40 NEliVK 



the underlying muscles. This is connective tissue, as is the 

 sheath of the muscle. We also see blood tubes running out to 

 the skin. And if we were to look very closely, we might see 

 fine nerve fibers passing from the skin to join the nerve trunk 

 in the legs and body. To skin an animal alive, then, would 

 mean terrible torture ; because all these nerve fibers would be 

 cut or torn across, and every injury to the skin, or to the 

 nerve fibers running inward from it, sends to the brain nerve 

 impulses that, in the normal, live animal, arouse sensations of 

 pain. 



We have already seen that the sciatic nerve is made up of 

 several spinal nerves, and that each of the nerves, just before 

 entering the spinal cord, divides into two roots, one entering 

 the cord nearer the back, the dorsal root ; the other the ven- 

 tral root. 



Nerve Roots and their Functions. Experiments on 

 the lower 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 Sensory root, and the ventral the Motor root. 

 Since the dorsal root always carries currents inward, it is also 

 called the Afferent root, while the ventral root, always carry- 

 ing currents outward, is called the Efferent root- 

 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, notion in the parts 

 whose muscles are supplied by the nerve ; second, sensation, 

 which is referred to the parts of the akin supplied by the 

 branches of the nerve. 



If, instead of simply stimulating the nerve, the nerve is 



