164 PHYSIOLOGY OF NERVE [CII. XV. 



Investigation of the Functions of a Nerve. 



There are always two main experiments by which the function 

 of a nerve may be ascertained. The first is section, the second is 

 stimulation. 



Section consists in cutting the nerve and observing the loss of 

 function that ensues. Thus, if a motor nerve is cut, motion of the 

 muscles it supplies can no longer be produced by activity of the 

 nerve-centre; the muscle is paralysed. If a sensory nerve is cut, 

 the result is loss of sensation in the part it comes from. 



Stimulation of the cut nerve is the opposite experiment. When 

 a nerve is cut across, one piece of it is still connected with the brain 

 or spinal cord ; this is called the central end ; the other piece, called 

 the peripheral end, is still connected with some peripheral part of 

 the body. Both the central and the peripheral end should be stimu- 

 lated ; this is usually done by means of induction shocks. In the 

 case of a motor nerve, stimulation of the central end produces no 

 result ; stimulation of the peripheral end produces a nervous impulse 

 which excites the muscles to contract. In the case of a sensory 

 nerve, stimulation of the peripheral end has no result, but stimula- 

 tion of the central end causes a sensation, usually a painful one, and 

 reflex actions, which are the result of the sensation. 



When a nerve is cut across, there are other results than the loss 

 of function just mentioned ; for even though the nerve is still left 

 within the body with a normal supply of blood, it becomes less and 

 less irritable, till at last it ceases to respond to stimuli altogether. 

 This diminution of excitability starts from the point of section and 

 travels to the periphery, but is temporarily preceded by a wave of 

 increased excitability travelling in the same direction (Bitter- Valli 

 law). 



This loss of excitability of nerve is accompanied with degenera- 

 tive changes which are of so great importance as to demand a separate 

 section. 



Degeneration of Nerve. 



Suppose a nerve is cut right across, the piece of the nerve left in 

 connection with the brain or spinal cord remains healthy both in 

 structure and functions ; but the peripheral piece of the nerve loses 

 its functions and undergoes what is generally called, after the dis- 

 coverer of the process, Wallerian degeneration. A nerve is made up 

 of nerve-fibres, and each nerve-fibre is essentially a branch of a nerve- 

 cell ; when the nerve is cut, the axis cylinders in the peripheral 

 portion are separated from the cells of which they are branches and 

 from which they have grown. These separated portions of the axis 

 cylinders die, and the medullary sheath of each undergoes a gradual 



