METABOLISM 



229 



so largely derived from katabolic changes that we place it in that 

 category. 



In respect to their maintenance of bodily warmth, animals are divided 

 into two classes, homotherms and poikilotherms, these terms corre- 

 sponding to the older designations " warm-blooded" and "cold-blooded" 

 animals, respectively. These phrases are scientifically misleading, for 

 a so-called cold-blooded animal may in summer, even in the temperate 

 zone, have a higher temperature than a ' 'warm-blooded" animal in the 

 same place. The terms homothermal and poikilothermal, on the contrary, 

 well represent the physiological conditions. The former in the Greek 

 means "of the same temperature," and indicates that such animals 

 namely, birds and mammals maintain their heat at a relatively constant 

 degree despite changing conditions, sometimes internal as well as external, 

 The term poikilothermal means "of a varied temperature," thus implying 



FIG. 123 



The variation of a poikilotherm temperature with that of its environment (tortoise). 

 With an initial temperature of 13, placed in a temperature of 37, in three hours there was a 

 rise to nearly 33. With an initial temperature of 33.5, placed suddenly in environment at 10, 

 its temperature in three hours had fallen to about 17.5 (broken-line curve). (Richet.) 



that the temperature of such animals is not constant but variable (with 

 the environment). As is usually the case in Nature, the dividing line 

 between the two classes is not absolute, for several animals on the lower 

 margin of mammalia, so to say, are only imperfect homotherms, while a 

 number of other mammals become poikilotherms during their periods 

 of hibernation. On the other hand, one or two of the reptiles, e. g., 

 the python, maintains its temperature 10 or 15 above that of the sur- 

 rounding air, showing a tendency to homothermy. In general, however, 

 the division practically holds that all animals save birds and mammals 

 are poikilothermous. The difference of these two sorts of animals is 

 sufficiently striking. Frogs, for example, are sometimes received in the 

 laboratories in winter with their abdominal fluids obviously frozen, 

 yet placed in tepid water for a few minutes the animals are as lively as 

 ever. Six months later one of the same frogs might have a temperature 



