Till < LBBON B U 551 



Several factors operate to explain these differences, and of the 

 following are of importance: 



1. The Body Temperature, [ncrease in body temperature entails in- 

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

 greater than thai of ;i mammal of the same size, for, as is well known, 

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

 other animals. Rise in bodj temperature also explaii s, in pari at least, 

 the increased metabolism observed in fev< 



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



distinguish between tl ffecl produced on warm-bl led and on <•■ 



blooded animals, since the body temperature of a cold-blooded animal 



is only o] p two degrees Centigrade above thai of its environmenl 



follows thai 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 constanl whatever changes may occur 

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

 dependenl on the fad thai the intensity of the combustion pi 

 inversely proportional to tho cooling eflfed of the atmosphere. Tl 

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

 the body will tend to become greater, and to maintain the body tempe 

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

 the result thai an increased excretion of carbon dioxide and in t 

 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 

 weighl of small as compared with large animals. The argumenl is that, 

 since the sin face of small animals relatively to their mass is much •_ 

 than in large animals, the cooling of the small animals will he |> inn- 

 ately greater. The relationship between surface ami ma-- is Bhown by tak- 

 ing t v ubes ami putting them together; tin- mass of the two cubes 



equal t<> double that <>f either cube, wi irface than 



double, since two aspects of the cuhes ha\e l.eeii broughl I T 



prove the contention, the respiratory exchange has been comput 

 square meter of surface instead of per kilo body weight, with suit 



thai a very elos rrespondence in the metabolism of .i aimals 



has been observed; hut this question has already been disc ssed, and we 



now know that the law of cooling can not he the oiil\ 

 extent of the respiratory exchang< <\\ 



3. Muscular Exercise. This has a most important influ 

 change ami it is particularly in connection with it that studies in 

 dioxide output and oxygen intake have been 



