90 COMPARATIVE ELECTRO-PHYSIOLOGY 



first, or A, stage of extreme sub-tonicity, we have absorption 

 without excitatory response. By this absorption the sub- 

 stance passes into the next, or B, stage, which is the stage of 

 transition, where response is converted from the abnormal to 

 normal. Above this stage the rate of molecular transforma- 

 tion is very rapid. From the residual after-effect of stimulus, 

 the substance now passes from the stage B to the stage C, 

 which is a condition of more or less stability. Further 

 stimulation carries the substance to stages D and E. Here 

 the molecular distortion from the normal equilibrium is very 

 great. Stimulation applied in this condition has little 

 further effect in inducing response. That is to say, excit- 

 ability is here reduced to a minimum. In this extremely 

 distorted position, moreover, the substance has a strong 

 tendency to revert to the position of normal equilibrium. 



In the A condition of extreme sub-tonicity, since there 

 is absorption without excitation, the response which we 

 obtain is abnormal positive. Intense or long-continued 

 stimulation carries the substance into the B stage, with its 

 normal negative response often preceded by diphasic. An 

 example of this has already been given in fig. 54, obtained 

 from the sub-tonic petiole of cauliflower. We shall meet, 

 however, with numerous other examples in a great variety 

 of tissues. Arriving at the B stage, the substance is still 

 somewhat sub-tonic, and the rate of molecular transformation 

 here is rapid. From the after-effect of stimulus the mole- 

 cules of the somewhat inert substance become incipiently 

 distorted in the same direction as that of normal response. 

 A proportion of the incident stimulus is thus utilised in 

 inducing a favourable molecular disposition. A repetition of 

 the original stimulus will now give rise to a greater excitatory 

 reaction than before. Thus at the B stage we obtain a stair- 

 case increase of response. This fact that by the after-effect 

 of previous stimulation the molecules may be incipiently dis- 

 torted in a direction favourable to excitatory response finds 

 illustration in stimuli individually ineffective being made 

 effective by repetition. The result here is evidently made 



