RESPIRATION AND EXCRETION 343 



twenty-four hours. At all times, a small amount of sweat is given 

 off, but this is evaporated or is absorbed by the underwear ; as 

 this passes off unnoticed, it is called insensible perspiration. In 

 hot weather or after hard manual labor the amount of perspira- 

 tion is greatly increased. 



Regulation of Heat of the Body. — The bodily temperature 

 of a person engaged in manual labor will be found to be but little 

 higher than the temperature of the same person at rest. We know 

 from our previous experiments that 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 that the bodily temperature does not differ greatly at 

 such times ? The temperature of the body is largely regulated by 

 means of the activity of the sweat glands. The blood carries 

 much of the heat, liberated in the various parts of the body by 

 the oxidation of food, to the surface of the body, where it is lost 

 in the evaporation of sweat. In hot weather the blood vessels of 

 the skin are dilated ; in cold weather they are made smaller by 

 the action of the nervous system. The blood thus loses water in 

 the skin, the water evaporates, and we are cooled off. The object 

 of increased perspiration, then, is to remove heat from the body. 

 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. 



Sweat Glands under Nervous Control. — The sweat glands, 

 like the other glands in the body, are under the control of the sj^m- 

 pathetic 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 deter- 

 mining, 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 diminishing or increasing the oxidation of the 

 tissues." — Foster and Shore, Physiology. 



Colds and Fevers. — The regulation of blood passing through 

 the blood vessels is under control of the nervous system. If this 

 mechanism is interfered with in any way, the sweat glands may not 



