The Skin and the Kidneys 



189 



and nerves. It is covered with epidermic cells which mul- 

 tiply rapidly, thus causing the rapid growth of the hair. 



Surmounting each papilla is a bulbous expansion, the hair 

 tyilb, from which the. hair begins to grow. 



n v r 284. How the Hair grows. The cells on the papillae form 

 the hairs. As these cells are pushed upwards by new ones 

 formed beneath, they are compressed, and the shape of the 

 follicle determines the cylindrical form of the shaft of the 

 hair. So closely are these cells welded to form the cylin- 

 der that even under a microscope the hair presents only a 

 fibrous appearance, except in the center, where the cells 

 are larger, forming the medulla, or pith (Fig. 107). 



(LjT 285. What gives the Color to the Hair. The medulla of 

 the hair contains the pigment granules, or coloring matter, 

 which may be of any shade between a light yellow and an 

 intense black. It is this that gives 

 the great variety in color. Generally 

 with old people the pigment is absent, 

 the cells being occupied by air ; hence 

 the hair becomes gray or white. 

 \l 286. The Nails. A nail is simply 

 a thick layer of epidermic cells or 

 horny scales built from the outer part 

 of the scarf skin. It lies upon very 

 fine and closely set papillae, forming 

 its matrix, or bed. It is covered at its 

 base with a fold of the true skin, called 



, . . ' . . . (It will be noticed that there 



its root, from beneath which it seems are only a few orifices O f 



to grow (Figs. 10 1, 102, and 108).. 



J 287. Structure of the Sweat Glands. 



Deep in the substance of the true 



skin, or in the fatty tissue beneath it, are the sweat glands. 



Each gland consists of a single tube with a blind end, coiled 



FIG. 98. Epidermis 

 of the Foot. 



the sweat glands in this 

 region. Magnified 8 

 diameters.) 



