580 COMPARATIVE ELECTRO-PHYSIOLOGY 



and by the kathode at break. Actinosphcerium, again, shows 

 excitation on make, at both anode and kathode, and on break 

 at the kathode only. 



Having thus demonstrated the fact that an excessively 

 strong E.M.F. induces a reversal of the normal polar effects, 

 it may not appear improbable that there should be a similar 

 reversal of these effects when the intensity of E.M.F. is 

 varied in the opposite direction, that is to say, when it is 

 very weak. I have already drawn attention, in many places, 

 to the importance of this factor of intensity in determining 

 the excitatory effect of a stimulating agent. A chemical 

 reagent, for instance, when administered in moderate or very 

 dilute doses, will induce one effect, say that of exaltation, 

 and, in greater quantities, the very opposite, or depression. 

 A poisonous reagent, again, which usually induces depression, 

 will, if given in sufficiently minute quantities, have the effect 

 of exaltation. These reversals, under varying intensities of 

 the external agent, are noticeable again in different physico- 

 chemical phenomena. Thus it is well known that in the 

 formation of the photographic image, while a moderate 

 intensity of light gives us the normal ' negative/ a stronger 

 intensity will produce a * positive,' and a still more intense 

 light, bring about a re-reversal. We may thus have a series 

 of recurrent reversals. 



Returning, then, to the question of polar action on 

 excitability, we find that the typical results of Pfliiger with 

 nerve and muscle preparations, were obtained when using 

 a moderately strong E.M.F. In this, which is sometimes 

 distinguished as the third stage, excitatory contraction of the 

 muscle is induced, only on the closure of the descending 

 current, or opening of the ascending. In the former case, 

 the kathode is nearest the muscle, and as there is no inter- 

 mediate block the excitation is clearly due to the make-action 

 of the kathode. In the second case, similarly, the break of 

 the anode, which is now near the muscle, causes excitation. 

 With very weak E.M.F., however that is to say, in his first 

 stage Pfliiger found that excitation took place by the make 



