318 HANDBOOK OF PHYSIOLOGY. 



The difference, therefore, between what are commonly called the 

 warm and the cold-blooded animals is not one of absolutely higher or 

 lower temperature ; for the animals which to us in a temperate climate, 

 feel cold (being like the air or water, colder than the surface of our 

 bodies), would in an external temperature of 100 (37.8 C.) have nearly 

 the same temperature and feel hot to us. The real difference is that what 

 we call warm-blooded animals (Birds and Mammalia), have a certain 

 "permanent heat in all atmospheres/' while the temperature of the 

 others, which we call cold-blooded, is " variable with every atmosphere." 

 (Hunter.) 



The power of maintaining a uniform temperature, which Mammalia 

 and Birds possess, is combined with the want of power to endure such 

 changes of body temperature as are harmless to the other classes; and 

 when their power of resisting change of temperature ceases, they suffer 

 serious disturbance or die. 



Sources and Mode of Production of Heat in the Body. Tho 



heat which is produced in the body arises from combustion, and is due 

 to the fact that the oxygen of the atmosphere taken into the system is 

 ultimately combined with carbon and hydrogen, and discharged from 

 the body as carbonic acid and water. Any changes, indeed, which occur 

 in the protoplasm of the tissues, resulting in an exhibition of their func- 

 tion, are attended by the evolution of heat and the formation of carbonic 

 acid and water. The more active the changes, the greater is the heat 

 produced and the greater is the amount of the carbonic acid and water 

 formed. But in order that the protoplasm may perform its function, the 

 waste of its own tissue (destructive metabolism) must be repaired by the 

 due supply of food material and therefore for the production of heat 

 food is necessary. In the tissues, therefore, two processes are continu- 

 ally going on: the building up of the protoplasm from the food (con- 

 structive metabolism), which is not accompanied by the evolution of 

 heat but possibly by the reverse, and the oxidation of the protoplastic 

 materials, resulting in the production of energy, by which heat is pro- 

 duced and carbonic acid and water are evolved. Some heat also is 

 generated in the combination of sulphur and phosphorus with oxygen, 

 but the amount thus produced is but small. 



It is not necessary to assume that the combustion processes, wliich 

 ultimately issue in the production of carbonic acid and water, are as 

 simple as the bare statement of the fact might seem to indicate. But 

 complicated as the various stages may be, the ultimate result is as simple 

 as in ordinary combustion outside of the body, and the products are the 

 same. 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 combination be rapid and direct, as in ordinary combustion, 

 or slow and almost imperceptible, as in the changes which occur in the 

 living body. And since the heat thus arising will be distributed wherever 



