THE NATURAL CURRENT AND ITS VARIATIONS. 123 



the natural current in Musa. It will be seen here that in the 

 first stage there was an enhancement, or positive variation, 

 of the existing current. In a later stage, however, this is 

 followed by a reversal, the resting-current now undergoing a 

 diminution. We have here an effect parallel to that of the 

 intermediate dose of sodium carbonate. Vapour of alcohol 

 also exerts an effect very similar ; that is to say, it induces a 

 preliminary exaltation, followed by a depression of the natural 

 current. 



In connection with this subject, of the changes induced 

 in the natural electro-motive difference between the two 

 surfaces, by the action of a chemical reagent, it is well to 

 distinguish between the effects of two different factors : 

 namely, the electrical variation caused by the chemical sub- 

 stance as such, and that brought about by the excitatory 

 reaction. Let us suppose both the electrical contacts to be 

 made on iso-electrical surfaces, with normal saline solution ; 

 there will then be no difference of potential, as between the 

 two. But this state of things will be disturbed, by the appli- 

 cation of another chemical solution, say acid, on either one 

 of the two contacts. The resulting disturbance may be dis- 

 tinguished as due to heterogeneity of chemical application. 

 But if the same chemical agent be applied at both the 

 contacts, no such chemical heterogeneity will ensue. If, 

 then, any electro-motive difference be induced, it must be due 

 primarily to some induced physiological change. The contact 

 which has been rendered more excitable will become increas- 

 ingly positive ; that which is more excited, on the other hand, 

 will become increasingly negative. That the induced electro- 

 motive variation under such circumstances is indicative ot 

 a variation of excitability or excitation, was seen in the fact 

 that the same chemical agent for example, Na 2 CO 3 caused 

 a positive variation when dilute, a negative when strong, and 

 positive followed by negative under the continued action of 

 an intermediate dose. This conclusion that the variation 

 of the existing current, by the simultaneous application 

 at the two contacts of the same chemical reagent, is due 



