254 ANIMAL HEAT. 



Active exercise, as already stated, raises the temperature 

 of the body. This may be partly ascribed to the fact, that 

 every muscular contraction is attended by the development 

 of' one or two degrees of heat in the acting muscle ; and 

 that the heat is increased according to the number and 

 rapidity of these contractions, and may be quickly diffused 

 by the blood circulating from the heated muscles. Possibly 

 also, some heat may be generated in the various move- 

 ments, stretchings, and recoilings of the other tissues, as 

 the arteries, whose elastic walls, alternately dilated and 

 contracted, may give out some heat, just as caoutchouc 

 alternately stretched and recoiling becomes hot. But the 

 heat thus developed cannot be great. 



Moreover, the increase of temperature throughout the 

 whole body, produced by active exercise is but small ; the 

 great apparent increase of heat depending, in a great 

 measure, on the increased circulation and quantity of blood, 

 and, therefore, greater heat, in parts of the body (as the 

 skin, and especially the skin of the extremities), which, at 

 the same time that they feel more acutely than others any 

 changes of temperature, are commonly by some degrees 

 colder than organs more centrally situated. 



That the increased temperature of the skin during 

 exercise is not accompanied by corresponding increase of 

 the heat of other parts, which are naturally much warmer, 

 is well shown by some observations of Dr. J. Davy. 



The influence of external coverings for the body must not 

 be unnoticed. In warm-blooded animals, they are always 

 adapted, among other purposes, to the maintenance of 

 uniform temperature ; and man adapts for himself such 

 as are, for the same purpose, fitted to the various climates 

 to which he is exposed. By their means, and by his com- 

 mand over food and fire, perhaps as much as by his capacity 

 for developing appropriate amounts of heat, he maintains 

 his temperature on all accessible parts of the surface of the 

 earth. 



