cerebral fissure of Bichat, or the great 
transverse or horizontal fissure (Cruveilhier.) 
It may be described as commencing at the fissure 
of Sylvius on one side, turning round the oppo- 
site cerebral peduncle, and ending at the oppo- 
site Sylvian fissure. The anterior and lateral 
portions of this fissure have already been no- 
ticed as the situations at which the pia mater 
enters the brain to form the choroid plexuses of 
the lateral ventricles. And it may be remarked 
here, how freely the subarachnoid fluid may pass 
along this fissure from before backwards. Pa- 
rallel to this fissure we find the fourth pair of 
nerves as it passes to its point of exit from 
the cranium. 
Not the least interesting and important of the 
objects presented at this central portion of the 
base of the brain is that remarkable arterial 
anastomosis, called the circle of Willis. This 
will be more particularly described by-and-bye ; 
but it may be stated here, that the anterior 
bifurcation of the basilar artery is immediately 
behind the interpeduncular space, on each 
side of which the posterior cerebral artery 
passes for a short distance. The posterior 
communicating artery is parallel to the inner 
edge of the middle lobe; the subdivision of 
the carotid corresponds to the commencement 
of the Sylvian fissure; and the anterior com- 
municating artery is at right angles with the 
longitudinal fissure immediately behind the 
anterior reflection of the corpus callosum. 
This anastomosis of arteries is bathed in the 
liquid which occupies the subarachnoid space 
in this situation. 
The tentorium cerebelli is situate on a plane 
a little beneath that of the middle segment of the 
base of the encephalon just described. It forms 
a septum between the posterior lobes of the ce- 
rebral hemispheres, which are continuous with 
the middle segment, and the posterior segment 
of the encephalon, which we now proceed to 
describe. 
The posterior segment, as occupying the pos- 
terior fossa of the cranium, is ona level con- 
siderably below that of the middle segment. 
The parts which are deserving of more par- 
ticular notice here, are, proceeding from before, 
the pons Varolii (n, fig. 382), the inferior and 
anterior surface of the mesocephale, which is 
situate immediately behind the interpeduncular 
space, the crura cerebri appearing to emerge just 
above its anterior border. From its posterior 
edge the medulla oblongata (0) extends down- 
2x 
