136 



COMPARATIVE PHYSIOLOGY 



the first, if adequate to set up a nervous impulse, for a measur- 

 able interval after the refractory phase leaves the point of 

 excitation in a more excitable state, so that with an appropriate 

 period intervening between the two stimuli a second one 

 of subminimal intensity may become an effective agent of 

 excitation. The time-relations in the case of the frog's sciatic 

 gastrocnemius preparation are represented by Adrian and 

 Keith Lucas, as in Fig. 34. Their possible bearing on the 

 phenomena of inhibition and summation in the central nervous 

 system will be dealt with later. We must first consider the 



•01 -02 



Time since previous stimulus {seconds) 



Fig. 34. — Excitability to second stimulus in the sciatic gastrocnemius 

 preparation of the frog (Adrian and Keith Lucas). 



light they throw on the nature of the local change which 

 precedes the propagated disturbance in nerve. To account 

 for both the refractory period and the supernormal phase we 

 may picture this change as a phenomenon of disintegration ; 

 if excitation involves dissolution of some constituent in the 

 neighbourhood of the electrode, we should expect no further 

 stimulus to have any effect so long as the latter state persists ; 

 and if the disintegrative process is reversible, it is possible 

 to conceive why this period should be followed by one in which, 

 restoration being incomplete, a less potent stimulus is required 

 to reverse the process. 



Further light is shed on the problem by taking into account 



