COORDINATION 77 



pain. Since these results never follow destruction of the 

 ventral roots, we must conclude that impulses enter the cord 

 solely by the dorsal roots precisely as they leave the cord solely 

 by the ventral roots. 



It has been stated above (p. 74) that there is a ganglion 

 on the dorsal root. Microscopic study of this ganglion shows 

 that the fibers of the dorsal root pass through it and that 

 each fiber gives off at right angles to itself a branch which 

 becomes continuous with a pear-shaped nerve cell of the 

 ganglion. These cells have no other processes. We may 

 express the relation between the pear-shaped cells of the 

 ganglion and the fibers of the dorsal root by saying that the 

 single axon from the main cell body divides into two in 

 the ganglion, one branch passing outward to the periphery, 

 the other passing centrally into the spinal cord (Fig. 43). 



7. Endings of the peripheral branches of the neurones of the 

 dorsal root in sense organs. The peripheral branch ultimately 

 ends in some " sense organ," one of the most important of 

 which, so far as the spinal nerves are concerned, is the skin. 

 The eye, the ear, the nose, the mouth, are examples of other 

 sense organs, and they all contain the peripheral endings of 

 afferent neurones. Each is sensitive to some special influence 

 from without, as the eye to light, the ear to sound, etc. ; 

 and when stimulated they start nerve impulses moving in- 

 ward along the nerves toward the brain or cord. 



8. Ending in the spinal cord of the central branch of the 

 neurones of the dorsal root. The other or central branch 

 passes into the spinal cord. It does not, however, like the 

 neurones of the ventral root, there become continuous with 

 the nerve cells of the gray matter, 1 but divides, on entering 

 the cord, into an ascending and a descending branch (Fig. 44), 

 each of which runs for a longer or shorter distance in the 

 white matter of the cord. Indeed, many of the ascending 



1 It is, as has already been pointed out on this page, part of a nerve cell 

 in the ganglion of the dorsal root. 



