PEODUCTION OF WOEK 177 



perature of the body in fact rises during labour, and 

 perspiration is increased. 



It was formerly supposed that muscular force was 

 produced by the oxidation of the nitrogenous con- 

 stituents of the muscle, and that a diet very rich in 

 albuminoids was necessary if hard labour was to be 

 maintained. This idea is now known to be erroneous, 

 it having been shown by repeated experiments that 

 labour does not necessarily increase the production of 

 urea, while it does in every case greatly augment the 

 amount of carbonic acid exhaled. 



The energy which produces work is generated by the 

 oxidation of organic matter in the muscular tissues ; 

 this organic matter may be derived from albuminoids, 

 but it far more usually consists of carbohydrates or 

 fat.^ Wolff found that it was indifferent whether the 

 digestible substance supplied to a horse consisted of 

 starch (3 lbs. per day were employed in one experi- 

 ment), or of linseed oil, or of the mixed constituents 

 occurring in ordinary horse foods ; the labour value of 

 the food was determined in every case solely by its 

 heat-producing 'power. The animal body has been 

 compared to a steam engine, in which food is burnt in 

 place of coal. The proportion of the generated energy 

 which finally appears as mechanical work is, however, 



^ For the sake of simplicity the function of glycogen in the animal 

 body has not been discussed. Glycogen is a carbohydrate formed in 

 the animal body which acts as a reserve of energy ; during rest it is 

 stored up in the muscle, it disappears during labour. Glycogen can 

 be formed in the animal from the carbohydrates of the food and from 

 albuminoids. We must not hastily conclude that glycogen, or the 

 sugar derived from it, is the only substance oxidised to produce 

 muscular energy ; all we can assert is that it takes an important part 

 in this work. 



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