THE KIDNEYS AND SKIN. 445 



these get pushed up by fresh ones formed beneath them, the 

 outermost layer become fiat- 

 tened and form the hair 

 cuticle; several succeeding 



layers elongate and form MaMmaHgaHH^g^s u 

 the cortex; while, in hairs 

 with a medulla, the middle 

 cells retain pretty much 

 their original form and size. 

 Pulled apart by the elongat- 

 ing cortical cells, these cen- 

 tral ones then form the *O 



medulla with its air Cavities. FIG. |137. Parts of two hairs imbedded 



rni . , , , , in their follicles, a, the skin, which is seen 



1 lie innermost layer OI the to dip down and line the follicle; 6, the sub- 



'A^*, ' v ' 4-1* f IT i cutaneous tissue; c, the muscles of the hair- 



epidermis lining the lOlllCle, follicle, which by their contraction can 



has its Cells projecting, With erect the hair ; o, sebaceous gland. 



overlapping edges turned downwards. Accordingly these inter- 

 lock with the upward directed edges of the cells of the hair- 

 cuticle; consequently when a hair is pulled out the epidermic 

 lining of the follicle is usually brought with it. So long as the- 

 dermic papilla is left intact a new hair will be formed, but not. 

 otherwise. Slender bundles of unstriped muscle (c, Fig. 137} 

 run from the dermis to the side of the hair-follicles. The latter- 

 are in most regions obliquely implanted in the skin so that 

 the hairs lie down on the surface of the Body, and the mus- 

 cles are so fixed that when they shorten, they erect the hair 

 and cause it to bristle, as may be seen in an angry cat, or 

 sometimes in a greatly terrified man. Opening into each hair- 

 follicle are usually a couple of sebaceous or oil glands. Hairs are 

 found all over the skin except on the palms of the hands and 

 the soles of the feet; the back of the last phalanx of the fingers 

 and toes, the upper eyelids, and one or two other regions. 



Nails. Each nail is a part of the epidermis, with its 

 horny stratum greatly developed. The back part of the nail 

 fits behind into a furrow of the dermis and is called its root. 

 The visible part consists of a body, fixed to the dermis be- 

 neath (which forms the bed of the nail), and of a free edge. 

 Near the root is a little area whiter than the rest of the nail 

 and called the lunula. The whiteness is due in part to the 

 nail being really more opaque there and partly to the fact 

 that its bed, which seen through the nail causes its pink 

 color, is in this region less vascular. 



