CHAP. IL] THE CONTRACTILE TISSUES. 157 



evolution of heat. On the other hand, the electric phenomena are so 

 prominent that some have been tempted to regard a nervous impulse 

 as essentially an electrical change; and this view is supported by 

 the facts mentioned above ( 86) as to the nerve not being 

 fatigued by work. But it must be remembered that the actual 

 energy set free in a nervous impulse is, so to speak, insignificant, 

 so that chemical changes too slight to be recognized by the means 

 at present at our disposal would amply suffice to provide all the 

 energy set free. On the other hand, the rate of transmission of a 

 nervous impulse, putting aside other features, is alone sufficient 

 to prove that it is something quite different from an ordinary 

 electric current. 



The curious disposition of the end-plates, and their remarkable 

 analogy with the electric organs which are found in certain animals, 

 has suggested the view that the passage of a nervous impulse from 

 the nerve fibre into the muscular substance is of the nature of an 

 electric discharge. But these matters are too difficult and too 

 abstruse to be discussed here. 



It may however be worth while to remind the reader that in 

 every contraction of a muscular fibre, the actual change of form is 

 preceded by invisible changes propagated all over the fibre, and 

 that these changes resemble in their features the nervous impulse 

 of which they are, so to speak, the continuation rather than the 

 contraction of which they are the forerunners and to which they 

 give rise. So that a muscle, even putting aside the visible termi- 

 nations of the nerve, is fundamentally a muscle and a nerve 

 besides. 



