950 NUTRITION AND HEAT REGULATION. 



iologists have been solved satisfactorily, but there remain, of course, 

 many more to interest this and succeeding generations. Investi- 

 gations in this field at present are directed mainly to an effort to 

 understand the details of the heat-regulating apparatus, on the one 

 hand, and, on the other, to comprehend more satisfactorily the 

 nature of the process of oxidation. This latter problem is one of 

 common interest at present in chemistry and in physiology. 



The Body Temperature. — We divide animals into the two 

 great classes of warm blooded and cold blooded, according as their 

 temperature is or is not above that of the surrounding air. In 

 this sense, birds and mammals are warm blooded and reptiles, 

 amphibia, and fishes are cold blooded. The names, however, are 

 badly chosen. The difference of deepest significance between the 

 mammals and birds, on the one hand, and the fishes, amphibia, and 

 reptiles, on the other, is that in the former the body temperature 

 is, within wide limits, independent of the outside temperature; it 

 remains practically constant during winter and summer, whether 

 the surrounding air is hotter or cooler than the body. They are, 

 therefore, constant-temperature animals (homoiothermous). The 

 reptiles, amphibia, and fishes, on the contrary, have a body tem- 

 perature that changes with the environment. On winter days 

 their temperature is low, approximately that of the surrounding 

 air or water, and in summer their body temperature rises to cor- 

 respond with that of the outside. Strictly speaking, they are cold 

 blooded only in cold surroundings. This group may be designated 

 as the changeable-temperature animals (poikilothermous). The 

 warm-blooded animals maintain a constant high body temperature 

 on account of their relatively active oxidations and the existence 

 of a heat-regulating mechanism. In the cold-blooded animals the 

 oxidations are not so intense and a heat-regulating mechanism is 

 absent or poorly developed. The hibernating animals form a group 

 intermediate in many ways between these two classes. They possess 

 a heat-regulating apparatus that maintains a constant body tem- 

 perature under most conditions, but breaks down in very cold 

 weather; so that during the period of winter sleep their tem- 

 perature is but little above that of the surrounding air. In 

 some of the cold-blooded animals the production of heat during 

 warm weather is more rapid than its loss; so that they exhibit 

 a body temperature slightly higher than the surrounding medium. 

 A hive of bees in activity may raise the temperature within 

 the hive through a number of degrees, and snakes and many 

 reptiles show a temperature of 2° to 8° C. above that of the 

 air. So also some reptiles possess a rudimentary means of pro- 

 tecting their bodies from too great a rise of temperature, — for 

 instance, by accelerated breathing, whereby more water is evap- 



