862 THE SKIN OK INTEGUMENT. 



corium, and the deeper the subcutaneous tissue ; the papillae of the corium make their 

 appearance in the fourth month. The epidermis, nails, hairs, sudoriferous and sebaceous 

 glands are of ectodermal origin. 



The epidermis at first consists of a single layer of cells, but by the end of the second 

 month it is duplicated, and then exhibits a superficial layer of irregular cells and a deeper 

 layer of more or less cubical cells. By the third month three strata are seen : (a) a deep 

 layer, consisting of a single layer of cubical cells the future stratum germinativum ; 

 (6) a middle layer, comprising two or three strata of irregular cells the future stratum 

 mucosum ; and (c) an outer layer, a double stratum of large cells. This outer layer 

 appears to be homologous with a thin membrane, termed the epitrichium^ first described 

 as covering the embryo of the sloth and overlying its hairs, but since shown to be present 

 also in birds and mammals. Over the hairy parts of the body it disappears about the 

 sixth month ; but over the free edge and root of the nails, and on the palms and soles, it 

 develops into several layers of cells, which, in these parts, probably persist to form the 

 thick stratum corneum. The part which persists over the root of the nail is termed 

 the eponychium, and covers the proximal part of the lunula (vide p. 859, Fig. 737). 



Nails. The first rudiment of the nails is seen about the beginning of the third month 

 of embryonic life, and consists of a thickening of the epitrichium over the ends of the 

 digits. Owing to the greater growth of the volar surfaces of the digits, the nail rudiment 

 comes to be placed dorsally, and, at its proximal edge, an ingrowth of the stratum 

 mucosum occurs to form its root, while the future nail is limited behind and at the sides 

 by a groove. The superficial cells of the stratum mucosum become keratinised to form 

 a thick stratum lucidum, the future nail proper, over the greater part of which the 

 epitrichium disappears. The latter persists in the adult as the eponychium across the 

 root of the nail, and, until fifth month, also forms a thick mass over the extremity of the 

 nail, and is continued into the stratum corneum over the end of the digit. The future 

 distal edge of the nail, at this stage, is continuous with the stratum lucidum in front of 

 it ; but this continuity is lost, and by the seventh month the nail presents a free border. 

 The nails grow in length, and are renewed, in case of removal, by a proliferation of the 

 cells of the stratum mucosum at the root of the nail, while an increase in their thickness 

 takes place from the part of the same stratum which underlies the lunula. 



Hairs. The hair rudiments appear about the third month of embryonic life as solid 

 downgrowths of the stratum mucosum, which pass obliquely into the subjacent corium. 

 The deep end of this column of cells expands to form the hair bulb, and is moulded on 

 a papilla derived from the corium ; the epidermis immediately overlying the papilla 

 becomes differentiated into the hair and its inner root sheath, while the %i peripheral cells 

 form its outer root sheath. The surrounding corium is condensed to form the fibrous 

 sheath of the hair follicle, the hyaline layer of which is continuous with the basement 

 membrane covering the corium. The hair gradually elongates, and, reaching the neck of 

 the follicle, its extremity lies at first under the epitrichium, but becomes free on the dis- 

 appearance of the latter. This takes place about the fifth month of foetal life, and the 

 first crop of hairs constitutes the lanugo, and is well developed by the seventh month. 

 The lanugo consists of very delicate hairs, some of which are shed before, the remainder 

 shortly after birth the last to drop out being those of the eyelashes and scalp and are 

 replaced by stronger hairs. Shedding and renewal of the hairs take place during life ; 

 prior to the shedding of a hair active growth and proliferation of the cells of the hair bull 

 cease, and the papilla becomes atrophied, while the hair root, gradually approaching th( 

 surface, at last drops out. New hairs arise from epidermic buds, which extend downwards 

 from the follicle, and their development is identical with that of the original hairs. 



Sebaceous Glands. These appear about the fifth month as solid outgrowths fron 

 the sides of the hair follicles, and consist of epidermal offshoots continued from the celL 

 of the outer root sheath. Their deep ends become enlarged and lobulated, to form the 

 secreting part of the gland, while the narrow neck connecting this with the follicle forms 

 its duct. The sebaceous secretion, together with the cast-off epidermal cells, is collectec 

 on the surface of the body during the last months of intra-uterine life, and forms a laye: 

 of varying thickness, termed the vernix caseosa or smegma embryonum. 



Sudoriferous Glands. These, like the hairs, arise as solid downgrowths of th< 

 stratum mucosum. They descend, however, perpendicularly, instead of obliquely, an< 

 are of a yellowish colour ; they appear on the palms and soles early in the fifth month 

 but much later over the hairy parts of the body. The downgrowths extend through th 

 corium, and, on reaching the subcutaneous tissue, become coiled up to form the body o 

 secreting part of the gland. The ducts of the glands do not open on the surface unti 

 the seventh month. 



