100 MUSCULAR AND NERVOUS ACTION. [BOOK i. 



becomes excessive and violent when a contraction takes place. 

 When rigor mortis sets in, the whole remaining contractile material 

 is decomposed. 



While in muscle the chemical events are so prominent that we 

 cannot help considering a muscular contraction to be essentially a 

 chemical process, with electrical changes as attendant phenomena 

 only, the case is different with nerves. Here the electrical pheno- 

 mena completely overshadow the chemical. Our knowledge of the 

 chemistry of nerves is at present of the scantiest, and the little we 

 know as to the chemical changes of nervous substance is gained by 

 the study of the central nervous organs rather than of the nerves. 

 We find that the irritability of the former is closely dependent on 

 an adequate supply of oxygen, and we may infer from this that in 

 nervous as in muscular substance a metabolism, of in the main an 

 oxidative character, is the real cause of the development of energy; 

 and the axis-cylinder (which is probably the active element of a 

 nerve-fibre, the medulla being useful for its nutrition and protec- 

 tion only,) undoubtedly resembles in many of its chemical features 

 the substance of a muscular fibre. But we have as yet no satis- 

 factory experimental evidence that the passage of a nervous impulse 

 along a nerve is the result, like the contraction of a muscular fibre, 

 of chemical changes, and like it accompanied by an 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. But it must be remembered that 

 the actual energy set free in a nervous impulse is so to speak in- 

 significant, 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 ai e 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 

 occupying the latent period, 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 terminations of the nerve, is funda- 

 mentally a muscle and a nerve besides. 



