98 THE CHEMISTRY OF THE FARM. 



circulation, respiration, and other vital processes; such 

 work is carried on even when the animal is at rest. In 

 man the whole of the blood is pumped through the heart 

 every half minute. The daily work performed by the 

 heart of an average man has been calculated as equal to 

 150 — 200 foot-tons ; that is to say, the power exerted by 

 the heart would raise 1 ton to the height of 150—200 feet. 

 The work performed by other organs, and by the muscles 

 when merely maintaining the body in an erect position, 

 has not yet been satisfactorily measured. 



It was formerly supposed that muscular force was 

 produced by the oxidation of the muscle, and that a diet 

 rich in albuminoids was consequently necessary if hard 

 labour was to be maintained. This idea is now known to 

 be erroneous, it having been shown by repeated experi- 

 ments that labour does not necessarily increase the pro- 

 duction of urea, while it does in every case greatly 

 augment the amount of carbonic acid and water exhaled. 

 Mechanical power is, in fact, produced not by oxidation 

 of the muscle, but of the organic matter in circulation ; 

 this organic matter may be indifferently either fat, carbo- 

 hydrates, or albuminoids. The animal body thus obtains 

 the power necessary for the performance of work in the 

 same manner as a steam engine, only that in the body 

 food is burnt in the place of coal. 



When labour is demanded from an under-fed animal, 

 the oxidation taking place in the circulatory system may 

 be in excess of the food supplied, and of the fat and carbo- 

 hydrates in circulation ; in such a case the albuminoids 

 of the animal body are oxidised, and the excretion of urea 

 becomes increased. A working animal ill supplied with 

 food will thus suffer seriously in condition. 



