348 COMPARATIVE ELECTRO-PHYSIOLOGY 



tends to give rise to a responsive reaction, whose sign is 

 opposite to that of excitation expansion instead of con- 

 traction. Now such an increase of internal energy could not 

 fail to be the result of the absorption of the chemically- 

 dissolved food. 



Another interesting consideration to be remembered in 

 connection with digestive organs is that periodically-acting 

 forces give rise to an induced periodicity, which persists for 

 a time, even in the absence of the periodically-exciting cause. 

 A well-known illustration of this is met with in the nycti- 

 tropic movements, so-called, of plants, induced as these are 

 by the periodic variation of night and day. These move- 

 ments persist for a certain length of time, even when the 

 plant is kept in continuous darkness. Similarly, animals 

 accustomed to the supply of food at regular intervals would 

 undoubtedly exhibit alternating phasic changes apparently 

 spontaneous, in the condition of the digestive organ in 

 consequence of the original periodicity of the exciting cause. 

 Such an organ, therefore, must necessarily exhibit periodic 

 electrical variations. 



