itt ii 
NERVOUS CENTRES. (Human Anatomy. Tat Menrnces.) 
uls-de-sac at the orifices through which the 
erves escape. ) 
Fig. 367. 
Transverse section of the same on a level with the fifth 
4 cervical nerves. (After Arnold. ) 
"The same parts are displayed as in the last figure, 
ind the reflection of the arachnoid at the exit of 
he nerves is seen. : 
af, anterior fissure ; m, ”, spinal nerves. 
In the interval between each pair of nerves, 
ve find a triangular process of fibrous mem- 
wane which is inserted by its apex into the 
mater. This process lies in the sub- 
arachnoid cavity and adheres by its base to the 
ia mater. It seems to pierce both layers of 
1e arachnoid, or to pin them down, as it were, 
) the dura mater. 
At the foramen magnum the spinal arach- 
noid may be seen to be continuous with that 
of the brain, and here its visceral layer invests 
the medulla oblongata loosely. Inferiorly we 
trace the membrane down quite to the lowest 
extremity of the dura mater, and in this region 
the visceral layer is particularly loose and free, 
as it lies over the cauda equina. 
When the dura mater is carefully slit up 
either the anterior or the posterior surface, 
‘the arachnoid sac is laid open. It does not 
always happen that the parietal layer separates 
very Seay from the visceral: frequently the 
two layers adhere firmly at several minute 
points, yet this adhesion is effected without any 
connecting membrane, and appears to arise from 
the two layers becoming dried at several corres- 
ponding points, and thus being, as it were, glued 
together. We may frequently observe this in 
specimens that have been some time kept in 
spirits. This point is deserving of notice, as 
these adhesions might be (and indeed they have 
been) noted as of a morbid nature. 
The visceral layer of the spinal arachnoid is 
connected to the pia mater by means of a num- 
ber of long filaments of fibrous tissue which in- 
terlace slightly, and in the areole thus formed 
the fluid is contained. This tissue is most dis- 
tinct and abundant in the cervical region, and 
exists in very small quantity in the dorsal. It 
ceases nearly altogether over the cauda equina. 
Numerous minute bloodvessels are also to be 
found in it passing from the pia mater to the 
arachnoid. Majendie gives to this tissue the 
name “ tissu cellulo-vasculuire sub-arachnoide.” 
In general the adhesion of the visceral layer 
of the arachnoid to the subjacent pia mater is 
closer along the posterior than along the ante- 
rior surface of the cord. 
Along the posterior surface of the cord on 
the median line, the sub-arachnoid space is 
divided by means of a septum, which is most 
rfect in the dorsal region, but which in the 
umbar and cervical regions is cribriform or 
> 
637 
pectiniform, as may be shown by pouring 
quicksilver on either side of it, which will be 
retained in the dorsal region, but will readily 
ass from right to left in the other situations. 
t is highly probable that this septum is a mo- 
dified portion of the sub-arachnoid tissue. 
The existence of this septum (erroneously 
described as complete) dividing the posterior 
part of the sub-arachnoid space into a right 
and a left portion, appears to have led to the 
opinion that this space is lined by another 
serous membrane, which has been called the 
internal arachnoid, by which the fluid is sup- 
posed to be secreted, and that the septum is 
formed by the reflection of its visceral into its 
parietal layer along the median plane. But there 
are many objections to this hypothesis. In the 
first place, if the septum were formed by the 
reflection of a serous membrane, it would be 
complete, and not a very imperfect one such as 
it is; it ought to resemble the mediastinum in 
the chest, or one of the processes of the perito- 
neum in the abdomen. Secondly, it is quite 
contrary to all experience to find the cavity of a 
serous membrane in the normal state traversed 
by a quantity of filamentous tissue, as the sub- 
arachnoid space is throughout a great part of its 
extent. Thirdly, were there a serous membrane 
in this space, the microscope ought to detect an 
epithelium on its inner surface, but sucha struc- 
ture does not exist here. Lastly, such a serous 
membrane must necessarily be continued into the 
encephalic sub-arachnoid space. But the close 
adhesion of the visceral layer of the arachnoid 
to the pia mater, opposite to the prominent 
parts of the cerebral convolutions, seems quite 
incompatible with such an arrangement. 
Cerebral arachnoid.—The cerebral portion 
of the arachnoid exhibits essentially the same 
general arrangement as the spinal portion. Its 
parietal layer adheres very intimately to the pia 
mater at certain points, leaving in the intervals 
a considerable space for the accumulation of 
liquid. If we trace it over the surface of the 
hemispheres, it will be found to give them that 
smooth and uniform character which is always 
distinct on the recent healthy brain. The 
arachnoid passes from convolution to convolu- 
tion, adhering closely to the pia mater over the 
most convex portions of those convolutions, 
but allowing that membrane to separate from 
it in the intervals between them, and to dip 
down to the bottom of the sulci. Hence liquid 
accumulated in the cerebral sub-arachnoid space 
will be found to take the direction of the inter- 
gyral sulci, and to cause the membrane to 
bulge opposite to them; and if air be blown 
underneath the arachnoid, it will be found to 
take the tortuous course of these sulci. 
The arachnoid sinks into the great longitu- 
dinal fissure of the brain, lining the surfaces 
which bound it on each side, and passing 
across from right to left beneath the inferior 
margin of the falx, and above the corpus cal- 
losum. 
On the base of the brain, the arachnoid has 
the same arrangement on those parts where 
there are convolutions, as on the superior and 
lateral surfaces of the hemispheres. It. passes 
