REVIF.W OF RESPONSE, SIMPLE AND MULTIPLE 715 



The excitability of a tissue which has not recovered fully 

 from previous strong stimulation is found to be impaired. 

 The fatigue-effect only disappears after the lapse of a period 

 of rest. If, then, the resting intervals between successive 

 stimuli be gradually shortened, the motile responses will be 

 found to be progressively diminished, and a time arrives 

 when the succeeding stimulus evokes no response, the tissue 

 having become as it were refractory. The minimum interval 

 during which the tissue remains thus irresponsive is known 

 as the refractory period. In BiopJiytiim, under normal condi- 

 tions, this period is about ten seconds in duration (p. 273). 



Conductivity. — It is usually supposed that the trans- 

 mission of the excitatory effect, as seen in sensitive plants 

 like Mimosa^ is merely the transmission of a hydro- 

 mechanical disturbance, and therefore unlike the transmis- 

 sion of the excitatory effect in animal tissues. It has, 

 however, been shown that this is not the case, for here, as in 

 the animal, transmi.ssion of excitation takes place by the 

 propagation of protoplasmic changes. It was shown further 

 that the hydrostatic disturbance was transmitted with a 

 relatively great rapidity, whereas the true excitatory effect 

 has a slower and definite velocity, characteristic of the 

 particular specimen. This velocity, moreover, is found to be 

 modified, and that in a manner precisely similar, by all those 

 agencies which modify the velocity of transmission of excita- 

 tion in animal tissues. Thus, cold, anesthetics, and fatigue 

 are all influences which reduce the velocity of transmission. 

 As an example of this, we saw, in a certain specimen of 

 Biophytiuii, in which the normal velocity was 3-8 mm. per 

 second, that slight cooling reduced it to 1-3 mm. per second, 

 or almost to one-third. Conversely, the raising of the 

 temperature from 30° C. to 37'' C. increased the velocity from 

 37 to 9-1 mm. per second. Strong stimulus is found to be 

 conducted further and more rapidly than feeble or moderate. 

 It is found in the case of animal tissues, again, that on 

 account of its physiologically depressing effect, the anode 

 acts as a block to the transmission of excitation ; and the 



