NERVOUS CENTRES. (Human Anatomy. Tue Mentnces.) 
cavernous sinuses of opposite sides across the 
basilar process of the occipital bone. Some- 
times there are two running parallel to each 
other. Cruveilhier states that the capacity of 
this sinus is much greater in old than in young 
subjects. 
Cavernous sinuses.—In point of shape these 
sinuses differ considerably from all the other 
sinuses of the dura mater. They are venous 
reservoirs, situated on each side of the sella 
Turcica, from which they are separated by the 
internal carotid arteries. Their name is derived 
from the spongy appearance which they present 
in their interior, owing to the existence of some 
filaments within them, which, by their inter- 
lacement with each other, form a reticular 
texture there. It was formerly supposed that 
the carotid arteries lay in the cavity of these 
sinuses and were bathed by their blood ; but it 
is easy to demonstrate by a little careful dis- 
_ section that the inner membrane of the sinus 
adheres loosely to the outer wall of the artery, 
and that the sixth nerve passes between them. 
In the outer wall of each cavernous sinus there 
are channels for the reception of those nerves, 
which pass from the cranium into the orbit. 
These are the third nerve which is placed 
highest up, the fourth nerve which holds the 
next place, and the ophthalmic portion of the 
fifth. The cavernous sinus receives at its ante- 
rior extremity the ophthalmic vein, which col- 
lects the blood from the eye-ball and other 
Structures within the orbit, and which commu- 
" Ricates also with the angular vein and with the 
frontal vein. (Hence the injected state of the 
vessels of the eye-ball when the brain is con- 
gested, as in fever.) Veins from the inferior 
Surface of the anterior lobes of the brain also 
Open into it, also some from the middle lobe 
and from the dura mater. Posteriorly it com- 
municates with both the petrosal sinuses, and 
veins from the cranial bones open into its 
superior wall. 
__ Circular sinus—A communication is esta- 
blished between the cavernous sinuses by means 
of the circulur or coronary sinus which em- 
braces the pituitary body, one portion lying in 
front of it and the other behind it, both open- 
ing by a common free orifice into the right and 
left cavernous sinuses. The posterior portion 
of the circular sinus is much larger than its 
anterior portion. Its size is much greater, 
according to Cruveilhier, in old subjects than 
in young ones. It receives small veins from 
the pituitary body, and also from the sphenoid 
bone and from the dura mater. 
It is impossible to examine this complicated 
atrangement of venous channels in connexion 
with the dura mater of the brain without ad- 
miring the beautiful provision which it affords 
against the undue accumulation of blood in 
the venous system within the cranium. In 
the first place, we observe that these veins do 
not admit of dilatation beyond a prescribed 
extent, by reason of their being enclosed be- 
tween layers of an inelastic and inextensible 
membrane. Next, we remark the safety pro- 
vision which is afforded by the frequent com- 
munication between them, so that if one chan- 
633 
nel were altogether closed or material con- 
tracted, there are many others by whitn the 
blood could return. Nor is a local congestion 
likely to take place to any extent, for such 
is the freedom of communication between the 
sinuses and the veins of the exterior of the 
cranium, that (all being devoid of valves) an 
overflow would readily be received by the 
latter without the least impediment. Lastly, 
we learn the great importance and value of 
local depletion as an agent for relieving vascu- 
lar fullness within the head, owing to the free 
communication between the extra- and the 
intra-cranial circulation, and especially of the 
veins; and we may infer from anatomy that 
local depletion would most probably be more 
serviceable than general, for although the latter 
would diminish the amount of the mass of 
circulating fluid, it would not affect the relation 
between the venous and arterial systems, whilst 
it is evident that the former must affect the 
venous system more directly than the arterial. 
Moreover, the free communication between the 
circulation within and that without the cranium 
may explain somewhat the advantage that is 
often derived from the application of an intense 
cold to the external surface of the head. 
Of the pia mater. ( Tunica intima vel vascu- 
losa..)—The pia mater is the most internal mem- 
brane of those which have been enumerated 
as belonging to the spinal cord and brain. 
Pia mater of the spinal cord.—This mem- 
brane stands in precisely the same relation 
to the spinal cord as the neurilemma does to 
the nerves; and as long as the spinal cord 
could be, as it formerly was, regarded merely 
as a bundle of nervous fibres, the analogy of 
this membrane to the nervous sheath would 
be perfect. It is composed almost entirely 
of white fibrous tissue; it closely invests the 
cord and supports the minute bloodvessels 
which carry the nutrient fluid to it. Not 
only does it thus form a complete sheath to the 
cord, but it likewise sends in processes which 
dip into the anterior and posterior median 
fissures of that organ. That which passes into 
the anterior median fissure is a true fold or 
duplicature of the pia mater; but the posterior 
fissure, which is much narrower than the ante- 
rior, is occupied only by a single and extremely 
delicate layer, which at some parts almost 
entirely disappears, and seems to consist merely 
of a few minute capillary vessels. The pia mater 
becomes continuous with the neurilemma of 
the roots of the nerves on each side of the cord, 
and at its inferior extremity it tapers in accor- 
dance with the shape of the spinal cord, and is 
prolonged as a delicate thread which is inserted 
into the extremity of the dura mater. This 
prolongation is quite gradual, so that at the 
upper part it encloses a portion of the medullary 
substance of the cord ; in the greater part of its 
extent, however, it is merely a membranous 
thread, and, therefore, goes by the name /fili- 
Jorm prolongation of the pia mater (filum 
terminale). The late Dr. Macartney used 
to regard it as highly elastic, but my friend 
Mr. Bowman has called my attention to the 
fact that it consists almost entirely of white 
