20 



HUMAN BIOLOGY 



FIG. 23. A, DEVELOPMENT OF 

 SWEAT GLAND; B, SWEAT 

 TUBE DEVELOPED. 



The oil glands empty into the hair follicles (colored Fig. I ). 

 They form an oil from the blood that keeps the hair glossy 



A and the surface of tJie skin soft 



t and flexible by preventing ex- 

 cessive drying. Hair oil should 

 never be used upon the hair, as 

 the oil soon becomes rancid, and 

 besides causes dust and dirt to 

 stick to the hair. 



The sweat glands (Fig. 23), 

 like the hair bulbs, are deep in 

 the lowest part of the dermis. 

 A sweat gland Jias the form of a 

 tube coiled into a ^//(colored Fig. i). This tube continues 

 as a duct through the two layers of skin, and its opening 

 at the surface is called a. pore (Fig. 24). The perspiration 

 evaporates as fast as it flows out through the pores, if the 

 secretion is slow ; but if poured out rapidly, it gathers into 

 drops (Exp. 3). The perspiration is chiefly water, contain- 

 ing in solution several salts, including 

 common salt and a trace of a white, 

 crystalline substance called urea. The 

 material for the perspiration is fur- 

 nished by the blood flowing around 

 the gland in a network of fine tubes. 

 The amount of the perspiration is con- 

 trolled in two ways : by nerves that 

 regulate the activity of the epithelial 

 cells lining the gland, and by nerves 

 that regulate the size of the blood ves- 

 sels supplying the gland (Fig. 25). 



FIG. 24. PORES on 

 ridges in palm of hand. 



THOUGHT QUESTIONS. Freckles, Warts, Moles, Scars, Proud Flesh, 

 Pimples, Blackheads. Use these names in the proper places below : 



