26 PHYSIOLOGY CHAP. 



combustion, must be in proportion to the loss of heat as determined 

 by the body- surface. 



This simple explanation will, however, appear inadequate if 

 we examine the phenomenon of the loss of heat more closely. As 

 we shall see in the next chapter, this loss does not depend 

 merely on the relative extent of the surface of the body, but 

 also upon the external temperature, the thickness of the layer 

 of adipose tissue, and the hair or clothing covering the skin. We 

 must also bear in mind that in cold-blooded animals, the tempera- 

 ture of which is variable, the loss of heat is not proportional to 

 the production, and, moreover, in these animals the consumption 

 differs greatly in animals of different weights. 



Voit accounted for this phenomenon by connecting it with 

 the fact, proved by Vierordt, that the blood circulates more 

 rapidly in small animals than in large ones, so that in the former 

 the same portion of the organs is supplied during a given time 

 with a more copious flow of blood than is the case in the latter. 

 Voit considers that the more, quickly the tissues are supplied 

 with blood, the more intense will be the metabolism. This theory 

 has, however, never been proved conclusively, nor does it afford 

 an explanation of the metabolism of cold-blooded animals. 



Hosslin suggested as an explanation of the greater metabolism 

 of small as compared ' with large animals that the former are 

 obliged to move more quickly than the latter in order to escape 

 their enemies and to catch their prey. Now it is easy to prove that 

 the work necessary for locomotion is proportional not to the size 

 of the body, but to the square of its cube root. It has been found 

 by experiment that the consumption per kgrm. of weight in 

 dogs of different sizes traversing the same distance is in inverse 

 proportion to the size of the animal. It follows that even in 

 a state of rest small animals must consume more material per 

 unit of weight. This teleological explanation does not, however, 

 account for the condition in homothermic animals, nor does it 

 hold good for poikilothermic ones. 



We must also remember that age, sex, and constitution exercise 

 an influence on the intensity of the combustion both in man and 

 animals, though this influence is not so great as that of the size 

 of the body. According to Magnus-Levy, the consumption per 

 unit of body -surface in healthy individuals is as follows : 



Males from 20 to 40 years of age 100 



Adult women 113-117 



Young women 115-135 



Boys of 2|- 12 years of age 137-160 



The influence of the constitution is obvious if we compare the 

 consumption of material in thin, muscular persons with that in 

 stout people ; it is much greater in the former than in the latter, 



