340 



THE SKIN. 



Structure of Hair and Nails. 



Hair. A hair is produced by a peculiar growth and modi- 

 fication of the epidermis. Externally it is covered by a layer 



FIG- 115a. 



FIG. 115. Sebaceous glands of the skin, after Gurlt : or, a, sebaceous glands opening 

 into the follicle of the hair by efferent ducts ; b, a hair on its follicle. 



FIG. 11 5a. Sweat-gland and the commencement of its duct. a. Venous radicles 

 on the wall of the cell in which the gland rests. This vein anastomoses with others 

 in the vicinity, b. Capillaries of the gland separately represented, arising from 

 their arteries, which also anastomose. The bloodvessels are all situated on the out- 

 side or deep surface of the tube, in contact with the basement-membrane. Magn. 

 35 diam. 



of fine scales closely imbricated, or overlapping like the tiles 

 of a house, but with the free edges turned upwards (Fig. 116, 

 A). It is called the cuticle of the hair. Beneath this is a 

 much thicker layer of elongated horny cells, closely packed 

 together so as to resemble a fibrous structure. This, very 

 commonly, in the human subject, occupies the whole of the 

 inside of the hair ; but in some cases there is left a small cen- 

 tral space filled by a substance called the medulla or pith, 

 composed of small collections of irregularly shaped cells, con- 

 taining fat- and pigment-granules. 



The follicle, in which the root of each hair is contained 

 (Fig. 117), forms a tubular depression from the surface of the 



