PRODUCTION OF HEAT. 



237 



is carried, every part of the body will be heated equally, or 

 nearly so. 



This theory, that the maintenance of the temperature of 

 the living body depends on continual chemical change, chiefly 

 by oxidation, of combustible materials existing in the tissues 

 and in the blood, has long been established by the demon- 

 stration that the quantity of carbon and hydrogen which, 

 in a given time, unites in the body with oxygen, is suffi- 

 cient to account for the amount of heat generated in the 

 animal within the same time : an amount capable of main- 

 taining the temperature of the body at from 98 to IOO, 

 notwithstanding a large loss by radiation and evaporation. 



Many things observed in the economy and habits of 

 animals are explicable by this theory, and may here briefly 

 be quoted, although no longer required as additional 

 evidence for its truth. Thus, as a general rule, in the 

 various classes of animals, as well as in individual ex- 

 amples of each class, the quantity of heat generated in 

 the body is in direct proportion to the activity of the 

 respiratory process. The highest animal temperature, for 

 example, is ^ound in birds, in whom the function of 

 respiration is most actively performed. In Mammalia, the 

 process of. respiration is less active, and the average tem- 

 perature of the body less, than in birds. In reptiles, both 

 the respiration and the heat are at a much lower standard ; 

 while in animals below them, in which the function of 

 respiration is at the lowest point, a power of producing 

 heat is, in ordinary circumstances, hardly discernible. 

 Among these lower animals, however, the observations of 

 Mr. Newport supply confirmatory evidence. He shows 

 that the larva, in which the respiratory organs are smaller 

 in comparison with the size of the body, has a lower tem- 

 perature than the perfect insect. Volant insects have the 

 highest temperature, and they have always the largest 

 respiratory organs and breathe the greatest quantity of air ; 

 while among terrestrial insects, those also produce the most 



