38o 



COMPARATIVE ELECTRO-PHYSIOLOGY 



passing off of excitatory effects of preparation. The record 

 of normal suction is then taken by means either of the ordi- 

 nary recorder or of photography. The variation induced in 

 the record after the application of stimulus, then, exhibits the 

 effect of excitation. 



In order to test the results in as many ways as possible, I 

 arranged for three different methods of record. The first 

 was without balance. Here the slope of the suction-curve 

 indicates the normal rate of suction, the enhanced rate under 

 stimulus being indicated by flexure and increased steepness. 



FIG. 224. Terminal 

 Mode of Applica- 

 tion of Stimulus 



FlG. 225. Sub-terminal 

 Mode of Application 

 of Stimulus 



A diminution in the rate of suction, or an expulsive effect, 

 would, on the other hand, be indicated by a corresponding 

 diminution of the slope, or reversal of the curve. 



The second method of record is carried out under exact 

 balance, and is, as already explained, an extremely delicate 

 means of detecting variations in the rate of suction. If this 

 rate be enhanced the curve rises suddenly from the balanced 

 horizontal line, a depression inducing, on the contrary, a 

 downward movement. The last method is that of Over- 

 balance, where, owing to a supply of water from the com- 



