186 



THE HUMAN MECHANISM 



The hairs, the sweat glands, and the nails are modified 

 portions of the epidermis. Of these the hairs and the sweat 

 glands are of sufficient importance to m^rit some description. 



6. Structure of a hair and a hair follicle. A hair grows from 

 the bottom of a pit, the hair follicle, which extends downward 



into the dermis or even into the 

 subcutaneous tissue. Microscopic 

 examination shows that this fol- 

 licle is lined with a continuation 

 of the epidermis, just as a gland 

 of the stomach or intestine is 

 lined by an ingrowth of the cells 

 of its surface. At the bottom of 

 the follicle is a papilla, and the 

 hair which grows out from this 

 papilla to the surface bears to 

 the cells of the papilla the same 

 relation that the horny layer of 

 the epidermis bears to the similar 

 underlying cells. We accord- 

 ingly find that the hair is com- 

 posed of horny scales closely 

 pressed together into the well- 

 known threadlike structure. 



Opening into the hair follicle, 

 one or more sebaceous glands dis- 

 charge an oily secretion which 

 lubricates the hair and the horny layer of the epidermis, and 

 so prevents drying and chapping (Figs. 84 and 85). 



7. The sweat glands are tubular prolongations of the epi- 

 dermis through the dermis into the subcutaneous tissue. Here 

 the tube becomes much coiled, forming the secreting recess, 

 which is richly supplied with blood vessels and also receives 

 nerves. It is a simple tubular gland formed as an ingrowth 

 from the epidermis (see Figs. 86 and 89). 



FIG. 85. Magnified section of the 



lower portion of a hair and hair 



follicle 



A, membrane of the hair follicle, 

 cells with nuclei and pigmentary 

 granules ; B, external lining of the 

 root sheath; C, internal lining of 

 the root sheath; D, cortical or 

 fibrous portion of the hair shaft; 

 JS, medullary portion (pith) of 

 shaft; F, hair bulb, showing its 

 development from cells from A 



