FUNCTIONS OF NERVE HOOTS. 41 



severed, the same two effects will be produced. We have 

 seen that if we now stimulate the end of the nerve still con- 

 nected 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 sensation 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. 



If we now turn to the roots of the nerve, and try similar 

 experiments, we get the following results : Stimulating the 

 dorsal root causes sensation 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 con- 

 nected with the spinal cord causes sensation ; but stimulating 

 the end of the root connected with the nerve gives no appre- 

 ciable 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 the 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. 



Severing all the spinal nerves destroys all power of sensa- 

 tion 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 

 and the body, and so far as motion and sensation in the body 

 generally are concerned, has the same effect as severing the 

 spinal cord below the head. 



The parts essential to reflex action of the spinal cord 

 are: 



