ANIMAL AND VEGETABLE 77 



correspond to the gutta-percha covering of the telegraph 

 wire, but in the case of the telegraph wire as in the nerve- 

 fibre no electrically isolating membrane really exists ; all 

 their parts conduct electricity, and conduction is merely a 

 matter of degree. A*substance which will not conduct low 

 tension may be an excellent conductor of high-tension 

 electricity, and there is an enormous difference between the 

 human electromotive force of four or five millivolts and 

 the voltage of an induction shock. 



As regards electric processes occurring within a nerve 

 we have in a nerve the process of intra-cellular action, 

 which does not take place in a wire in the same way or to 

 anything like the same extent, even if it occurs at all. 

 There are many points of similarity between nerve- 

 circuits and telegraph-circuits, but the two are not identical. 



In regard to inhibition it is at least conceivable that by 

 the action of certain ganglion-cells an opposing E.M.F. is 

 set up in or communicated to a nerve-fibre or fibres so as 

 to produce a lessening of action or diminution of impulse. 

 It is known that " an impulse will in some cases travel 

 both ways." This would necessarily occur in a circuit in 

 which there was inductive capacity, and a mere cursory 

 examination of such physiological diagrams as show the 

 direction given to nerve impulses by different combinations 

 of ganglion-cells in sensory and motor paths should 

 sufficiently convince the student that such action does 

 occur. 



Macdonald reduces the phenomenon of nervous con- 

 duction to electrolytic dissociation and association of 

 inorganic ions, but I fail to see how this can be caused 

 by potassium salts in organic combination within the axis 

 cylinder, as suggested by him, though some such action 

 may occur within a cell. A more reasonable explanation 

 is electrical action set up between oxygen and some 

 eiement electro-positive to it in the cell contents. 



