CHAPTER XXIX. 



THE PEODUCTION AND KEGULATION 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 difference may 

 not be noticeable unless the heat production is considerable. 

 A frog or a fish is a little hotter than the air or water in 

 which it lives, but not much; the little heat that it pro- 

 duces 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 cliangeaUe-temperatured 

 (poikilo-thermous) 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 temperature of their own, much as a fire does 

 whether it be burning in a warm 'or a cold room; the heat 

 production during any given time balancing t K e loss a nor* 

 mal body temperature is maintained, and usually one con- 

 siderably higher than that of the medium in which they 

 live; such animals are therefore known as animals of con- 

 stant temperature (liomo-tliermous), or more commonly 

 " warm-blooded" animals. The latter 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 

 usually is; but on the cold day the lizard becomes cold, 

 while the average temperature of the healthy Human Body 



