590 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Young animals produce more heat, weight for weight, than the mature. This 

 is owing chiefly to the greater activity of the metabolic processes in the former, 

 and in part to the relatively larger body surface, young animals generally 

 being smaller than the matured and thus having, in proportion to body-weight, 

 larger radiating surfaces. 



Heat-production is more active in the robust than in the weak, other con- 

 ditions being the same. 



The weight of the body is obviously a most important factor in relation to 

 the quantity of heat produced, especially as regards the weight of the active 

 tissues in relation to inactive structures such as bone, sinew, and cartilage. 

 Two animals of the same weight may produce very different quantities of 

 heat per diem, other things being equal. Thus, a fleshy animal should 

 naturally be expected to produce more heat than one with very little flesh and 

 an abundance of fat, which is an inactive heat-producing structure. While, 

 therefore, the relation of heat-production to body-weight does not seem to be 

 definite, yet the experiments by Reichert l and by Carter 2 indicate that heat- 

 production bears, broadly speaking, a direct relation to body-weight. 



Heat-production is greater relatively in homothermous than in poikilother- 

 mous animals ; it varies materially in intensity in different species, especially in 

 warm-blooded animals ; and it is closely related to the intensity of respiration. 

 Moreover, it is probable that each species, and even each individual of the 

 species, has its own specific thermogenic coefficient, that is, a mean standard of 

 heat-production for each kilogram of body-weight or for each square centime- 

 ter of body-surface. The following figures giving the heat-production per 

 kilogram per hour, compiled by Munk, 3 are of interest both as regards species 

 and size and weight of the animal in relation to heat-production : 



Horse 1.3 kilograradegrees. 



Man 1.5 " 



Child (7 kilograms) . . 3.2 



Dog (30 " ) . . 1.7 



Dog (3 " ) . . 3.8 



Guinea-pig 7.5 " 



Duck ........ 6.0 kilogramdegrees. 



Pigeon 10.1 



Eat . 11.3 



Mouse 19.0 " ' 



Sparrow 35.5 " 



Greenfinch . . 35.7 



These figures have an additional interest when compared with the respira- 

 tory activity of different species (p. 537). The intensity of respiration has a 

 marked significance both in connection with the species and the individual. 

 The larger the quantity of oxygen consumed the greater relatively is the 

 activity of oxidation processes, and, consequently, the more active is heat-pro- 

 duction (see p. 537). Therefore, all circumstances which affect respiratory 

 activity tend to affect thermogenesis. The intensity of respiratory activity and 

 the extent of body-surface in relation to body- weight are closely related (p. 

 538). 



Increased activity of the circulation is favorable to increased heat-produc- 



1 University Medical Magazine, 1890, vol. 2, p. 225. 



2 Journal of Nervous and Mental Diseases, 1890, vol. 17, p. 782. 



3 Phydologie des Menschen und der Saugethiere, 1892, p. 302. 



