THE AFFERENT PATHS OF SENSORY IMPULSES 



807 



The dorsal roots contain afferent fibers not only from the skin, but 

 from the receptors of deep sensibility located in the muscles and viscera. 

 We will see that the primitive segmentation of the muscles is consider- 

 ably disturbed in the course of development (page 890). The afferent 

 nerves from the muscles enter the cord, however, in the segment from 

 which the muscles originally arose. The same is also true of the viscera. 

 The heart, for example, which arises in the cervical part of the embryo 

 has moved to a lower position, retaining, however, its innervation from 



Fig. 214. Diagram showing the segmental arrangement of the sensory nerves. (From Purves 



Stewart.) 



the cervical segments. Pain from the heart is referred consequently to 

 the neck and arm. 



Because of this separation of the muscular and cutaneous distribution 

 of the fibers composing a single dorsal root', injuries to the peripheral 

 nerves may destroy cutaneous sensibility without affecting deep sensi- 

 bility. This is particularly liable to occur because of the collection to- 

 gether in nerve trunks, especially in the limbs, of fibers from adjacent 

 dorsal roots which have a common (overlapping) distribution. When 

 this occurs the cutaneous sensibility to light touch, two dimensional 

 localization, heat and cold, and the prick of a pin, may be destroyed over 



