396 HUMAN PHYSIOLOGY 



called the hair follicle. A hair is formed by the rapid multiplica- 

 tion of the cells covering a little projection of the epidermis at 

 the bottom of the pit. Scattered through the dermis, but always 

 connected with the hair follicles, are tiny oil-secreting glands, the 

 sebaceous glands. The function of the sebaceous gland is to keep 

 the hair and surface of the skin soft. The scales of fishes and 

 snakes, the feathers of birds, and the hoofs and horns of cattle, 

 are all examples of outgrowth from the epidermis. They may 

 then be considered as modifications of hair or nails. 



Structure and Use of Sweat Glands. — Examine the surface of 

 your skin with a lens. Notice the surface is thrown into little ridges. 

 In 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 epider- 

 mis; there, coiling around upon itself several times, it forms the 

 sweat gland. Close around this coiled tube are found many capil- 

 laries. From the blood in these capillaries, cells lining the wall 

 of the gland tube take water, and with it a little carbon dioxide, 

 urea, and some salts (table salt among others). This forms 

 the excretion known as sweat. It has been estimated that there 

 are over two million five hundred thousand sweat glands in the 

 skin of an average-sized person. A combined secretion from these 

 glands amounts normally to a little over a pint during twenty-four 

 hours. The amount secreted varies greatly. At all times, a small 

 amount of sweat is given off, but this is evaporated or is absorbed 

 by the underwear; as it passes off unnoticed it is called insen- 

 sible perspiration. In hot weather or after hard manual labor 

 the amount of perspiration is greatly increased. 



Home or Laboratory Experiment. — Take your temperature by inserting a 

 clinical thermometer in the mouth just under the tongue immediately before 

 and immediately after some hard manual labor. Note the difference in 

 temperature if any. 



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

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

 but little higher than his temperature when at rest. When a 

 man works, he releases energy by oxidizing food material or tissue 

 in the body. Thus we know from our previous experiments that 



