THE MEMBRANES OF THE BRAIN.. 
463 
brane in our other patients, except that the cavernous sinuses are 
more extensive in the ox, and that they likewise contain a kind 
of arterial resorvoir derived from the carotids. 
The Arachnoid Membrane. —Beneath the dura mater, and 
separated from it by a small portion of fluid, or at least of mois- 
dure on the surface of the membranes, is the arachnoid tunic, so 
called from being as fine as a spider’s web. It likewise envelops 
the brain. It is said to float loose between the dura mater and 
the pia mater, but it is tied down by those cords and vessels 
from the dura mater which penetrate through it in order to reach 
the brain ; and at these points of union, where we are occasionally 
enabled to make a happy dissection, we find a portion of cellular 
substance of almost inconceivable delicacy surrounding the cord, 
and giving some strength to the attachment of so fragile a mem- 
.brane. The arachnoid tunic is most easily demonstrated at the 
base of the brain superiorly to the origins of the optic nerves. 
Of its use I cannot confidently speak; it may either be a secret¬ 
ing membrane to supply the fluid interposed between the menin¬ 
ges of the brain, or it maybe destined in some measure to cut off 
the communication between the brain and the dura mater, and to 
prevent its too much sympathizing with any inflammation or 
injury of the membranes of the cranium. 
It does not appear that either the dura mater or the arachnoid 
membrane is endued with sensibility ; and in none of our patients 
have we proof of that inflammation of the tunica arachnoidea of 
which medical men have spoken. 
The Pia Mater. —Beneath these is the proper investing mem¬ 
brane of the brain—the pia mater —and which not only covers 
its external surface, but penetrates into every sulcus, lines every 
ventricle, and clothes every irregularity and portion and part of 
the brain. So far as we can judge of it by maceration, it is 
composed of delicate but condensed cellular tissue; and, like 
the cellular tissue generally, it penetrates everywhere, being the 
vehicle by means of which the bloodvessels and the absorbents 
are conveyed, by which strength is given to every part, and by 
which they are at once divided and united. The external sur¬ 
face of the pia mater is smooth and polished; the internal sur¬ 
face cannot be demonstrated ; it may occasionally be torn off, 
but it cannot be dissected ; it is, however, necessarily roughened 
by the multitude of vessels which it conveys into the substance 
of the brain. Both the pia mater and the arachnoid membrane 
are prolonged through the whole extent of the spinal cavity. 
The Bulk of the Brain. —We now arrive at the brain itself; 
and the first thing which strikes us is its comparatively diminu¬ 
tive size in our domesticated animals. Look at this brain of the 
