OF THE ANIMAL BODY. 21 



It is evident that the supply of the heat lost by 

 cooling is effected by the mutual action of the 

 elements of the food and the inspired oxygen, which 

 combine together. To make use of a familiar, but 

 not on that account a less just illustration, the 

 animal body acts, in this respect, as a furnace, 

 which we supply with fuel. It signifies nothing 

 what intermediate forms food may assume, what 

 changes it may undergo in the body, the last 

 change is uniformly the conversion of its carbon 

 into carbonic acid, and of its hydrogen into water ; 

 the unassimilated nitrogen of the food, along A\ith 

 the unburned or unoxidised carbon, is expelled in 

 the urine or in the solid excrements. In order to 

 keep up in the furnace a constant temperature, we 

 must vary the supply of fuel according to the exter- 

 nal temperature, that is, according to the supply of 

 oxygen. 



In the animal body the food is the fuel ; with a 

 proper supply of oxygen we obtain the heat given 

 out during its oxidation or combustion. In winter, 

 when we take exercise in a cold atmosphere, and 

 when consequently the amount of inspired oxygen 

 increases, the necessity for food containing carbon 

 and hydrogen increases in the same ratio ; and by 

 gratifying the appetite thus excited, we obtain the 

 most efficient protection against the most piercing 

 cold. A starving man is soon frozen to death ; and 

 every one knows that the animals of prey in the arctic 

 regions far exceed in voracity those of the torrid zone, 



