42 PHYSIOLOGY OF MUSCLES AND NERVES. 



they accomplisli no work in a mechanical sense. Yet 

 everyone knows the difficulty of holding a weight long 

 in this position ; the sense of weariness which very 

 soon makes itself felt shows that work in a physiological 

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

 plished may be spoken of as the internal work of the 

 muscle, 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 durirtg its active con- 

 dition, we find that chemical processes occur witLin 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 acts 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 active state, 

 and the energy released by this chemical process is 

 the source of the work accomplished by the muscle. 

 The production of warmth in a muscle can be shown 

 even during a single pulsation; but this production 

 of warmth is far more noticeable during tetanus ; 

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

 infer from this that the whole force resulting from 

 the chemicnl process is converted into warmth during 

 tetanus ; while during the raising of a weight at the 



