638 
over the fissure of Sylvius from the anterior to 
the middle lobe, and here its distinctness from 
the pia mater may be clearly demonstrated ; 
here too it appears much stronger and more 
opaque than elsewhere, which is probably 
due to the existence of an increased quantity 
of fibrous tissue beneath it. 
In that space on the base of the brain which 
is bounded on each side by the middle lobes, 
and which is limited in front by the optic 
nerves and behind by the pons Varolii, the 
arachnoid membrane stretches across from one 
middle lobe to the other, leaving a considerable 
space between the tuber cinereum and the pons, 
in which it is connected to the pia mater by 
several long filaments similar to those which 
are met with on the surface of the spinal cord. 
This s is favourable for the accumulation 
of fluid, and it communicates in front with the 
fissures of Sylvius and other deep fissures into 
which fluid might make its way. Cruveilhier 
calls it the anterior sub-arachnoid space, and 
regards it as the principal reservoir of the cra- 
nial serosity. Immediately in front of it we 
observe that the arachnoid membrane is conti- 
nued around the infundibulum to the pituitary 
body. 
re tracing the arachnoid backwards from the 
great longitudinal fissure of the brain, we ob- 
serve that it stretches down from the posterior 
edge of the corpus callosum to the superior 
surface of the cerebellum, crossing over the 
tubercula quadrigemina. At this situation the 
arachnoid is reflected upon the vene magne 
Galeni as they pass to the straight sinus. It 
was at this place that Bichat described the 
canal which goes by his name, through which, 
as he thought, a process of the arachnoid mem- 
brane was carrried in to line the interior of the 
ventricles. 
The arachnoid covers the superior surface of 
the cerebellum and also its inferior surface, 
stretching across the longitudinal fissure from 
one hemisphere to the other, and it is also ex- 
tended downwards, and a little forwards from 
the superior surface of the cerebellum to the 
terior surface of the medulla oblongata, 
low the inferior extremity of the fourth ven- 
tricle. A considerable space is thus left, situate 
posteriorly between the cerebellar SA NRC 
and bounded in front and inferiorly by the 
medulla oblongata, which also forms a conside- 
rable reservoir for cerebral fluid, and communi- 
cates freely with the sub-arachnoid space of the 
spinal canal; but as the arachnoid is tied down 
somewhat more closely over the posterior sur- 
face of the spinal cord, there is an appearance 
of constriction where the cerebral passes into 
the spinal arachnoid. This space is called by 
Cruveilhier the posterior sub-arachnoid space 
(posterior conflux of Majendie). It commu- 
nicates with the anterior sub-arachnoid space 
through the furrows around the crura cerebelli. 
Of the cerebro-spinal fiuid—In examining 
such a dissection of the membranes of the 
spinal cord as that above described, we shall 
find that at various points the visceral layer 
of the arachnoid membrane appears raised up 
by fluid or by a bubble or two of air from 
NERVOUS SYSTEM. (Nervous Centres. Tar Mentnoes.) 
If a small po ion of 
the subjacent viscus, 
this layer be taken up in a id 
blow-pipe be introduced into it, air may be 
blown underneath it, raising it up all a 
the spinal cord to a considerable distance fron 
that organ. The inflation is more easily e 
fected in the cervical and in the lumbar region: 
than in the dorsal, and the air will pass dow 
quite to the lowest part of the canal of the dur; 
mater, where the connexion of the rn 
membrane to the cauda equina is particulal 
loose. In the same way col Auid, 
some material which may assume the solid forr 
as size, tallow, &c, may be injected to demo 
strate this anatomical arrangement. If now v 
examine a transverse section, it will be observe: 
that a considerable interval exists between th 
visceral layer of the arachnoid and the pi 
mater of the cord, and that this interval 1 
much greater in the neck and in the loins 
in the back. We observe too that the spit 
cord is by no means of sufficient size to fill 
spinal canal, and that as a considerable intery 
exists between its surface and the visceral lays 
of the arachnoid, so also a still greater one 
found between it and the inner surface of 
dura mater. Now as it is of the very natu 
of a serous membrane that its free smo¢ 
surfaces should always be in contact (for it 
in that way that it favours the movements 
the viscus with which it is connected), it 
plain that the sub-arachnoid space in the sp 
must, during life, be kept in a state of distens 
otherwise the object of a serous mem 
would not be attained. 
Moreover, in tracing the arachnoid mi 
brane upwards over the medulla oblongata : 
the other parts of the encephalon, we obs 
an evident continuity between the spinal ; 
the cranial sub-arachnoid cavity, which is m 
evident at the base of the brain, where 
latter possesses the greatest dimensions, so tl 
air or fluid may be readily made to pass | 
one to the other. This is most conspicu 
old subjects, in which the brain being : 
and more or less shrunken, leaves a cons 
able interval between its surface and the 
ceral layer of the arachnoid. ‘ 
On opening the spinal canal in a bod 
cently dead, the visceral layer of the arac 
will almost always be found raised by 
When a portion of the posterior wall 
spinal canal is removed in a living an 
in one just killed, the dura mater is fe 
be quite tense from the fluid which is ae 
lated within it. In a horse, whose spina 
I opened in the dorsal region imme Jiatels 
he had been knocked down in the k 
yard, I found the dura mater perfectly 
and semi-transparent from bei etc 
firmly over fluid. Upon making a @ 
puncture in it, a fine stream of clear fluid 
ejected with much force to a considerable 
tance, and immediately the dura mater be 
quite flaccid. By a little careful diss 
through the dura mater and parietal | 
arachnoid, it may be shewn that this 1 
not contained in the arachnoid sac, bat 
sub-arachnoid cavity. we 
