ANIMAL HEAT. 271 



actual depression of the temperature of the body is followed by such 

 serious results, and since its warmth is maintained during health at the 

 normal standard, notwithstanding exposure to varying degrees of cold, 

 it is evident that the living organism possesses the power of increasing 

 its internal production of heat, to compensate for greater loss without. 

 It is a matter of common observation, that moderate external cold, if 

 not too long continued, produces a sense of warmth and increased 

 vigor, instead of depression. The atmosphere of a winter's day, or a 

 cold shower-bath, acts as a stimulant to the vital processes ; and 

 although the exposed parts of the skin may be reduced below their 

 normal temperature, the body, as a whole, does not experience a loss 

 of warmth, but maintains its natural condition of vitality. It is 

 certain that in these circumstances more heat must be produced under 

 the influence of external cold. 



The mode in which this result is accomplished has not been deter- 

 mined with precision. It is plain that the nervous system has its 

 share in the process, perhaps by directly stimulating the molecular 

 changes which are active in heat-production. There are, however, two 

 sources of heat-supply, which evidently play an important part in main- 

 taining the temperature of the body under exposure to cold. 



The first of these is muscular actii'itu. It has been shown that the 

 muscles produce a considerable quantity of heat in their own tissue, and 

 that this quantity is increased by muscular contraction. The total pro- 

 duction of heat, therefore, for the whole body, must be augmented 

 when the voluntary muscles are thrown into activity. Experience 

 shows that this is, in fact, one of the requisite conditions of resistance 

 to cold. The stimulus of cool air upon the skin excites the desire for 

 active movement, and muscular exercise produces a compensating 

 quantity of internal heat. But if the body be exposed to even mod- 

 erate winter weather without voluntary motion, it must either be 

 protected by an unusual quantity of clothing, or it will soon feel a 

 depressing effect from the loss of its animal heat. 



Secondly, an increased production of warmth is provided for by 

 increased supply of food. The requisite materials for beat-produc- 

 tion, in the substance of the tissues, are primarily derived from the 

 ingredients of the food. Even a recent in.srestion of food, as shown 

 by Senator, increases perceptibly, in the dog, the amount of heat 

 generated within a given time ; and for longer periods, the influence 

 of an ample or a scanty supply is abundantly manifest. In "animals 

 which are insufficiently fed or ill-nourished, the capacity for resistance 

 to cold is much less than in those which are in good condition and 

 which have received a fair quantity of food. The effect of moderate 

 exposure to cold in the healthy condi " n, is to increase the appetite. 

 A larger quantity of food is habitually taken during the winter than 

 in summer ; and among the inhabitants of northern and arctic re- 

 gions, the daily consumption is greater than in temperate and tropical 

 climates. 



