256 STUDIES IN PHYSIOLOGY 



From either side of this nerve center pass off numerous 

 bundles of nerve fibers; these are sometimes called nerve 

 trunks or simply nerves. As they approach the different 

 organs of the body they divide into branches, and thus the 

 nerves become smaller and smaller. Finally, the microscope 

 is needed to trace the individual nerve fibers to their end- 

 ings in muscle, gland, or sense organ. By means of these 

 countless nerve fibers all parts of the body are put in com- 

 munication with the nerve centers (see Fig. 115). 



Since the structure and functions of the brain are exceed- 

 ingly intricate and difficult to understand, we will first 

 study the least complicated part of the cerebro-spinal nerve 

 center, namely the spinal cord. 



1. ANATOMY OF THE SPINAL CORD 



Shape and Size. The spinal cord is more or less cylin- 

 drical in shape. Its length in an adult is about a foot and 

 a half. If one measures this distance posteriorly from the 

 base of a man's head, one will find that the cord terminates 

 in the small of the back (near the first lumbar vertebra). 

 Its average diameter from side to side is about three- 

 quarters of an inch. Since its dorsal and ventral surfaces 

 are somewhat flattened, a piece of the cord might be com- 

 pared in its general form and size to one's little finger. 



The spinal cord is not of the same size, however, through- 

 out its whole extent. In the lower neck region its diameter 

 increases considerably ; this is the so-called cer'vi-cal enlarge- 

 ment. A second expansion, the lum'bar enlargement, occurs 

 near its posterior end. These two enlargements are found 

 where collections of nerves run off to the arms and the legs. 

 Posterior to the lumbar enlargement the cord tapers off and 

 ends in a slender thread (see Fig. 121). 



Fissures. Along the ventral surface of the cord runs a 

 deep groove, and a corresponding though much shallower 

 groove furrows the dorsal surface. By these so-called dorsal 



