274 PHYSIOLOGY 



currentf required in the same ratio for all durations. If then the change of 

 temperature is such that the two opposite effects are exactly balanced at a 

 certain medium duration of current; it follows that for currents 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 



FIG. 121. Tracing of muscle contractions to show effect of cooling a nerve on 

 its excitability. The lower line indicates the changes in temperature of the 

 excited part of the nerve. The muscle responded only when the nerve was 

 cooled, the stimulus becoming ineffectual when the nerve was warmed. 



(GOTCH.) 



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 histological changes in the nerve, which we shall describe 

 later. 



