DETECTION OF LATENT STIMULUS 465 



would have culminated in the restoration of the original rate 

 of growth ; but we actually find, on the contrary, that, 

 immediately following the responsive retardation, there is 

 an acceleration of growth above the normal, and the true 

 recovery, or restoration of the normal rate, takes place only 

 after this. Thus there is here, instead of a run down, an 

 actual increase, of the energy of the system. 



Again, from this constancy of the sum of the immediate 

 and the after-effects of stimulus, we can see that, as in an 

 inorganic system, so also in a living organism, the law of the 

 Conservation of Energy holds good. For while at the 

 optimum point the entire stimulus finds expression in direct 

 response (stimulus being here equal to response), below the 

 optimum the direct response is less than the stimulus, the 

 missing fraction being left to find expression in the negative 

 after-effect. 



Relation between stimulus and response in different 

 tonic conditions. — The experiments which I have described 

 were carried out under the thermal form of stimulation, which 

 is, as already explained, the most satisfactory in practice. 

 We shall hereafter come across instances of the after-effect of 

 accelerated growth, as caused by stimulus of light. And a 

 similar after-effect has already been seen to be caused by 

 electrical stimulus (p. 436). But for the clear demonstration 

 of this particular effect, electrical stimulus is not very suit- 

 able, inasmuch as it is apt to induce fatigue and, through 

 electrical polarisation, a certain amount of tissue-change. I 

 shall, however, describe an experiment, using this mode of 

 stimulation, which, by its responsive indications, will demon- 

 strate certain differences in the internal conditions of tissues, 

 according as they are sub- or super-tonic. 



I have shown that the general excitability of the tissue at 

 30° C. is superficially the same as that at 2)7° C. or thereabouts ; 

 that is to say, the direct responses at these two temperatures 

 are in some cases approximately the same ; but we have seen 

 that there is a difference of molecular condition at these two 

 points, for at 30" C. the tissue is capable of holding a portion 



II H 



