PRODUCTION OF HEAT. 193 



from the air in the luags, of the disposal of which no account 

 can be given except by regarding it as combining, for the 

 most part, with the excess of carbon and hydrogen, and being 

 excreted in the form of carbonic acid and water. In other 

 words, the blood of warm-blooded animals appears to be always 

 receiving from the digestive canal and the lungs more carbon, 

 hydrogen, and oxygen than are consumed in the repair of the 

 tissues, and to be always emitting carbonic acid and water, for 

 which there is no other known source than the combination of 

 these elements. 1 By such combination, heat is continually 

 produced in the animal body. The same amount of heat will 

 be evolved in the union of any given quantities of carbon and 

 oxygen, and of hydrogen and oxygen, whether the combina- 

 tion be rapid and evident, as in ordinary combustion, or slow 

 and imperceptible, as in the changes which occur in the living 

 body. And since the heat thus arising will be generated wher- 

 ever the blood 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 demonstration 

 that the quantity of carbon and hydrogen which, in a given 

 time, unites in the body with oxygen, is sufficient to account 

 for the amount of heat generated in the animal within the 

 same time ; an amount capable of maintaining the temperature 

 of the body at from 98 to 100, 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 ani- 

 mals, as well as in individual examples 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 found in birds, in whom the func- 

 tion 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 



1 Some heat will also be generated in the combination of sulphur 

 and phosphorus with oxygen, to which reference has been made 

 (p. 177) ; but the amount thus produced is but small. 



