120 PRINCIPLES OF ANIMAL BIOLOGY 



furnish oxygen, just as one of the principal ends of digestion is to provide 

 foods, for this reciprocal reaction whose object is the release of energy. 



Carbohydrates require less oxygen from outside sources for their 

 combustion, because they furnish some of their own. The carbon of the 

 sugar molecules unites with the oxygen which the same molecules con- 

 tain and with oxygen of respiration. Carbon dioxide, the end product 

 of this combustion, contains little stored energy. Most of the energy 

 residing in the sugar is thus liberated. 



Fats, which are also primarily fuels, are burned in the same way; 

 but since they contain relatively little oxygen, more oxygen of respira- 

 tion is required for their combustion. Again carbon dioxide is the 

 energy-poor end product. As stated in the preceding chapter, fats are 

 not readily burned unless carbohydrates are being oxidized at the same 

 time; the reason for this connection is not known. 



Proteins, which are primarily material for construction, may also be 

 burned. To some extent they are utilized as a normal source of energy, 

 but in times of starvation this use is stepped up markedly. Since 

 proteins are not stored to any extent in animals, combustion of them is 

 at the expense of the body tissues. Animals literally burn themselves 

 at such times. Part of the living organism is being destroyed to main- 

 tain the rest of it. Proteins are intermediate l:>etween fats and carbohy- 

 drates in the amount of outside oxygen they require for their oxidation. 



Heat. — One of the important uses to which energy is put in some 

 animals is the development of heat. This heat comes mostly from 

 oxidations occurring in muscle. If the amount of heat is regulated in 

 some way, so that a fairly constant temperature is maintained, an 

 especially advantageous situation is produced. Many physiological 

 processes bear a time relation to one another, and the speed of most such 

 processes is accelerated by high temperatures and retarded by low ones. 

 If the speeds of various processes are not equally affected, a change of 

 temperature destroys a nice adjustment among them. Hence a con- 

 stant temperature is an advantage. 



Many invertebrate animals have no heat regulation; and, when their 

 muscular movements are slight, as in clams and snails, their temperatures 

 are almost identical with that of othcn* things around them. Such 

 animals are said to be cold-blooded. Among the vertebrates, the fishes, 

 amphibia, and reptiles are all regarded as cold-blooded because their 

 temperatures rise and fall with changes in external temperature; but some, 

 perhaps most, of them have temperatures somewhat above that external 

 to them. 



The higher mammals, including man, are warm-blooded (as are also 

 the birds) and have very marked regulation of temperatiu'e. The tem- 

 perature of the human body in health seldom rises much above 38° or falls 



