1 76 COMPARATIVE ELECTRO-PHYSIOLOGY 



But when A is killed, the current of injury c is B -> A, the 

 responsive current is also B -> A, constituting a positive 

 variation (fig. 117, b). 



Second we take an instance where, owing to some physio- 

 logical difference, an intermediate point A is less excitable 

 than B or B f (fig. 118, a). The primary natural current will 

 here be from the less to the more excitable : that is to say, C will 

 be A-> B' and A -> B. If stimulus be now applied at x on the 

 right, an identical excitatory current R, flowing away from the 

 excited point from right to left, will cause seemingly opposite 

 effects : that is to say, a negative variation of A->B' and a 

 positive variation of A -* B. 



FIG. 118. Typical Cases of Variation of Current of Rest and Action- 

 Current ; intermediate point naturally less or more excitable than 

 either of terminal. 



(a) Intermediate A, less excitable, shown by vertical shading ; current of 

 rest A-B and A->B' ; when right-hand point x excited, action-current 

 R from right to left, gives rise to negative variation of A-B', and 

 positive variation of A-*B. (b) Intermediate B, more excitable, shown 

 by horizontal shading ; current of rest AHB and A'-H ; action-current 

 on excitation at x , from right to left, giving rise to positive variation 

 of A'->B and negative variation of A-B. 



Again, we may have the intermediate point B naturally 

 more excitable than A' or A. The natural current C will 

 be A-B and A'-B (fig. 118, b}. Stimulation at x will now 

 give rise to an excitatory current R, from right to left. The 

 results here will, however, appear to be exactly the reverse 

 of those in the last case : that is to say, an identical current, 

 R, will give rise to a positive variation of A' -> B, and negative 

 variation of A-B. Instances of these effects will be given 

 in Chapter XVIII. 



And, lastly, we may have a typically anisotropic tissue, 

 composed of two halves, which are unequally excitable as, 

 for instance, the upper and lower halves of the pulvinus of 

 Mimosa, or the muscle and gland in a muscle-and-gland 



