STBIULATION AND TRANSMISSION 291 



interval also appears to be determined by the same 

 conditions. 



SUMMATION 



The phenomenon of summation is of great importance 

 in the analysis of the stimulation process. It shows 

 clearly that a single sul^minimal stimulus produces an 

 effect on the tissue, but that this eiTect is transient; 

 within a certain brief time the tissue resumes the same 

 condition as before the stimulus. "But if before this 

 time has elapsed a second similar stimulus is apj)lied. 

 its effect is added to that of the first, and the critical 

 level of disturbance required to initiate an excitation- 

 wave may be reached. The second stimulus, in order 

 to be effective, must be sent in before the etTect of the 

 first has subsided; and the more rapid the rate of this 

 subsidence the shorter is the summation-interval. The 

 summation-interval is therefore defined as the longest 

 interval separating the successive subminimal stimuli 

 of an effective series of two or more such stimuli.' 



This interval is shorter than the least duration of the 

 exciting current of threshold intensity; and its precise 

 duration varies with the intensity of the subminimal 

 stimuli employed. Lucas gives the following intervals 

 for different frog's tissues at 13°, using two sui)minimal 

 electric stimuli (induction shocks) which were 5 per cent 

 below the strength required for stimulation by single 

 stimuli.^ 



Motor nerve (sciatic) 0004-.0005 sec. 



Muscle (sartorius) 001 1-.0019 sec. 



Ventricle ooS sec. 



^ Cf. Lucas, Journal of Physiology, XXXIX (1910), 46a. 

 ^ Op. cit. (19 10), pp. 466 fl. 



