THE SKIN 



239 



they are partly closed so that the blood is kept inside 

 the body, one feels cold. 



The larger part of the heat of the body is produced in the 

 muscles when they contract, though a great deal seems to 

 >e formed in certain glands like the liver. This heat warms 

 ;he blood flowing through these organs. Then the blood 

 goes to the cooler parts of the body, warming them in much 

 the same way as hot water warms the different rooms of a 

 house, as it goes to them through pipes from a heater. The 

 blood thus distributes, but does not produce heat. 



Cold and Warm-Blooded Animals. The cold-blooded an- 

 imals frogs, alligators, lizards etc. are never much warmer 

 than the air in which they live. While oxidation of course 

 takes place in them as in other animals, and heat is thus 

 produced, there seems to be no special regulation of body 

 heat, and they have a very low temperature on a cold day 

 and a high temperature on a hot day when lying in the sun. 

 They are apt to be sluggish at any time, but are sure to be so in 

 cold weather. 



In warm-blooded animals, i. e. birds and mammals, the 

 amount of heat produced is always great, and the temperature 

 of the body does not change with the temperature of the air 

 but remains constant if the animal is in uniform health. 

 Such animals are called warm-blooded because their blood is 

 generally warmer than the air, although on a hot summer 

 day it may be cooler than the air. To maintain a constant, 

 high temperature a considerable amount of heat must be 

 produced and consequently a large amount of food must 

 be oxidized. Warm-blooded animals thus demand much 

 more food than cold-blooded. A turtle's activities are so 

 slight that the little energy stored in its body is sufficient to 

 keep it alive for six months without food while a warm-blooded 

 animal must have a large supply of food, and can live only 

 a comparatively short time if deprived of it. 



The Regulation of Temperature. The temperature of the 



