RESPONSE OF EPITHELIUM AND GLANDS 323 



may be observed when the stimulus is either very feeble or 

 excessively strong. 



We have seen (p. 83) that when the intensity of stimulus 

 is below the critical degree which is sufficient to induce 

 response, its effect is to increase the internal energy of 

 the tissue. We have also seen that the sign of this 

 increased internal energy is galvanometric positivity, being 

 thus opposite to the excitatory effect. Hence, in a dif- 

 ferentially excitable tissue, we may expect to find instances 

 in which stimulus that falls below the threshold of true 

 excitation will act by inducing a greater galvanometric 

 positivity of the more excitable, whereas, under normal 

 intensity of stimulus, the more excitable would have 

 become galvanometrically negative. We can thus see the 

 possibility of response being reversed under very feeble 

 stimulus. 



It must be remembered that the excitability of both 

 the contacts is a factor in the response, which has hitherto 

 been overlooked. A second very important factor, which 

 has not yet been taken into consideration, is the difference 

 between the characteristic curves of the tissues at the 

 two different surfaces. By characteristic curve is here meant 

 the curve which shows the relation between intensity 

 of stimulus and response. This difference will be better 

 understood from the diagram of the theoretical curves given 

 below (fig. 197). This exhibits all the cases that can possibly 

 exist. 



Let the curve A a a' a" represent the characteristic curve 

 of the surface A. Let the curve B b b' b" similarly represent 

 the characteristic curve of the surface B. Of these two 

 surfaces, B is under moderate stimulation, normally the more 

 excitable. In the middle portion of the curve, representing 

 response under moderate intensity of stimulus, the induced 

 galvanometric negativity of B is thus greater than that of A. 

 Under moderate excitation, therefore, the current is b'-*a' 

 through the tissue in the direction from B to A. But below 

 the threshold of true excitation, B would be positive, and A 



