STIMULATION OF MUSCLE AND NERVE 135 



The apparent deviation from the law of polar 

 excitation (cathodal on closure, anodal on open- 

 ing) is explained by the presence of - a physi- 

 ological anode and cathode at each electrode, 

 as already mentioned. The appearance of cath- 

 odal closing contraction before anodal closing 

 contraction is due to the fact that when the 

 negative electrode lies over the nerve the physi- 

 ological cathode will be found on the side of the 

 nerve next the electrode. The nearer the elec- 

 trode, the greater the current density, and hence 

 the earlier the threshold value is reached. When, 

 however, the positive electrode lies over the 

 nerve, the physiological cathode will be found 

 on the side of the nerve farther from the elec- 

 trode, where the density is less, owing to the 

 divergence of the current lines. The threshold 

 value will be reached first at the point of higher 

 density, and consequently the first contraction 

 will appear while the negative electrode rests 

 over the nerve. The anodal opening contraction 

 appears before the cathodal opening contraction 

 for a similar reason. 



Reaction of Degeneration. Whenever a nerve 

 is severed, the portion separated from the cell of 

 origin of the nerve " degenerates." The degener- 

 ation does not begin at the section and advance 

 to the terminal branches, but takes place al- 



