THE NAILS 



1293 



and its follicle (figs. 1050, 1051). Immediately beneath this is the narrowest part of the follicle 

 the neck [collum foUicuIi pili], especially important as the position of the nerve ending of the 

 hair. 



The hair is formed of epithelial cells arranged in two and sometimes three layers; an outer 

 single-celled layer of transparent over-lapping cells, the cuticle, an intermediate layer several 

 cells thick formed of irregular fusiform hornj^ cells containing pigment and arranged in fibrous 

 strands, the substantia corticalis, and in some of the larger hairs an internal two or three celled 

 layer of angular cells occupying the center of the hair shaft for only part of its length, the 

 substantia medullaris. Between both the cortical and medullary cells are spaces containing air. 

 In the hair bulb, where the cells are larger and softer the layers are not distinguishable. The 

 cells here being in process of division and being gradually transformed into the horny cells of 

 the shaft. 



Many of the hairs have in connection with their folHcle round or flat bundles 

 of unstriped muscle fibres, the arrectores pilorum (figs. 1050, 1052). These are 

 situated on the side toward which the hairs point, their deep ends being attached 

 to the hair follicle beneath the sebaceous glands which the}^ more or less embrace 

 and their superficial ends connected with the papillary layer of the skin. Con- 

 traction of the arrectores not only causes the hairs to become more erect and the 

 skin around them to project somewhat causing ''goose flesh," but also compresses 

 the sebaceous glands which are situated between the follicle and muscle and helps 

 to empty the glands of their secretion. 



Fig. 1052. — Vertical Section of the Skin from Scalp. ( X 20.) 



Hair root 



Hair-follicle 



Sebaceous gland 



Fat and connec- 

 tive tissue 



The blood supply of the hairs. — -The hair follicles are surrounded by a capillary network 

 of arteries connected with those of the corium and the papillae are also supplied with loops of 

 arteries. 



The nerves of the corium supply branches to the hairs. Some of these branches enter the 

 papillae, others surroimd the follicle at its neck and are distributed among the cells of the outer 

 root sheath. 



Development.— The hairs are developed from the epidermis by thickenings and down- 

 growths into the corium of plugs of epithelium. The deepest parts of these plugs become swol- 

 len to form bulbs and from these the hairs are produced. The central cells of the epithehal 

 downgrowths disintegrate producing the lumen of the follicle. The hairs continue to grow from 

 the deeper cells and protrude from their follicles between the fifth and seventh foetal months. 

 Abnormally they may be scanty at birth and rarely entirely absent, alopecia. The lanugo hairs 

 which cover all the hairy parts of the body at birth are soon shed and replaced by new hairs in 

 the old follicles. Throughout life also the hairs are being constantly shed and replaced by new 

 ones. This is accompanied by cornification of the bulb and fibrillation of the deep end of the 

 hair (fig. 1051). Thinning of the hair and baldness occur when the shed hairs cease to be 

 replaced. This is common in old age and a premature baldness appears to run in certain fam- 

 ilies. The rate of growth is normally from 1 to 1.5 cm. per month, but is subject to variation. 



B. THE NAILS 



The nails [ungues] are thin, semi-transparent, horny epidermic plates upon the 

 dorsal surfaces of the distal phalanges of the fingers and toes. Through their 

 hardness they serve as protective organs not only by covering the nerve endings 

 and other delicate structures of the skin; but also by acting as natural weapons. 

 On the fingers they form useful tools. They are four-sided plates presenting a dis- 



