MEMBRANES. 105 



arachnoid and pia mater, which protect it from injury and supply it with 

 blood vessels. 



The Brain and Spinal Cord are composed of both white fibres and 

 collections of gray cells, and are, therefore, to be regarded as conductors of 

 impressions and motor impulses, as well as generators of nerve force. 



MEMBRANES. 



The Dura Mater, the most external of the three, is a tough membrane, 

 composed of white fibrous tissue, arranged in bundles, which interlace in 

 every direction. In the cranial cavity it lines the inner surface of the 

 bones, and is attached to the edge of the foramen magnum ; sends pro- 

 cesses inward, forming the falx cerebri, falx cerebelli, and tentorium cere- 

 belli, supporting and protecting parts of the brain. In the spinal canal it 

 loosely invests the cord, and is separated from the walls of the canal by 

 areolar tissue. 



The Arachnoid, the middle membrane, is a delicate serous structure 

 which envelopes the brain and cord, forming the visceral layer, and is 

 then reflected to the inner surface of the dura mater, forming the parietal 

 layer. Between the two layers there is a small quantity of fluid which 

 prevents friction by lubricating the two surfaces. 



The Pia Mater, the most internal of the three, composed of areolar 

 tissue and blood vessels, covers the entire surface of the brain and cord, to 

 which it is closely adherent, dipping down between the convolutions and 

 fissures. It is exceedingly vascular, sending small blood vessels some dis- 

 tance into the brain and cord. 



The Cerebro-spinal Fluid occupies the sub-arachnoid space, and the 

 general ventricular cavities of the brain, which communicate by an opening, 

 the foramen of Magendie, in the pia mater, at the lower portion of the 4th 

 ventricle. This fluid is clear, transparent, alkaline, possesses a salt taste 

 and a low specific gravity ; it is composed largely of water, traces of albu- 

 men, glucose and mineral salts. It is secreted by the pia mater; the 

 quantity is estimated from two to four fluid ozs. 



^fo function of the cerebro-spinal fluid is to protect the brain and cord, 

 by preventing concussion from without ; by being easily displaced into the 

 spinal canal, prevents undue pressure and insufficiency of blood to the 

 brain. 



