116 HUMAN PHYSIOLOGY. 



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 processes 

 inward, forming the falx cerebri, falx cerebelli, and tentorium cerebelli, 

 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 albumen, 

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

 estimated from two to four fluid ozs. 



The 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. 



SPINAL CORD. 



The Spinal Cord varies from 16 to 18 inches in length; is half an inch 

 in thickness, weighs i^ oz., and extends from the atlas to the 2d lumbar 

 vertebra, terminating in the filum terminale. It is cylindrical in shape, 

 and presents an enlargement in the lower cervical and lower dorsal regions, 

 corresponding to the origin of the nerves which are distributed to the 

 upper and lower extremities. The cord is divided into two lateral halves 



