4U niY.-aoLoiiY or MUSCLES AND M.UYKS. 



they accomplish no work in a mechanical sense. Yet 

 everyone- kno\\> the difficulty of holding a weight long 

 in this position; the sense of weariness which verv 

 soon make.- itself frit, shows that work in ;i phvsiologjeal 

 smse is really done. The kind of work thus accom- 

 plished may be spoken of as the i nh-nml work of the 

 iiiii>ele, as distinguished from the external work accom- 

 plished in the raising of weights. 



5. We must now inquire on what the labour accom- 

 plished by the muscle as a whole depends. We are 

 justified in assuming that here also, as in other cases, 

 the work done does not originate in itself, but comes 

 into existence in consequence of the exercise of some 

 force. On examining a muscle during its active con- 

 dition, we find that chemical processes occur within it 

 which, though the details are not indeed fully known, 

 must, since they are connected with the production 

 of warmth and the evolution of carbonic acid, depend 

 on the oxidation of a portion of the muscle-substance. 

 Thus, the muscle art > like a steam-engine, in which work 

 is accomplished in the same way by the evolution of 

 warmth and the production of carbonic acid. So far all 

 is clear; a portion of the substances of which the 

 muscle is composed is oxidised during its artive state, 

 and the energy released by this chemical process is 

 (lie source of the work accomplished by the muscle. 

 The production of warmth in a muscle can be shown 

 even during a single pupation; but, this production 

 of \\armth is far more noticeable during tetanu- : 

 and as warmth is but another form of motion, we jnav 

 infer from this that the \\lmle force resulting from 



the rheiuieal proccs- i - r, ,\\ \ rH ( ( 1 into warmth during 



letanu-; while during the raiding of a weight at tin; 



