COLDS AND FEVERS 415 



urea, and some salts (common salt among others). The com- 

 bined secretions from these glands amount normally to a little 

 over a pint during twenty-four hours. At all times a small amount 

 ' of perspiration is given off, but this is evaporated or is absorbed 

 by the underwear. Since this passes off unnoticed, it is called 

 insensible perspiration. 



Regulation of the heat of the body. The body 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. The 

 muscles, equal to 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. The temperature of the body is largely 

 regulated by 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, and as the water evaporates, 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, per- 

 spiration is increased; with a small amount, it is diminished. 

 Hence, we have in the skin an automatic regulator of body tem- 

 perature. 



Practical Exercise 8. Why is the amount of perspiration noticeably in- 

 creased in hot weather and after doing hard work? 



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, as for example through 

 bacterial toxins released in the body, the sweat glands may not 

 do their work, perspiration may be stopped, and the heat from 

 oxidation held within the body. The body temperature goes up, 

 and a fever results. 



If the blood vessels in the skin are suddenly cooled when full of 

 blood, they contract and send the blood elsewhere. As a result an 

 increase of blood in the internal organs or a congestion may follow. 



