NERVOUS SYSTEM. 33 



anything comes quickly toward the eye, dodging, jumping 

 when suddenly touched by anything hot or when pricked 

 by a pin, but also the adjustments of the essential processes 

 of life, circulation, respiration, and digestion, are brought 

 about through reflex action. 



Destination of Nerve Fibers. The sciatic nerve is 

 composed of many fibers. If this nerve is traced outward, 

 it is found to be continually subdividing, and sending small 

 branches to the muscles, and finally in the muscles one 

 fine nerve fiber goes to each muscle fiber. (See Fig. 13.) 

 Many fibers go on past the muscles to the skin. We can 

 feel in any part of the skin, and we can tell just where we 

 are touched. These fibers from the skin, then, carry 

 nerve impulses inward, as those going to the muscles 

 carry impulses outward. 



Nerve Roots and their Functions. Observations 

 made on animals, and accidents in the case of man, show 

 that all the fibers of the nerves that carry currents to the 

 muscles pass out from the spinal cord into the ventral 

 root, and that all the fibers that carry currents inward 

 enter the spinal cord through the dorsal root. Hence, the 

 dorsal root is often called the afferent root, and the ventral 

 the efferent root. Since ingoing impulses produce sensa- 

 tion, the dorsal root is called the sensory root, while the 

 ventral root, carrying currents outward to produce motion, 

 is called the motor root. 



Effect of Stimulating a Spinal Nerve. Experiments 

 have shown that if, in an uninjured animal, a nerve, or 

 more properly a nerve trunk, as the sciatic nerve, be 

 stimulated, for instance, by a suitable electric shock, two 

 effects are produced : first, motion in the parts whose 



