82 INDUCTION SHOCKS 
the course of the experiment and so do not enter as 
modifying factors. 
Stimulation of Corresponding Tissues in Different 
Animals. Next in importance are cases- in which it is 
desired to impart comparable stimuli to corresponding 
tissues through a series of experiments. Cases of this 
sort arise frequently in the course of physiological re- 
search, and I have therefore given them special consid- 
eration. 
While this subject was before me there was being car- 
ried on in the laboratory at Harvard an investigation 
which involved, among other things, determining in a 
series of cats the threshold stimulus for producing ex- 
tension of the wrist, when the stimulus was applied to 
the deep branch of the radial nerve below the elbow; 
and reflex flexion of the hind leg through stimulation of 
the tibial. Here was presented a typical example of the 
class of experiments described in the paragraph heading, 
and I therefore utilized it in the study of my problem. 
In several cases the threshold stimulus was determined 
when the tissue only was in the secondary circuit, and 
immediately afterward, the threshold when an additional 
resistance of 10,000 ohms had been introduced. I was 
thus able in these cases to compute the value of the con- 
stant A , and from it to obtain the solution of the equa- 
ZA 
tion for ''specific" irritability, /3 = ~^~~~r' I n the ex- 
