1078 THE SKIN 



and smaller hairs in the two sexes is too familiar to require description. The 

 direction taken by the hairs in the various parts of the body is fairly constant and 

 may be traced in each region to or from certain centres or vortices, such as those 

 upon the crown of the head, at the external auditory meatus, in the axilla, the 

 fold of the groin, and elsewhere. 



The hair not only varies in character and develo})nK'nt in difTerent jiarts of the 

 surface, but presents considerable racial and individual variations in each region. 

 Taking the hair of the head as a type, we may find it either straight, wavy, curled 

 into incomplete or complete spirals, or minutely tufted like wool, and each of these 

 varieties is associated with i)eculiarities in the transverse sectional area; thus, in 

 the straightest hair the section is circular or nearly so, while in the wavy, curly, and 

 woolly hair it is oval or elliptical, the greatest difference between the largest .and 

 smallest diameters being found in the woolly hair in Avhich the radius of the curve 

 is smallest. In length the highest development is attained in straight hair, and the 

 growth is nearly always greater in the female than in the male. In some regions 

 the growth becomes luxuriant from infancy; in others, as upon the i)ubic regions 

 in both sexes and on the lips and cheeks of the male, its full development is deferred 

 until puberty or later. As age advances, the hair-bulbs are liable to undergo pre- 

 mature atrophy upon the sunmiit of the head in men, while the groAvth may con- 

 tinue to progress in other parts of the body, even to extreme senility; and a loss 

 of pigment in the hair, usually beginning on the head and extending later in a 

 somewhat capricious way to the other regions of the body, is another sign of local 

 failure of nutrition which may or may not be associated with senile degeneration 

 of the tissues generally. Pathologically, the hair is subject to many changes, into 

 which it is not necessary to enter here. 



The first growth of hair begins about the fifth month of fa^tal life, but this croj) 

 (lanugo) is entirely shed within a few^ months of birth. The process of shedding 

 and new formation goes on throughout life. 



The typical hair has its root in a dermic papilla sunken at the bottom of a deep 

 follicle which runs more or less obliquely through the whole or greater part of the 

 thickness of the skin. The hair-follicle is cylindrical or oval in section and fairly 

 uniform in diameter in the greater part of its length, but expanding below where it 

 contains the bulb of the hair. Into it open the orifices of the sebaceous glands 

 appended to the hair; very rarely also the duct of a sweat-gland. 



Structurally, the follicle consists of an invagination of the elements of the skin, 

 with the addition of a partial lining derived from the generative papilliv of the hair; 

 and its wall comprises three laminae. The external or dermic coat is continuous 

 Avith the derm, and is composed of two layers of fibrous tissue, the more external 

 being longitudinal in direction, the inner transverse. At the bottom of the follicle 

 it is reflected upwards as the papilla, which constitutes the essential and generative 

 element of the hair-root. The middle or hyaline coat is an invagination of the 

 structureless basement meml^rane, upon Avhich rest the deei)est cells of the epidermis. 

 This becomes lost upon the papilla. The internal epithelial coat is divided into 

 two secondary layers called, respectively, the outer and inner root-sheaths. The 

 outer root-sheath, from the opening of the folhcle down to the openings of the 

 .sebaceous ducts, includes all the layers of the epidermis, Init lielow this i)oint loses 

 the stratum granulosum and the stratum conifmn, and is reduced to the stratum 

 Malpighii and stratum l)asilare. The inner root-sheath, unrei)resented in the 

 ei)iderm and ])robably a derivative of the hair-papilla, consists of three layers: tlie 

 layer of Henle, formed by a single set of polyhedral cells; the layer of Huxley, 

 similarly constituted, but with the component cells more elongated and less closely 

 compacted; and the cuticular layer, composed of a single set of tliin, imbricating 

 cells, clear and transparent, with atrophic nuclei. 



The hair proper is a cylindrical or oval shaft ex])anded l)elow into a bulb, 

 where it cai)S the pa])illa, at the bottom of the follicle. Tlie shaft, fairly uni- 

 form in diameter in the scalp-hairs, consists of a medullary axis, surrounded 

 by a cortical coat and invested by a cuticular layer. The medulla, constituting 

 about one-fourth of the entire diameter of the hair, is a solid cylinder, white l»y 

 reflected light, dark by transmitted light, and is composed of clf)sely compacted 

 nucleated cells, filled with pigmentary and fatty granules and air-bui)bles. It is 



