METABOLISM IN ANIMALS OF DIFFERENT SIZE 



117 



is for a time independent of the external temperature. But since, as Voit 

 showed most clearly, the human body does undoubtedly react against the 

 influence of a falling temperature by a more active production of heat, the 

 conclusion we are to draw from the experiments just mentioned is rather 

 that visible muscular movements constitute the real cause of increased 

 metabolism. 



7. METABOLISM IN ANIMALS OF DIFFERENT SIZE 



It is evident that, other things being equal, the total metabolism must 

 vary with the mass of the body. But if, in animals of different size the 

 metabolism be calculated according to the unit of weight, it proves to be 

 relatively greater in small animals than in large ones. 



This proposition may best be tested on fasting animals because the metab- 

 olism in them is constant, not being influenced by the kind and quantity of 

 food. The following table, with the exception of experiment number one, is 

 compiled from Rubner: 



It is fairly evident that some variations between the different species of ani- 

 mals occur, probably because the covering of the body and some other bodily 

 characters are not the same in all species. With this exception, however, the 

 table shows a very uniform relationship, the cause of which will be apparent from 

 the following considerations. 



The smaller an animal is, the greater is its superficial area in proportion to 

 its volume and weight. Suppose we have two balls, the one, A, 2 cm., the other, B, 

 4 cm. in diameter, the surface of A is then 12.56 sq. cm., that of B 50.24 sq. cm. ; 

 their volumes are 4.18 and 33.49 cc. respectively. In the smaller ball the ratio 

 of volume to surface is as 1 : 3, in the larger as 1 : 1 .5. Now the animal body 

 loses its heat very largely (about four-fifths) through the skin, and the quan- 

 tity of heat thus given off is evidently proportional to the skin surface. In 

 order that the temperature may remain constant, the production of heat i. e., 

 the metabolism must also be proportional to the skin surface. If, therefore, 



1 [It has been shown that when the rabbit's ears are excluded this animal forms no 

 exception. ED.] 



