392 RESPIRATION AND EXCRETION 



found many capillaries. From the blood in these capillaries, cells 

 lining the wall of the gland take water, and with it a little carbon 

 dioxide, urea, and some salts (common salt among others). This 

 forms the excretion known as sweat. The combined secretions 

 from these glands amount normally to a little over a pint during 

 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 perspiration 

 is greatly increased. 



Relation of Bodily Heat to Work Performed. — The bodily tem- 

 perature of a person engaged in manual labor will be found to be 

 but little higher than the temperature of the same person at rest. 

 When a man works, he releases energy by oxidizing food material 

 or tissue in the body. Thus we know from our previous experi- 

 ments 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? 



Regulation of Heat of the Body. — 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 re- 

 move 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 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 



