586 METABOLISM 



Several factors operate to explain these differences, and of these the 

 following are of importance: 



1. The Body Temperature. Increase in body temperature entails in- 

 creased combustion. This explains why the metabolism of a bird is 

 greater than that of a mammal of the same size, for, as is well known, the 

 temperature of a bird is two or three degrees centigrade above that of 

 other animals. Rise in body temperature also explains, in part at least, 

 the increased metabolism observed in fever. 



2. The Temperature of the Environment. In considering this we must 

 distinguish between the effect produced on warm-blooded and on cold- 

 blooded animals. Since the body temperature of a cold-blooded animal 

 is only one or two degrees Centigrade above that of its environment, it 

 follows that the metabolic activity will be directly proportional to the 

 temperature of the latter. In a warm-blooded animal, on the other hand, 

 the body temperature remains constant whatever changes may occur 

 in that of the environment, this constancy of body temperature being 

 dependent on the fact that the intensity of the combustion processes is 

 inversely proportional to the cooling effect of the atmosphere. Thus, 

 suppose the external temperature should fall, then the loss of heat from 

 the body will tend to become greater, and to maintain the body tempera- 

 ture at a constant level, the body furnaces must burn more briskly, with 

 the result that an increased excretion of carbon dioxide and intake of 

 oxygen will occur. 



This influence of the surrounding atmosphere on the metabolic activ- 

 ity of warm-blooded animals has, as already pointed out, been used by 

 several investigators to explain the greater combustion per kilo body 

 weight of small as compared with large animals. The argument is that, 

 since the surface of small animals relatively to their mass is much greater 

 than in large animals, the cooling of the small animals will be proportion- 

 ately greater. The relationship between surface and mass is shown by tak- 

 ing two cubes and putting them together; the mass of the two cubes is 

 equal to double that of either cube, whereas the surface is less than 

 double, since two aspects of the cubes have been brought together. To 

 prove the contention, the respiratory exchange has been computed per 

 square meter of surface instead of per kilo body weight, with the result 

 that a very close correspondence in the metabolism of different animals 

 has been observed ; but this question has already been discussed, and we 

 now know that the law of cooling can not be the only one that determines 

 the extent of the respiratory exchange (see page 577). 



3. Muscular Exercise. This has a most important influence on the ex- 

 change and it is particularly in connection with it that studies in carbon- 

 dioxide output and oxygen intake have been of great practical value, par- 



