104 THE POPULAR SCIENCE MONTHLY. 



thicker in the palm of the hand and sole of the foot, where most 

 resistance is needed. When we look at the skin of the hand, we 

 notice delicate grooves in it, which, examined through a magnify- 

 ing glass, are seen to be pierced with small orifices ; and if the 

 hand be warm, minute shining drops of perspiration will be seen 

 issuing from them. 



The glands for the secretion of the perspiration are set in the 

 lower side of the inner skin and are in connection with the capil- 

 lary network of blood-vessels which cover the surface of the 

 body. The gland or duct which conducts the perspiration to the 

 surface of the skin is about a quarter of an inch in length, and is 

 straight in the true skin, but becomes spiral while traversing the 

 outer skin. Over thirty-five hundred of these small ducts have 

 been found to exist in a single square inch of the skin, and it has 

 been computed that the aggregate length of the sudoriferous ducts 

 in the body of an ordinary-sized man is about twenty-eight miles. 

 These little glands and ducts perform the important function of 

 throwing off the moisture produced during the combustion of 

 waste tissue by the blood-borne oxygen of the body, and secrete 

 about twenty- three ounces of perspiration in the twenty- four 

 hours, which under ordinary conditions evaporates, without our 

 noticing it, into the air, but under conditions of considerable ex- 

 ertion or unusual heat accumulates as beads of perspiration. 



The throwing off of the perspiration and its evaporation on 

 the skin is a beautiful natural contrivance for regulating the 

 temperature of the body, as the conversion of the perspiration 

 into vapor renders latent an enormous amount of heat, which, 

 being principally derived from the body, keeps it in a comparative 

 state of coolness even when subjected to high temperatures. 



In the twenty-three ounces of liquid so secreted in the course of 

 the twenty-four hours there will be found rather more than an 

 ounce of solid matter, which is left when the liquid portion of the 

 perspiration evaporates, and tends to clog the pores of the skin, 

 and it is the removal of this by the morning tub and rough towel 

 which is responsible for a considerable portion of the refreshing 

 influence of the bath. 



Besides these sudoriferous glands, however, there is a second 

 set, called the sebaceous glands, the ducts of which are spiral, and 

 open generally into little pits, out of which the fine hairs which 

 stud the skin grow, and these glands secrete an oily or waxy sub- 

 stance, which nourishes the hair, and also keeps the outer skin 

 smooth and pliant. This waxy substance is developed in largest 

 quantity inside the ear, where it serves to protect the more deli- 

 cate portions of that organ ; and, next to the ear, these glands are 

 found most abundantly on the face and other portions of the 

 body which are exposed to external influences and friction. 



