v a. 



" 







VARIOUS TYPES OF RESPONSE IO5 



hitherto horizontal base-line. This bifurcation causes the 

 first contractile response of the now growing organ sixth of 

 the series to be smaller than usual. But, as a favourable 

 tonic condition is gradually established by the absorption of 

 energy and the molecular mobility of the responding organ 

 is increased, the contractile response becomes larger, and 

 growth goes on at a certain steady rate. This constitutes 

 an instance in which stimulus, so far from lowering the 

 energy of the responding system, has actually raised it 

 above par. 



It would thus appear that while the theory of assimilation 

 and dissimilation is insufficient for the explanation of the 

 various characteristics of response, the difficulties there en- 



untered are, on the contrary, satisfactorily explained, on 

 taking full account of the influence on response of the 

 molecular condition of the responding substance. From 

 the chemical hypothesis of an explosive molecular change, 

 with its attendant dissimilation and run-down of energy, it 

 would follow that previous stimulation should always induce 

 a depression of the subsequent responses. Instead of this, 

 however, it is found that previous stimulation sometimes 

 exalts, and at other times depresses, the subsequent re- 

 sponses. This apparent anomaly we have seen to be ex- 

 plained by the consideration of molecular transformation. 

 From the sluggish condition A, we have seen tissues trans- 

 formed, by the impact of moderate stimulus, to condition B, 

 with its greater excitability. It is only when the molecular 

 condition has been brought to D or E, that the responses 

 undergo a diminution or reversal. 



The molecular condition, then, undergoes a continuous 



nsformation, in consequence of the action of stimulus, from 

 the extreme of sub-tonicity A to the overstrained molecular 

 conditions D and E. In the A stage, there is no true ex- 

 citatory expression, response to stimulus being here by the 

 abnormal positive variation. The substance is next trans- 

 formed into stage B, where response exhibits a staircase 

 character. In the next stage C, the responses are uniform. 



