CH. XLIIL] 



HEAT PRODUCTION 



637 



Heat Production. 



(1) Effect of Changes of External Temperature. In theory there is 

 a fundamental difference between cold- and warm-blooded animals in 

 their reactions to external temperature. A cold environment, since 

 it lowers the temperature of the poikilothermic creature, reduces the 

 metabolism of all its tissues, and thus reduces its heat production. 



The warm-blooded individual reacts in precisely the opposite way. 

 Since his temperature remains constant, his heat production increases, 

 in order to neutralise the effect of his cold surroundings. This has 

 been demonstrated in the case of fasting dogs. An example may be 

 given. 



In practice it is doubtful whether any such exact relation can be 

 discerned in man, as it may be masked by other factors. We have 

 already insisted upon the equality between the respective energy 

 values of the food eaten and of the heat produced, and upon the 

 advantage of an ample diet. In practice it is the amount of food 

 taken which controls the heat production, rather than the reverse. 

 The majority of well-to-do people, whose appetite is stimulated by 

 their palate, maintain a constant body-temperature by regulating 

 the loss rather than the production of heat. In this connection the 

 following figures, derived from observations made upon a dog who 

 was fed upon considerable quantities of meat, may be compared with 

 those obtained when the same animal was fasting. 



In the fasting dog a lowering of the surrounding temperature, 

 increases heat production in the animal; in the well-fed dog this 

 is hardly noticeable. 



On the other hand, it is instructive to note the types of food eaten 

 by the natives of different climates. The Indian, who eats rice yets 



