34 PHYSIOLOGY. 



muscles are supplied by the nerve ; second, sensation, 

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

 branches of the nerve. 



Effect of Severing a Spinal Nerve. If. instead of simply stimu- 

 lating the nerve, the nerve is severed, the same two effects will he pro- 

 duced. After severing the nerve, if we stimulate the end of the nerve 

 still connected with the limb, we get action of the muscles in that limb. 

 If we stimulate the end of the nerve connected with the body, a sensa- 

 tion will be produced, and this sensation will be referred to the parts 

 from which the nerve fibers arise, probably in the skin of the limb. 



Effect of Stimulating the Ends of Severed Nerve Roots. If we 

 now turn to the roots of the nerve, and make similar experiments, we 

 obtain the following results : Stimulating the dorsal root causes sensa- 

 tion referred to some outer surface, and no other effect is noticed. 

 Cutting the dorsal root also causes sensation. Stimulating the end of 

 this root still connected with the spinal cord causes sensation: but 

 stimulating the end of the root connected with the nerve gives no 

 appreciable result. 



Stimulating or cutting the ventral root causes motion in the parts 

 whose muscles are supplied by fibers from this root. After severing 

 this root, if the end connected with trie spinal cord be stimulated, no 

 effect is noticed ; but stimulating the end still connected with the nerve 

 is followed by shortening of the muscles supplied. 



Effect of Severing All the Spinal Nerves. Severing 

 all the spinal nerves destroys all power of sensation and 

 voluntary motion in all parts of the body except the head. 

 After severing all the dorsal roots, no sensation would be 

 produced by stimulating any part of the body, and after 

 severing all the ventral roots no act of the will can cause 

 any of the muscles of the body to act. Severing all the 

 nerves, or severing all the roots, cuts off all communication 

 of the brain with the body, and so far as motion and sensa- 

 tion in the body generally are concerned, has the same 

 effect as severing the spinal cord below the head. 



