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COMPARATIVE ELECTRO-PHYSIOLOGY 



relatively negative to it. Hence there would here be 

 a reversal of response, the direction of the responsive current 

 a^b through the tissue being now from A to B. This current 

 will be recorded by the galvanometer, provided the induced 

 difference between A and B be sufficiently great. 



Having thus inferred the different effects possible under 

 sub-minimal and moderate stimuli, we shall next consider 

 the differential effect which may sometimes be induced by 

 excessively strong stimulus. In the middle part of the curve, 



FIG. 197. Possible Variations of Responsive Current, as between Two 

 Surfaces A and B, shown by Means of Diagrammatic Representations of 

 Characteristic Curves 



A, a, a', a", characteristic curve of surface A ; B, b, b', b", that of B. 

 Under moderate stimulation, B is the more excitable, its induced 

 galvanometric negativity being greater, and the direction of current 

 from b 1 to a' , as in the middle part of the curve. Under sub- minimal 

 and super-maximal stimulation the direction of the responsive current 

 is reversed to a - b and a" -> b" respectively. 



B b b' b" is seen to be very much steeper than A a a' a" ; 

 that is to say, the excitatory effect increases very rapidly 

 with the stimulus, in the more excitable of the two surfaces. 

 But this increase may sooner or later reach a limit, that curve 

 tending to become horizontal, aided in this process, possibly, 

 by growing fatigue. The curve A a a' a", however, though 

 not so steep, may yet continue to rise throughout a longer 

 abscissa, representing increasing intensity of stimulus. In 

 such a case, there would be a second crossing-point, and 



