OH. xv.] DEGENERATION OF NERVE. 167 



end has no result, but stimulation 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 tem- 

 porarily preceded by a wave of increased excitability travelling in 

 the same direction (Hitter- Valli law). 



This loss of excitability of nerve is accompanied with degen- 

 erative 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 discoverer 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 process of disintegration into droplets 

 of myelin, which are ultimately absorbed and removed by the 

 lymphatics. At the same time there is a multiplication of the 

 nuclei of the primitive sheath. This degenerative process begins 

 one or two days after the section has been made. In the case 

 of the non-medullated fibres, there is no medullary sheath to 

 exhibit the disintegrative changes just alluded to ; and the nuclei 

 of the sheath do not multiply ; there is simply death of the axis 

 . cylinder. The degeneration occurs simultaneously throughout 

 the whole extent of the nerve ; it does not start from the section 

 and travel to the periphery. 



A great amount of attention has been directed to this process 

 of degeneration, because it has formed a valuable method of 

 research in tracing nervous tracts, and ascertaining the nerve-cells 

 from which they originate. It must not, however, be regarded as 

 an isolated phenomenon in physiology ; it is only an illustration 

 of the universal truth that any portion of a cell (in this case 



