NEUROLOGY. 109 



2. The Pia 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 spinal cord. This membrane stands in precisely the 

 same relation to the spinal cord as the ncurilcnia does to the nerves. It is 

 'composed 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 nnd density. When quite 

 ]-ecent, 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 

 membrp,ne 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 supei'ficial grey matter. At certain situations also, this 

 jnembrane is continued into the ventricles of the encephalon, constituting 

 the choroid plexitses. 



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 tlie 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 tlie 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 lies 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, . . . 0.801 



Animal matter and Phosphate of Soda, . . . 0.036 



Carbonate of Soda and Phosphate of Lime, . . 0.017 



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 



815 



