CHAPTER XVII. 



THE MAINTENANCE OF THE ANIMAL HEAT. 



As long as the body is alive heat is being produced in 

 it. Even a muscle that may be showing not a bit of con- 

 traction is nevertheless developing a certain amount of heat. 

 Heat is a constant product of the transformation of energy 

 in the body. The most noteworthy fact in examining this 

 heat-production is its steady maintenance under normal 

 conditions. Thus the temperature of an average adult is 

 about 37 l lioC.) and this temperature is maintained through 

 summer, through winter, and through all of the vicissitudes 

 of every-day life with almost mathematical exactness. 



Animals which have such a constant temperature are 

 spoken of as homothermous ; that is, as the etymology of 

 the word shows, of constant temperature. Animals pos- 

 sessing a constant temperature have one which is usually 

 above the temperature of their surroundings. They are 

 warm-blooded. This is true of nearly all the higher ani- 

 mals. As we descend the scale, however, this condition 

 changes, and we find the temperature of the lower animals 

 determined almost wholly by their immediate environment. 

 Their temperature rises or sinks with the temperature of 

 their surroundings. Such animals are called poikilother- 

 mous\ that is, of several temperatures. More frequently 

 such animals are spoken of as cold-blooded animals, but it 

 not infrequently happens that the temperature of such a 

 cold-blooded animal may even be higher than that of a 

 warm-blooded animal, provided the medium in which it 

 lives rises above that temperature. Examples of cold- 

 blooded animals are the fishes, amphibians and reptiles. 

 (376) 



