26 PHYSrOLOGY 



the arms. In the region of the loins is the lumbar enlarge- 

 ment, where the nerves are given off to supply the poste 

 rior limbs. The cord is not so long as the cavity of the 

 spinal column, and the space posterior to the cord is occu- 

 pied by the nerves extending to the posterior limbs, and 

 these nerves are given off at a very sharp angle, and con- 

 tinue backward for some distance before they emerge 

 from the cavity of the spinal column. But in the region 

 of the shoulders the nerves spring off at about a right 

 angle with the cord. The outside of the cord is white, but 

 the central portion consists of what is called gray matter. 

 The white portion is made up of fibers, but the gray matter 

 consists of nerve cells as well. 



The Spinal Nerves. These are given off in pairs from 

 the sides of the spinal cord, passing out between the suc- 

 cessive vertebrae. In the regions of the shoulders and 

 loins the spinal nerves are large, as they supply the large 

 muscles of the limbs ; but in the middle of the back, where 

 only the muscles of the body wall are supplied, the nerves 

 are small. We have thirty-one pairs of spinal nerves. 



The Roots of the Spinal -Nerves. Each spinal nerve 

 arises by two roots, one nearer the back, called the dorsal 

 root, the other nearer the ventral surface, the ventral root. 

 These two roots soon unite to form one spinal nerve. 



The Ganglion of the Dorsal Root. On the dorsal 

 root, just before it unites with the ventral root, is a swell- 

 ing, the ganglion of the dorsal root. Like all ganglions, it 

 is largely made up of nerve cells, being a center of con- 

 trol rather than a means of communication. This ganglion 

 appears to control the nutrition of the adjacent nerve fibers, 

 and is not concerned in the process of reflex action. 



