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THE NATURAL CURRENT AND ITS VARIATIONS 121 



is well known to be modified by the amount of the 

 dose. 



Much light appears to be thrown on this subject, when 

 we consider the electrical reactions of the chemical agents as 

 due to their physiological action. If a drop of hydrochloric 

 acid be applied to the pulvinus of Mimosa, the leaf falls, 

 showing that the more excitable side has undergone a greater 

 excitatory contraction. We have also seen that when a drop 

 of this acid is applied on any tissue in the neighbourhood of, 

 but not directly touching, an electrical contact, it induces an 

 excitatory galvanometric negativity. If now we apply it in 

 solution, say, of 10 per cent, on the two diametrically opposite 

 contacts of Musa, we shall expect that the greater excitatory 

 action induced on the concave side will give rise on that 

 side to a relative galvanometric negativity, resulting in a 

 negative variation of current of rest. On the application of 

 the reagent this is found to be the case, the responsive 

 current flowing in a direction opposite to that of rest : that 

 is to say, it flows from the more excited concave to the less 

 excited convex. 



It is by considering chemical agents from the point of 

 view of their physiological reaction, that we are able to 

 explain their diversity of effects, according to the strength of 

 the dose and the duration of application. We have seen that 

 while a strong stimulus induces the excitatory effect of 

 negativity, a feeble stimulus will bring about the opposite, 

 or positivity. This abnormal positive response, however, by 

 the continued action of moderately feeble stimulus becomes 

 converted into normal negative. Now a chemical substance 

 which in a certain strength acts as an efficient excitatory 

 agent, may, when sufficiently diluted, act as a feeble stimulus, 

 inducing a positive response. If the same agent again were 

 applied in a slightly greater concentration, its immediate 

 effect might be positive, to be succeeded under continued 

 application by the normal negative. 



We might thus expect, using a strong solution of a given 

 chemical reagent, to obtain a negative variation of the current 



