68 



THE SIGNS OF LIFE 



[LECT. 



for our further experiments, i.e., with its scale to take in a 

 range of about 0.05 volt. 



42. Summation. The positive response of the skin is ordi- 

 narily of considerable duration ; in general, the stronger the 

 stimulus, the longer the response ; five minutes has been a very 

 ordinary duration in these experiments. If, then, a second 

 stimulus is applied at the end of one or two minutes, before 

 the first response has fully subsided, we shall obtain a second 

 response superposed on the first, and so on for a third and 

 fourth and n th response, which give a summation of effects by 

 successive deflection remainders. Here is a record that will 

 make this point clear. 



vott 



o 



mins. 



FlG. 31. Frog's skin. Summating effect of excitation at intervals of 2 mins. 

 (The first response is to a weaker excitation.) 



With stimuli at shorter intervals, the summation is steeper, 

 and the individual effects of which it is composed may be 

 indistinguishable. There is a fusion of deflections analogous 

 with the tetanic fusion of muscular contractions with which we 

 are all familiar. 



By reason of this summation, and of the fact that both 

 directions of induction shock arouse response in one and 

 the same (positive) direction, the rapidly alternating currents 

 of an induction coil as ordinarily used for tetanisation give 

 rise to a much larger (positive) response than does a single 



