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 

 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 Archivfur Physiologie, 1883, vol. xxx. p. 190. 



