THE MOLECULAR THEORY OF EXCITATION 



589 



or to new conditions of alignment, it is clear that contraction 

 must be brought about by a one-directioned, and expansion 

 by the opposite-directioned, change. This is evident in the 

 present case, since the polar stimulating agents are opposite 

 in their characters, and the opposition of their effects must 

 correspond to this. Now it is necessary to distinguish these 

 two responsive effects by opposite terms, which must needs 

 be somewhat arbitrary. 

 The contractile effect has 

 thus been taken as the 

 normal excitatory and 

 negative. Having once 

 adopted such a nomencla- 

 ture, it is of course im- 

 portant that it should be 

 strictly adhered to. Thus, 

 if contraction be the nor- 

 mal response, then any- 

 thing which tends to 

 enhance it must be re- 

 garded as excitatory, and 

 anything which opposes or 

 retards it as depressing. 

 This word ' depressing ' is, 

 however, unfortunate, since 

 by it might be indicated a 



permanent depreciation of the tissue, while diminution of 

 the normal response is possible without such depreciation. 

 Moderate rise of temperature, for example, with its 

 expansive tendency, will lessen the contractile response 

 without necessarily depreciating the tissue (p. 187). Revert- 

 ing once more to the kathodic mode of stimulation, we 

 know that a certain intensity of kathode is necessary, for 

 the visible initiation of contractile response. 1 Should the 

 intensity employed be just short of this, there will be an 



FIG. 360. Polar Effects of Currents due 

 to Localised Application on Upper 

 Half of Pulvinus of Erylhrina indica 



Up-curve represents expansion and con- 

 vexity. Down-curve represents con- 

 traction and concavity. Continuous 

 curve represents the action at make. 

 The dotted curve shows the effect at 

 break. A.m convexity induced at 

 anode-make. A& responsive con- 

 cavity at anode-break. Km = induced 

 concavity at kathode-make. K = 

 expansion induced at kathode-break. 

 The time-marks represent minutes. 



1 These kathodic and anodic effects refer to the normal moderate range of 

 E.M.F, within which Pfliiger's Law is applicable. 



