320 PROTOPLASMIC ACTION AND NERVOUS ACTION 



Considering the fact that the potentials as measured 

 must be less than those actually existing in the living 

 tissue — because of the partial cross-circuiting of the 

 current through the fluids of the tissue — it seems probable 

 that in typical active tissues the normal fluctuations of 

 potential in single cells have a range of 50 to 100 milli- 

 volts. The bioelectric currents are thus weak; their 

 intensity, however, is amply sufficient to excite sensitive 

 tissues, as is demonstrated in the laboratory experi- 

 ments in which nerves and muscles are excited by 

 demarcation and action currents ('^rheoscopic frog" 

 experiments) . 



In each special instance the duration, the rate of vari- 

 ation, and the rhythm — in the cases where the variations 

 are rhythmical — exhibit special features characteristic of 

 the tissue and of the species. Comparison of the condi- 

 tions in different tissues and animals reveals the exist- 

 ence of highly significant correlations between the time- 

 relations of the electrical response and of the normal 

 functional response or mode of activity of the tissue. 

 When the bioelectric variation develops rapidly and is of 

 brief duration, the response of the tissue to stimulation is 

 also rapid; e.g., in a muscle the duration of the latent 

 period and of the single twitch is brief, the chronaxie 

 is also brief, and the propagation of the excitation-wave 

 is rapid; the refractory period and the summation- 

 interval are also brief. On the other hand, tissues with 

 slowly developing bioelectric variations exhibit a slower 

 rate of response and a slower subsidence of their activity; 

 the muscular twitch, the chronaxie, the refractory 

 period, and the summation-interval are relatively pro- 

 longed and the transmission is slow. 



