ANIMAL HEAT. 591 



tion, this being due to several factors : (1) A more abundant supply of blood 

 may be accompanied by increased metabolic activity. (2) Increased circulatory 

 activity is favorable to increased heat-dissipation by causing a larger supply 

 of blood to the skin, thus facilitating loss by radiation and indirectly tending to 

 increase thermogenesis. (3) Increased circulatory activity also excites the respi- 

 ratory movements and the secretion of sweat, thus increasing heat-loss and in- 

 directly favoring heat-production. (4) The more active the circulation the 

 larger the amount of heat produced by the heart and the movement of the 

 blood. The diurnal fluctuations of the pulse-rate are said to be more or less 

 closely related to similar changes of body temperature. 



A rise of internal temperature (bodily temperature) is favorable to increased 1 

 metabolic activity (p. 540) and, therefore, to an increase of heat-production ; 

 conversely, a fall of bodily temperature reduces heat-production. The influ- 

 ences of bodily temperature are, as a whole, less important than those of ex- 

 ternal temperature. 



The influences of external temperature are in a measure different upon homo- 

 thermous and poikilothermous animals. In the former, heat-production is in 

 inverse relation to the temperature of the surrounding medium, so that the 

 cooler the ambient temperature the greater the heat-production ; in the latter 

 heat-production increases with an increase of external temperature, because 

 with the rise of the latter bodily temperature increases, which in turn increases 

 metabolic activity (pp. 540, 541). Consequently, in warm-blooded animals heat- 

 production is greater in cold climates and seasons than in the opposite conditions, 

 while in cold-blooded animals the opposite is the case. Cold applied to the skin 

 increases heat-production by reflexly exciting muscular activity (shivering, etc., 

 p. 541) ; moderate heat exerts the opposite influence unless the bodily tem- 

 perature is affected, as shown by the results of studies of respiration (p. 541). 

 The character of the food is important. Danilewsky 1 has estimated that the 

 following quantities of heat are produced under different diets, etc. : 



On a minimum diet 1800 kilogramdegrees. 



On a reduced diet (absolute rest) 1989 



On a non-nitrogenous diet 2480 



On a mixed diet (moderate work) 3210 



On an abundant diet (hard work) 3646 



On an abundant diet (very laborious work) 3780 



The influence of the quantity and quality of the diet must be potent when 

 it is remembered that 1 gram of proteid yields about 4937 calories, 1 gram of 

 fat about 9312 calories, and 1 gram of carbohydrate about 4116 calories. In 

 cold climates fats enter very largely into the diet because of the greater loss 

 of heat and the consequent increased demand for heat-producing substances. 



During the periods of digestion more heat is produced than during the in- 

 tervals, this increase being due chiefly to the muscular activity of the intestinal 

 walls (p. 540). Langlois' experiments indicate that during digestion heat- 

 production may be increased 35 to 40 per cent. 



1 Pfliiger's Archivfiir Physiologic, 1883, vol. xxx. p. 190. 



