74 



THE HUMAN MECHANISM 



Dorsal 



Dorsal Root 



Dorsal 

 Ganglion 



ventral and dorsal nerve roots 1 respectively. On the dorsal 

 nerve root, some distance from the cord, there is a slight 

 enlargement, or ganglion. Just outside this ganglion the 

 two roots unite, and from their union nerves pass to the 

 skin, the muscles, the blood vessels, the viscera, etc. 



The spinal cord itself in cross section shows a darker 

 central core, known as the gray matter, surrounded by an 

 outer lighter portion, the white matter. The white matter 



consists essentially of 

 nerve fibers which run 

 lengthwise of the cord 

 and here and there send 

 branches into the gray 

 matter; it may be re- 

 garded as a large nerve. 

 The gray matter, on the 

 other hand, contains a 

 mesh of fibers and, in ad- 

 dition, numerous nerve 

 cells. There is the same 

 difference everywhere 

 between the white and 

 gray matter of the nerv- 

 ous system ; the arrange- 

 ment in the brain is not 

 so simple as in the cord, but here also the white matter con- 

 sists of fibers running from one part of the nervous system 

 to another, while the masses of gray matter always include 

 collections of nerve cells. 



5. Fibers of the ventral, or anterior, nerve root. These 

 fibers may be traced into the spinal cord. It is then found 

 that the nerve cells from which they arise lie in the gray 

 matter in the immediate neighborhood of the root to which 



1 The older anatomical terms and those even to-day more generally used 

 are "anterior" and "posterior," instead of "ventral" and "dorsal." 



Dorsal 



Dorsal Root 



Ganglion 



FIG. 42. The origin of the dorsal and 



ventral nerve roots of a segment of the 



spinal cord 



