THE SKIN AS AN ORGAN OF EXCRETION 189 



be oxidized, so, combined with other elements into urea and other 

 wastes, it is ehminated through the kidneys. 



The Skin as an Organ of Excretion. — We have already learned 

 that the skin is an organ of protection. Let us now see how it aids 

 in excretion. If you examine the 

 palm of your hand with a lens, you 

 will notice the surface is thrown into 

 little ridges. Between these ridges 

 may be found a large number of 

 very tiny pits ; these are the pores 

 or openings of the sweat-secreting 

 glands. From each opening a little 

 tube penetrates deep within the 

 dermis ; there, coiling around upon 

 itself several times, it forms a sweat 

 gland. Close around this coiled 

 tube are found many capillaries. 

 From the blood in these capillaries 

 cells lining the wall of the gland 

 take water, and with it a little car- 

 bon dioxide, urea, and some salts 



(common salt among others) . These form the excretion known as 

 sweat, or perspiration. 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 evap- 

 orated or is absorbed by the underwear; as this passes off un- 

 noticed, it is called insensible perspiration. In hot weather or after 

 hard manual labor the amount of perspiration is greatly increased. 



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. Muscles, 

 nearly one half the weight of the body, release about five sixths Oi 

 their energy as heat. At all times they are giving up some heat. 

 How is it that the body temperature does not vary greatly at 

 different 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 



Diagram of sweat gland, duct, 

 pore in epidermis. 



and 



