NORMAL PROCESSES OF ENERGY METABOLISM 601 



tractions more food and reserve substances are placed in requisition and by 

 this means the deposit of reserve fat in the whole body, and especially 

 in the subcutaneous tissues, is made more difficult, so that the protection 

 against cooling which a thick layer of fat prevents fails in part amongst 

 the very animals which need it most." Even Kassowitz is obliged to 

 admit, however, that "in warm-blooded animals which are in a position 

 to maintain their own body temperature under the most diverse conditions, 

 one can claim the appearance of some justification that their living parts 

 produce heat in order to protect the body against loss by radiation, etc." 



Whether this is a real justification or only the appearance of one will 

 not trouble the practical physiologist so long as the generalization that hu- 

 man beings of different size produce heat in proportion to surface rather 

 than weight, and therefore, require food energy in proportion, helps him 

 to understand his feeding problems; and there is no doubt that the law 

 of surface area has been immensely useful in this connection. It explains 

 the much higher basal metabolism per unit of weight of the small individual 

 in comparison with the large, better than the so-called causal explanation 

 cited by Kassowitz. It explains also much better the need for conservation 

 of heat in the infant, and the role which subcutaneous fat plays in this con- 

 nection. 



3. Heat Production as Affected by External Temperature. a. In 

 Cold-blooded Animals Varit Hoff's Law. Increased activity in living 

 tissues is almost invariably accompanied by an increased evolution of heat. 

 Since this heat is derived from the chemical changes which proceed in the 

 living cells, and since all chemical processes are quickened by a rise of tem- 

 perature, we should expect to find that the heat, produced in the metabolic 

 processes of the organism would tend of itself to quicken these processes. 

 This is found in fact to be the case. In most chemical reactions a rise 

 of 10 C. would increase the velocity of the reaction from two and a half 

 to three times (Van't Hoff's Law), and the same law is, within the limits 

 of the stability of living tissues, found to apply to the process of oxidation. 

 For example, in the early growth of a lupine seedling it has been found 

 that the output of CO 2 bears to the temperature the following relationship : 



C 6 milligrams per hour 



10 C 18 " " " 



20 C 44 " " " 



30 C 86 " " " 



The same relationship has been found to obtain for the production of CO 2 

 in the snail, the leech, and the earthworm. Perhaps the absorption of 

 oxygen is a still better measure of the heat production. Within the range 

 of 5 to 21 C. it has been observed that the f actor (Q 10 ), which in biological 

 literature expresses the number of times the process is accelerated for a 

 rise of 10, has, for the absorption of oxygen by the crayfish, a value of 



