NEUROLOGY. 109 
2. Tak PrA Mater. The pia mater, tunica intima vel vasculosa, is the 
most internal membrane of those enumerated as belonging to the cerebro- 
spinal axis. 
Pia mater of the spmal cord. This membrane stands in precisely the 
same relation to the spinal cord as the neurilema does to the nerves. It is 
fcomposed almost entirely of white fibrous tissue, closely investing the cord, 
and supporting the minute blood-vessels which feed it. Not only does it 
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. The 
spinous pia mater possesses considerable strength and density. When quite 
recent, it may be readily dissected off from the cord, its adhesion being 
through the medium of numerous exceedingly minute capillary vessels. 
On its exterior, the pia mater adheres to the visceral layer of the arachnoid 
membrane by means of a loose fibrous tissue. 
Pia mater of the brain. In tracing the pia mater of the spine upwards, 
it will be found gradually to become thinner as it passes from the medulla 
oblongata to the hemispheres of the brain. Here it adheres closely to the 
whole surface of the brain, cerebellum, and connecting parts, numberless 
vessels passing from it into the nervous substance beneath. On the surface 
of the brain, it dips down into the furrow between the convolutions, and 
adheres to the superficial grey matter. At certain situations also, this 
membrane is continued into the ventricles of the encephalon, constituting 
the choroid plexuses. 
3. THE ARACHNOID MEMBRANE, intermediate between the two last 
described, is a great serous membrane pervading the entire cranio-spinal 
cavity. Its parietal layer adheres intimately to the inner surface of the 
dura mater, and its visceral to the outer surface of the pia mater. Like all 
serous membranes, it is composed of two layers, the interval between which 
is called the arachnoid bag or sac, and rarely contains fluid. Between the 
visceral layer of the arachnoid and the pia mater, is the sub-arachnoid cavity, 
usually including a considerable quantity of fluid. The object of this fluid 
is to protect the nervous centres with which it hes in immediate contact. 
By the interposition of a liquid medium between the nervous mass and the 
walls of the cavity in which it is placed, provision is made against a too 
ready conduction of vibrations from one to the other. Its composition, 
according to Lassaigne, is as follows: 
Water, . : : : : ‘ : . 98.564 
Albumen, : : : : : : : , 0.088 - 
Osmazome, ’ : : 0.474 
Chloride of Sodium and of Potassium, . : str, Oe0e 
Animal matter and Phosphate of Soda, . * ) 00ers 
Carbonate of Soda and Phosphate of Lime, < ntn 
99.980 
Having thus treated in brief terms of the membranes of the cerebro- 
spinal axis, we come next to the essential structure of this part of the 
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