Page:Popular Science Monthly Volume 46.djvu/114

104 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 magnifying 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 capillary 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 exertion 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 substance, 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 delicate 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.