EXCRETION 397 



heat is released. Muscles, nearly one half the weight of the body, 

 release about five sixths of their energy as heat. At all times, 

 they are giving up some heat. How is it, then, that the bodily 

 temperature does not differ greatly at such times? 



Regulation of Heat of the Body. — The temperature of the body 

 is largely regulated by means of the activity of the sweat glands. 

 If a large amount of perspiration is produced, it takes a correspond- 

 ingly large amount of heat to evaporate it. Thus it is that when we 

 perspire, we are cooler afterward. The object of increased per- 

 spiration, then, is to remove heat from the body. In hot weather, 

 the blood vessels of the skin are dilated ; in cold weather, they are 

 made smaller by the action of the nei'vous system. 



With a large amount of blood present in the skin, perspiration 

 is increased; with a small amount, it is diminished. Hence, we 

 have in the skin an automatic regulator of bodily temperature. 

 Bodily temperature is also helped to be kept at a constant level, be- 

 cause the amount of heat produced in the body is, to some extent, 

 regulated by the temperature outside of the body. Cold air 

 stimulates the body to produce more heat. A high temperature 

 outside the body retards its heat production. 



Sweat Glands under Nervous Control. — The sweat glands, like the 

 other glands in the body, are under the control of the sympathetic nervous 

 system. Frequently the nerves dilate the blood vessels of the skin, thus 

 helping the sweat glands to secrete, by giving them more blood. 



"Thus regulation is carried out by the nervous system determining, on 

 the one hand, the loss by governing the supply of blood to the skin and the 

 action of the sweat glands; and on the other, the production by diminish- 

 ing or increasing the oxidation of the tissues." — Foster and Shore, Physi- 

 ology} 



Comparison with Cold-blooded Animals. — We have seen the bodily 

 temperature of a frog remain nearly that of the surrounding medium. 

 Fishes, all amphibious animals, and reptiles are alike in this respect. This 



* "Some warm-blooded animals, as bears, hiherjiate, that is, sleep all through 

 the winter and take no food. They feed well in the warm weather, and are quite 

 fat at the close of autumn, when they seek some sheltered place to winter in. This 

 shelter and their warm, furry coats make the loss of heat very little; the aniraal, 

 except for its breathing and the beat of its heart, hardly ever moves during the 

 winter, and even those necessary movements are reduced to the fewest possible, 

 the breathing and heart beat being much slower than during the summer. With 

 return of warm weather the creature wakes up again, but is then lean and weak, 

 having burnt up its fat and part of its muscles during its winter sleep." — MartiNj 

 The Human Body. 



