EXCITATORY CHARACTER OF SUCTIONAL RESPONSE 387 



The first part of the record shows the normal down-curve of 

 over-balance. Stimulus of two seconds' duration was now 

 applied, at the place marked in the record with a vertical 

 line. This is seen to induce a growing diminution in the 

 slope of the curve culminating in reversal ; and afterwards, 

 owing to the persistence of the after-effect of stimulus, the 

 record becomes horizontal. A second stimulus of two 

 seconds.' duration was now applied. This is seen to induce a 

 further enhancement of the rate, which is shown by the up- 

 curve. The slope of this curve undergoes a slow decline with 

 the waning of the immediate effect of stimulus. But on 

 account of that component of the stimulus which remains 

 latent in the tissue, there is induced a more or less persistent 

 after-effect, which is greater than the after-effect due to the 

 first stimulus. For while the after-effect of the first stimulus 

 was seen to make the record horizontal, the second after- 

 effect renders the curve slightly ascending. From these 

 and other facts previously enumerated it will be under- 

 stood that the effect ot latent stimulus derived from external 

 sources is to increase suctional activity up to a certain 

 limit. 



Having now described the various effects induced in 

 a slightly sub-tonic tissue, under the simplest mode of 

 stimulation, namely, the terminal, we shall proceed to inquire 

 as to what are the effects induced under a somewhat more com- 

 plex mode of stimulation. This is the case with sub-terminal 

 stimulus, where the point stimulated is not on the external 

 extremity, but within the tissue, though near the lower end. 

 Under these conditions the excitatory wave will proceed in 

 two opposite directions, upwards and downwards. The 

 short terminal zone, however, being close to the directly 

 stimulated area, will be more intensely affected than the 

 extended upper region. For this reason there is likely to 

 be a predominant expulsion of water from the lower end. 

 Under continued strong stimulation, however, the more 

 intensely excited lower zone may become fatigued. The 

 natural upward suction, probably enhanced by the action of 



c c 2 



