218 



COMPARATIVE ELECTRO-PHYSIOLOGY 



understand why two electrical waves correspond to one 

 mechanical pulsation. First, we know that an excitatory 

 change at a given point will have, as its concomitant, an 

 electro-motive variation of galvanometric negativity. Second, 



the intensity of this electro- 

 motive variation depends 

 on the intensity of the 

 excitatory change. And 

 lastly, on the cessation of 

 excitation there is an elec- 

 trical recovery. 



Now we have seen from 

 the spark-record (fig. 143) 

 that the leaflet during one 

 complete mechanical pul- 

 sation is subjected to two 

 excitatory impulses, occur- 

 ring in the upper and 

 lower halves of its pul- 

 vinule alternately. It is 

 these two excitations 

 which give rise to the two 

 electrical disturbances of 

 galvanometric negativity. 

 And the different ampli- 

 tude and period in the two 

 cases are fully accounted 



a b (upper figure) represents up-movement for b 7 the different period 

 of leaflet ; a b (lower figure) corre- 

 sponding electrical subsidiary wave ; 



FIG. 144. Photographic Records of 

 Simultaneous Mechanical and Elec- 

 trical Pulsation of Desmodium Leaflet 



b a' (upper figure) down-movement 

 of leaflet ; b a' (lower figure) corre- 

 sponding principal electrical wave. 



and intensity of the two 

 excitatory impulses. It 

 will be remembered that 

 the excitatory impulse 

 which produced the up-movement was the feebler and more 

 protracted of the two. It is consequently attended by an 

 electrical disturbance of moderate intensity and correspond- 

 ing persistence. On the cessation of the up-movement, as we 

 have seen, there is a pause, and during this time we find that 



