124 COMPARATIVE ELECTRO-PHYSIOLOGY 



to a physiological reaction, the positive variation being a 

 sign of relatively increased, and the negative of decreased, 

 excitability will be verified by an independent mode of in- 

 quiry, to be described in the following chapter. 



It follows from the experimental demonstration which has 

 just been given that the phenomena of the natural current 

 and its variations may be summarised in general as follows : 



1. Under normal conditions, the current of rest flows in 

 the tissue from the less to the more excitable. In other 

 words, the more excitable is galvanometrically positive to the 

 less excitable. 



2. Increase of internal energy induces an increase or 

 positive variation of the existing current ; and diminution of 

 internal energy induces a negative variation. 



3. External stimulus induces a negative variation of the 

 true or natural current of rest. 



The natural current and its variations under normal con- 

 ditions have now been studied. We shall next proceed to trace 

 out those conditions under which abnormal results may occur. 

 Excessive cooling, by diminishing internal energy, may thus 

 reverse the normal current, which reversal may become more 

 or less persistent. It has been shown, moreover, that in an 

 anisotropic organ, external stimulus gives rise to a current 

 opposite in direction to the natural current. By this excita- 

 tory reaction. the more excitable side, hitherto positive, is 

 rendered negative, and if the excitatory reaction be great, it 

 may remain for a considerable period in this reversed condition 

 of galvanometric negativity. 



We have seen that under normal conditions, the direction 

 of the natural or true current of rest is from the less to the 

 more excitable, and that external stimulus causes a responsive 

 current in the opposite direction, which thus constitutes a 

 negative variation of the current of rest. This state of 

 things we shall distinguish as the primary condition. It 

 frequently happens, however, in consequence of previous 

 stimulation, with its after-effect, that extremely varied effects, 

 appearing at first very anomalous, occur, with regard to the 



