The epidermis or cuticle is the portion of the skin intervening between the corium and the external world Though structurally in most intimate relation with the true skin, by derivation it is totally distinct from it, being developed from the epiblast of the embryo, whereas the corium springs from the meso blast. As the epidermis is avascular, nourish ment is provided by means of juice-spaces in the deeper portions. The lower cells are there fore soft and succulent, and the extent of the change necessary to transform them into the dry, horny, and flat cells of the skin surface is readily apparent.
For purposes of description the epidermis is divided from within outward into four layers, the stratum mucosum, the stratum granulosum, the stratum lucidum, and the stra tum corneum. The stratum mucosum, called also the rete mucosum or Malpighii, the mucous or prickle layer, consists of several superim posed strata of cells, which fill in the inter papillary spaces of the corium, and present an undulating, plane surface to the layer next above. The cells in general are rich in protoplasm, have clearly defined nuclei, are columnar and polygonal in shape, becoming flattened as the surface is approached, and are provided with radiating, spine-like, protoplasmic '1)6 / processes or es,* which anastornose and d interdigitate one frith another. A viscid inter- . cellular cement-substance is present, permeated by channels for the passage of nUtritive fluidS, The layer of cells in immediate contact withothe corium has been aptly termed the stratum germinativum, since it is by multiplitation of these that both 'physiologic exfoliation - and pathologic loss of- the epidermis are restored. The pigment .whits gives color ti- the skin is found in this layer, and nerve fibrils are abundantly present between and within the cells. The stratum granulosum is a narrow band of cells distinguished by the appearance of granules within the cell protoplasm. These are highly retractile, and are composed of keratohyalin, a substance believed by some to be the antecedent of keratin, the horny material int the cells of the skin surface. The stratum lutidum is also narrow, and is made up of more oil less flattened cells; with faint "nuclei and a harttogerieolis and lucid-appearing protoplasm. The _granules of the preceding layer have totally' -disappeared, and instead a substance called etitidiii is found diffused throughout the cell body. This 'is re larded by some as a solution of the keratohyalin in the cell protoplasm. Others consider it a totally distinct body and not related to the product keratin. The stratum corneum is the most superficial layer. It is relatively thick, and its component cells are flat, dry, horny, and resistant, from the presence of keratin. Nuclei are absent or very faintly outlined. By special treatment the remains of the °prickles* may be detected. A well-grounded assertion has been made that the succulent protoplasmic spicules of .the rete, by desiccation and other change, give origin to the keratin of the cor neum. Constant exfoliation of the superficial cells of the skin, is in progress, the loss of substance being compensated by growth from below.
The vascular supply of the skin is abun dant. The numerous arterial branches emerg ing from the deeper tissues divide and sub divide to form an intricate horizontal network of vessels in the subcutaneous tissue. A second plexus is formed by division of radicles from the first at about the level of the papillary bases. From these arterioles capillary loops extend into the papillz. The distribution of veins and venules for the return of the blood is practically the same as that of the vessels for the arterial flow; the venous channels, how ever, are considerably larger. For the passage of lymph, juice-spaces and lymph-vessels exist. The former are mere excavations in the inter cellular cement-substance, communicating with one another, and eventually emptying into lymphatic vessels. These latter are channels with definite endothelial walls; they appear first at the bases of the papillz as blind terminals, and by anastomoses from dense plexuses in the corium and subcutaneous tissues. The vessels receive the lymph from
the juice-spaces through openings between the vascular endothelia, and eventually discharge their contents into the veins. The lymph which nourishes the epidermis flows from the papillary apices, percolates between the cells of the rete, and returns by way of the interpapillary spaces.
The nerves of the skin are (sensory) medullated and (vegetative) non-medullated. The latter form a dense plexus immediately beneath the rete mucosum; thence fibrils as cend, penetrating as far as the granular layer, and terminating either within or between the cells of the rete. Special nerves of non medullated variety, known as vasomotor nerves, ace distributed to the muscle-fibres of the vessel-walls. The medullated nerves are confined to the papillary and subcutaneous layers, and are peculiar in that they terminate in special end-organs. Three varieties of these are found in man: the corpuscles of Krause, the corpuscles of Meisner, and the Paccinian corpuscles. While these differ in minute struc ture, in general all may be described as round or oval bodies, made up of concentric layers of connective tissue, enclosing the blunt or coiled termination of a nerve which enters at one, pole of the mass. The first two kinds are found within and beneath the papilla, especially in sensitive parts, while the Paccinian corpuscles occur in the subcutaneous tissue throughout the body. Trophic nerves are present in the *in, they belong to the vegetative system and non-meclullated. Muscle-fibres of the smooth or unstriped variety are found in the corium of certain parts, especially the face. The principal muscular structures in the skin, however, are the arrectores pilorum. These are smooth muscle-bundles which arise froth the lower portion of the walls of the hair-fol licles and extend obliquely upward to be in serted into the pars papillaris of the corium_ Contraction of these muscles produces erection of the hair, or on theglabrous skin the condi. lion kfiown as "goose flesh.' The pigment of the skin resides in the lower cells of the rete mucosum. It appears as black granules of melanin within the cell protoplasm, chiefly about the nuclei. In the dark races the pigment is found in the superficial cells, in some cases even in the corneum. The tint of the skin is also influenced by vascularity. Many theories have been advanced as to the origin of the skin pig ment; it seems most probable that it is derived from the coloring matter of the blood in the corium, and thence carried to the rete by wan dering cells. Two kinds of glands exist in the skin; the sweat, or sudoriparons glands, and the sebaceous or oil glands. The sweat glands are found in the hypoderm and lower most portion of the corium. Each consists of a single long tubule, with a blind extremity. coiled many times upon itself to form a round or oval mass, from the upper part of which the excretory duct ascends. This is merely a straight portion of the tube slightly modified; it enters the rete opposite an interpapillary space, and after an irregular and spiral course through the epidermis opens upon the free• sur face of the skin, forming the sweat-pore. Be sides the single layer of secretory cells, an in vesting membrane, unstriped muscle fibres and nerve filaments compose the tubule, and many blood-vessels are distributed in and about the coils. The duct has a lining of several layers of cells, and a more distinct investing mem brane; in the rete these disappear, and through the corneum no definite lining exists. The sebaceous or oil glands are usually found in connection with the hair-follicles, but may .occur where hair is absent, as on the lips. Their seat is the corium; each consists of one or several, sometimes many, pear-shaped lobules, lined with epithelium and invested with fibrous tissue. All the lobules open into a common gland-cavity, which in turn discharges into the upper portion of a hair-follicle. The product of the gland, called sebum, is formed by fatty degeneration of the epithelial cells within the lobules; its extrusion is favored by the con traction of the arrestor pilorum, which com presses the gland against the hair root.