CHAPTEE XXX. 



THE PRODUCTION AND REGULATION OF THE HEAT OF 

 THE BODY. 



Cold- and Warm-blooded Animals. All animals, so 

 long as they are alive, are the seat of chemical changes by 

 which heat is liberated; hence all tend to be somewhat 

 warmer than their ordinary surroundings, though the differ- 

 ence may not be noticeable unless the heat production is 

 considerable. A frog or a fish is a little hotter than the air 

 or water m which it lives, but not much; the little heat that 

 it produces is lost, by radiation or conduction, almost at once. 

 Hence such animals have no proper temperature of their own; 

 on a warm day they are warm, on a cold day cold, and are 

 accordingly known as changeable-temperatured (poikilo-tlier- 

 mous) or, in ordinary language, " cold-blooded " animals. 

 Man and other mammals, as well as birds, on the contrary, 

 are the seat of very active chemical changes by which much 

 heat is produced, and so maintain a tolerably uniform tem- 

 perature of their own, much as a fire does whether it be burn- 

 ing in a warm or a cold room; the heat production during 

 any given time balancing the loss, a normal body temperature 

 is maintained, and usually one considerably higher than that 

 of the medium in which they live; such animals are com- 

 monly named " warm-blooded. " This name, however, does 

 not properly express the facts; a lizard basking in the sun 

 on a warm summer's day may be quite as hot as a man usu- 

 ally is; but on the cold day the lizard becomes cold, while 

 the average temperature of the healthy Human Body is, 

 within a degree, the same in winter or summer; within the 

 arctic circle or on the equator. Hence it is better to call 

 such animals " homothermous " or of uniform temperature. 



Moderate warmth accelerates protoplasmic activity; com- 

 pare a frog dormant in the winter with the same animal ac- 

 tive in the warm months: what is true of the whole frog is 

 true of each of its living cells. Its muscles contract more 



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