SEC. 6. THE ENERGY OF MUSCLE AND NERYE, AND 

 THE NATURE OF MUSCULAR AND NERYOUS ACTION. 



87. We may briefly recapitulate some of the chief results 

 arrived at in the preceding pages as follows. 



A muscular contraction itself is essentially a translocation of 

 molecules, a change of form not of bulk. We cannot however say 

 anything definite as to the nature of this translocation or as to 

 the way in which it is brought about. For instance, we cannot 

 satisfactorily explain the connection between the striation of a mus- 

 cular fibre and a muscular contraction. Nearly all rapidly contract- 

 ing muscles are striated, and we must suppose that the striation is 

 of some use ; but it is not essential to the carrying out of a 

 contraction, for as we shall see the contraction of a non-striated 

 muscle is fundamentally the same as that of a striated muscle. But 

 whatever be the exact way in which the translocation is effected, it 

 is in some way or other the result of a chemical change, of an 

 explosive decomposition of certain parts of the muscle substance. 

 The energy which is expended in the mechanical work done by the 

 muscle has its source in the energy latent in the muscle substance 

 and set free by that explosion. Concerning the nature of that ex- 

 plosion we only know at present that it results in the production 

 of carbonic acid and in an increase of the acid reaction, and that 

 heat is set free as well as the specific muscular energy. There is 

 a general parallelism between the extent of metabolism taking 

 place and the amount of energy set free; the greater the de- 

 velopment of carbonic acid, the larger is the contraction and the 

 higher the temperature. 



It is important to remember that, as we have already urged, 

 relaxation, the return to the original length, is an essential 

 part of the whole contraction no less than the shortening itself. 

 It is true that the return to the original length is assisted by the 

 stretching exerted by the load, and in the case of muscles within 

 the living body is secured by the action of antagonistic muscles or 



