1 64 HUMAN PHYSIOLOGY. 



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

 which envelops 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 distance into the 

 brain and cord. 



The cerebro-spinal fluid occupies the subarachnoid 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 fourth 

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

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

 bumin, glucose, and mineral salts. The quantity is estimated from two to 

 four fluidounces. 



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 sixteen to eighteen inches in length; is | 

 of an inch in thickness, weighs i^ ounces,' and extends from the atlas to 

 the second lumbar vertebra, terminating in the filum terminate. It is cylin- 

 dric in shape, and presents an enlargement in the lower cervical and lower 

 dorsal regions, corresponding to the origin of the nerves which are distrib- 

 uted to the upper and lower extremities. The cord is divided into two 

 lateral halves by the anterior and posterior fissures. It is composed of both 

 white or fibrous and gray or vesicular matter, the former occupying the ex- 

 terior of the cord, the latter the interior, where it is arranged in the form of 

 two crescents, one in each lateral half, united by the central mass, the gray 

 commissure; the white matter being united in front by the white commissure. 



Structure of the Gray Matter. The gray matter is arranged in the 

 form of two crescents, united by a commissural band, formng a figure re- 

 sembling the letter H. Each crescent presents a ventral and a dorsal 

 horn. The center of the commissure presents a canal which extends from 

 the fourth ventricle downward to the filum terminale. The ventral horn 

 is short and broad and does not extend to the surface. The dorsal horn 

 is narrow and elongated and extends quite to the surface. It is covered and 



