BODY TEMPERATURE 411 



the temperature rises again to a point which will permit the 

 organs to perform their functions. 



840. Birds and Mammals present a special adaptation in 

 this regard in the comparatively high and constant temperature 

 maintained by the body. This is done by the expenditure of a 

 considerable amount of energy, specifically for keeping the 

 body warm, and for this reason Birds and Mammals require 

 considerably more food than do other animals of corresponding 

 size. The temperature of the body is kept constant by a 

 control mechanism by which the amount of heat lost is con- 

 trolled. Much heat is lost by evaporation of moisture from 

 the lungs and respiratory passages and from the mouth cavity 

 also when the animal is panting. Panting greatly increases 

 the amount of vaporization and the accompanying loss of heat. 

 The feathers and hair, when lying close, prevent the loss of 

 heat, but they may be raised on end by the action of special 

 muscles in the skin. This permits free circulation of air and 

 increases the loss of heat by radiation, convection and vaporiza- 

 tion. The skin of Mammals is provided with numerous tubu- 

 lar glands which discharge their secretion on the surface. This 

 secretion is chiefly water, which evaporates from the surface as 

 rapidly as it is formed, or may accumulate in small droplets of 

 perspiration. The function of the sweat glands seems to be 

 primarily temperature control. 



841. The heat of the body is distributed by the blood. The 

 amount of blood brought to the surface, and there cooled, is 

 regulated by the expansion and contraction of the blood vessels 

 of the skin. The supply of blood to the skin and the activ- 

 ity of the sweat glands are both controlled through nervous 

 stimulation. 



842. Some Mammals have in a measure reverted to the primi- 

 tive condition of variable temperature. Bears and many 

 others pass a considerable portion of the winter in a deep sleep, 

 during which the temperature of the body falls to a low point 



