182 HUMAN PHYSIOLOGY. 



line, possesses a salty taste and has a low specific gravity ; it is 

 composed largely of water, traces of albumin, 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 insuffi- 

 ciency of blood to the brain. 



SPINAL CORD. 



The spinal cord varies from sixteen to eighteen inches in length ; 

 is Y-Z of an inch in thickness, weighs i l / 2 ounces, and extends from 

 the atlas to the second lumbar vertebra, terminating in the filum 

 terminate. It is cylindric 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 by the 

 anterior and posterior fissures. It is composed of both zvhite 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, forming 

 a figure resembling the letter H. Each crescent presents an anterior 

 and a posterior horn. The center of the commissure presents a canal 

 which extends from the fourth ventricle downward to the filum 

 terminale. The anterior horn is short and broad and does not extend 

 to the surface. The posterior horn is narrow and elongated and 

 extends quite to the surface. It is covered and capped by the 

 substantia gelatinosa. The gray matter consists primarily of a 

 framework of fine connective tissue, supporting blood-vessels, lym- 

 phatics, medullated and non-medullated nerve-fibers, and groups 

 of nerve-cells. 



The nerve-cells are arranged in groups, which extend for some 

 distance throughout the cord, forming columns more or less continu- 

 ous. The first group is situated in the anterior horn, the cells of 

 which are large, multipolar, and connected with the anterior roots 



