ELECTRICAL STIMULATION 309 



of longer duration the net result will be to reduce the current required 

 for excitation, while for currents of shorter duration the net result 

 will be to increase the current required. The effect of temperature 

 therefore on the minimum exciting current will vary from tissue to 

 tissue according as the two factors, rate of subsidence of excitatory 

 change and the initiation of a propagated disturbance as a result of the 

 excitatory change, are relatively affected by change of temperature. 



THE EFFECT OF INJURY. The irritability of the nerve of a 

 muscle-nerve preparation is not equal in all parts of its course, but 

 is greater at the upper end, probably in consequence of the proximity 

 of the cross-section. 



Some time after a motor nerve is divided the increased irritability 

 at the upper end gives way to a decreased irritability, and this decrease 

 goes on till the nerve is no longer excitable. The diminution in 

 excitability gradually extends down the nerve fibre, so that the part 

 of the nerve nearest the muscle remains excitable the longest. This 

 progressive change in the irritability of a nerve after section is spoken 

 of as the Hitter- Valli law. It is soon followed by definite } istological 

 changes in the nerve, which we shall describe later. 



