MULTICELLULAR ANIMALS 191 



heat are divided into two rather sharply defined groups, the warm- 

 and the cold-blooded animals, or more properly, the poikilother- 

 MAL and the homoiothermal. The body temperature of the cold- 

 blooded animals is therefore very nearly that of their environment. 

 Now the rate of a chemical reaction depends on the temperature, 

 the higher the temperature the more rapid the reaction, the rate 

 being slightly more than doubled for each rise of ten degrees. The 

 general metabolism of the cold-blood animal is therefore at the 

 mercy of the environmental temperature. Hence at low tempera- 

 tures they become quiet, for their metabolic processes transform 

 energy slowly. On the other hand, warm-blooded animals are inde- 

 pendent of the surrounding temperature and their rate of energy 

 transformation is fairly constant. One may with some accuracy 

 think of a warm-blooded animal as producing an excess o£ heat and 

 maintaining a constant temperature by means of varying a release 

 mechanism, while the cold-blooded animals have no heat-retaining 

 power nor any control over their temperatures. Obviously the 

 warm-blooded animals are better equipped to cope with tempera- 

 ture changes in their environment. 



Regulation of Body Temperature in Man. Control of 

 body temperature in the human body is effected by heat-releasing 

 mechanisms that are constantly dissipating heat. Most important of 

 these is the skin, which constantly radiates heat when the environ- 

 mental temperature is lower than that of the body. When the sur- 

 rounding air is warmer, the sudiferous glands excrete sweat that 

 by evaporation tends to lower the temperature. Also under such 

 conditions the walls of the fine capillaries in the corium of the skin 

 expand and more blood is brought near the surface where heat loss 

 may take place. When the air temperature is low these vessels are 

 contracted and thus the heat in the blood is conserved. Moreover, 

 low temperature is said to set up a stimulation by way of the skin 

 that is carried by a nerve mechanism to these glands whose products 

 accelerate oxidative metabolism. 



