734 THE SKIN OR INTEGUMENT. ™ 
appears to be homologous with a thin membrane, termed the epetrichium, first described 
as covering the embryo of the sloth and overlying its hairs, but since shown to be present 
in birds and mammals. Over the hairy parts of the body it ultimately disappears ; 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 (de p. 731). The stratum lucidum is differentiated from 
the cells of the epitrichium, and, where the latter is lost, possibly forms the superficial 
layer of the epidermis as it does the horny part of the nails. 
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 extremity 
of the digits. ‘Owing to the growth of the palmar aspect of the digits, the nail rudiment 
comes to be pli aced dorsally, and, at its proximal edge, an ingrow th of the stratum 
mucosum occurs to form its root, while the future nail becomes limited behind and later- 
ally 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, and, until the 
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 ultimately this continuity is 
lost and the edge becomes free. The nails grow in length, and are renewed, in case of 
remoyal, 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 becomes expanded to form the hair bulb, and rests 
on a papilla derived from the corium, the epidermis immediately overlying which becomes 
differentiated into the hair and its inner root sheath, while the peripheral cells form its 
outer root sheath. The surrounding corium becomes 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 recone free on the disappearance of 
the latter. This takes place about the fifth month, and the first crop of hairs constitutes 
the /anugo, which 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, the renewal being of course absent 
in the case of baldness. Prior to the shedding of a hair, active growth and proliferation 
of the cells of the hair bulb cease and the papilla becomes atrophied, while the hair root, 
gradually approaching the 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 from 
the sides of the hair follicles, and consist of epidermal offshoots continued from the cells 
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 collected 
on the surface of the body during the last months of intrauterine life, and forms a layer 
of varying thickness, termed the vernix caseosa or smegma embryonum. 
Sweat Glands.—These, like the hairs, arise as solid downgrowths of the stratum 
mucosum. They descend, however, perpendicularly, instead of obliquely, and 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 the corium, and, 
on reaching the subcutaneous tissue, become coiled up to form the secreting part of the 
gland. The lumen of the gland does not open on the surface until the seventh month. 
