670 FOOD AND LABOUR. [Book n. 



from the amount set free as heat, the two forms of energy be- 

 ing so related that an increase of work done is accompanied by 

 a greater or less increase of heat set free, it is obvious that a 

 man who is doing a hard day's muscular work needs a larger 

 income of energy for the day than does an idle man. What we 

 have learnt concerning muscular metabolism further shews us 

 that the additional energy needed is not necessarily to be sup- 

 plied by an increase in the proteid components of the diet ; the 

 energy of muscular contraction does not come as was once 

 thought from proteid metabolism (§ 423). The fact that it is 

 the carbon metabolism which is augmented in muscular work 

 may suggest that the extra food for extra work should be 

 exclusively carbon compounds ; and if, as we have seen to be 

 probable, the carbohydrates are more readily and directly avail- 

 able for the functional metabolism of muscle than are the fats, 

 we might be further led to recommend an increase in carbo- 

 hydrates to form a diet especially suited for labour. This view 

 seems directly supported by the experimental result that even 

 a small quantity of sugar taken by the mouth has an immediate 

 favourable effect on the power of the muscles. But several 

 considerations have to be taken into account in this matter. 

 A muscle is not a machine within the body which can be loaded 

 and tired off irrespective of the rest of the body. In the per- 

 formance of muscular labour, the condition of the muscle, the 

 amount of energy available in the muscle itself, is of course of 

 prime importance ; but, and this perhaps especially holds good 

 in severe labour, of great importance also, we might almost say 

 of no less importance, is as we have urged (§ 317) the power 

 of the body as a whole to avail itself of the energy latent in 

 the muscle. The power of doing work hangs not on the muscle 

 alone, but on the heart, the lungs, the nervous system and 

 indeed on the whole body. It is very doubtful whether we 

 ever, even in supreme efforts, draw upon more than a portion 

 of the capital of energy lodged in the muscle itself ; fatigue is 

 far more a nervous than a muscular condition, and even the 

 distinctly muscular fatigue is as we have seen (§ 81) partly at 

 least the result of the accumulation of products and not alone 

 the using up of available energy. In choosing a diet for mus- 

 cular labour we must have in view not the muscle itself but 

 the whole organism. And though it is possible that future 

 research may suggest minor changes in the various components 

 of a normal diet such as would lessen the strain during labour 

 on this or that part of the body, on the muscles as well as on 

 other organs, our present knowledge would rather lead us to 

 conclude that what is good for the organism in comparative 

 rest is good also for the organism in arduous work, that the 

 diet, normal for the former condition, would need for the latter 

 a limited total increase but no striking change in its composi- 



