REGULATION OF THE ANIMAL TEMPERATURE. 815 



stimulant to vital processes ; and, even although the exposed parts of 

 the skin may be reduced considerably below their normal temperature, 

 the body, as a whole, does not experience a loss of warmth, but main- 

 tains its natural condition of vitality. It is certain that under these 

 circumstances more heat than usual must be produced from the influ- 

 ence of external cold. 



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

 mined with precision by experimental means. It is plain that the 

 nervous system has its share in the mechanism of the process, perhaps 

 by directly stimulating the molecular changes which produce the evolu- 

 tion of animal heat. There are, however, two sources of heat supply, 

 which evidently play an important part in maintaining the temperature 

 of the body when exposed to cold. 



The first of these is muscular activity. It has been shown that the 

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

 that this quantity is increased by the contraction of the muscular fibres. 

 The total production of heat, therefore, for the whole body, must be 

 considerably augmented when all the voluntary muscles are thrown into 

 a condition of unusual functional activity. Experience shows that this 

 is, in fact, one of the requisite conditions of resistance to cold. The 

 stimulus of the cool air upon the skin excites the desire for active move- 

 ment, and muscular exercise produces a compensating quantity of 

 internal heat. But if the body be exposed to even moderate winter 

 weather without voluntary motion, it must either be protected by an 

 unusual quantity of clothing, or it will soon feel the depressing effect 

 of a loss of its animal heat. 



Secondly, the increased production of warmth, when required, is pro- 

 vided for by an increased supply of food. The materials for the chemical 

 changes requisite for heat-production are supplied directly by the tissues 

 or the blood, but primarily, of course, from the ingredients of the food. 

 Even a recent ingestion of food, as shown in the experiments of Senator, 

 increases perceptibly the amount of heat generated, in the dog, within 

 a given time ; and for longer periods, the influence of an ample or a 

 scanty supply is abundantly manifest. In animals which are scantily 

 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 suffi- 

 cient quantity of food. The immediate effect of a moderate exposure 

 to cold in the healthy condition, is to increase the appetite. A larger 

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

 summer season; and among the inhabitants of northern and arctic 

 regions, the daily consumption of food is much greater than in the 

 temperate and tropical climates. 



It is not necessary to assume that the food, thus required for main- 

 taining a greater heat-production, is directly employed to furnish the 

 necessary warmth by its consumption. The heat is no doubt generated 

 from the activity of all the nutritive changes in the different tissues of 

 the body, and these changes are enabled to continue indefinitely only by 



