RESPONSE BY POSITIVE VARIATION 179 



end is rendered relatively the more excitable, and galvano- 

 metrically positive, compared with the inexcitable injured end. 

 In a naturally anisotropic organ, we have a state of things 

 which is analogous. In this case, in the primary condition, 

 the more excitable surface is galvanometrically positive. But 

 under the excitation due to preparation, or accidental dis- 

 turbance, this more excitable surface becomes the more excited, 

 and, relatively to the other, gaivanometrically negative. These 

 varying changes in the direction of the so-called resting 

 current, or current of reference, are the cause of the existing 

 anomalies in the interpretation of response by the positive 

 or negative variation. 



But the direction of the action-current under normal 

 conditions is always the same. On diffuse stimulation it is 

 always from the more excited B to the less excited A. 

 The differential excitability or anisotropy, may be either 

 natural, or artificially induced, as by injuring one end of an 

 isotropic tissue. There are two different conditions under 

 which the normal effect may undergo reversal, those, namely, 

 of great sub-tonicity or excessive fatigue. But the statement 

 that the responsive current is always from the more excited to 

 the less excited, remains universally true. Numerous illustra- 

 tions, in verification of the cases laid down, will be met with 

 in the course of subsequent chapters. 



N 2 



