122 PHYSIOLOGY OF MUSCLES AND KERVES. 



point of section. These changes in the excitability 

 may, therefore, be thus conceived : that when the 

 nerve is cut some influence makes itself felt from this 

 cut, and that this first increases the excitability of the 

 nerve, then decreases, and then extinguishes it. If 

 this view is right, we must assume that the high 

 degree of excitability of a freshly cut nerve is also 

 only the result of the incision which is made. This is 

 not, however, exactly the case. The nerve with the 

 muscle of a living frog may be freed and prepared up 

 to the vertebral column without separating it from the 

 dorsal marrow. On irritating the various points in such 

 a nerve, differences, slight indeed but yet observable, 

 are noticed in the excitability, the upper parts being 

 always more excitable than the lower. Uninjured 

 human nerves may also, as we have seen, be irritated 

 at various points in their course, and in this case also 

 it is found that irritation is invariably more easily effec- 

 tive in the upper than in the lower parts. 



Pfliiger, who first called attention to the differences 

 of excitability at the various points of the nerve, thought 

 that the explanation of this is that the irritation evoked 

 at one point in the nerve, in propagating itself along 

 the nerve, gradually increases in strength; he spoke 

 of it as an avalanche-like increase in the excitement 

 within the nerves. This explanation appears to contra- 

 dict the above-mentioned fact as to the effect of cutting 

 on the nerve, for in such cases it appears that the irri- 

 tation is strengthened by the cutting away of the 

 higher portion of the nerve, even though the length of 

 that portion of the nerve which is traversed by the 

 irritation remains unaltered. It must at any rate be 

 admitted that at one and the same point in the nerve 



