1. THE ANATOMY OF THE SKIN



The horny layer, which is, as we have said, of variable but nearly always of considerable thickness, if formed of a number of layers of cells which, differentiated already in the lowest layers, have that differentiation completed as these pass upward. The upper, outer portion of this horny layer is continually being shed or rubbed off in the form or flakes of variable size. Each flake upon examination, as for instance after dissociation by maceration or with the help of alkalies, is found to be composed of elements which can no longer be recognized as cells, and which may be spoken of as scales. Every scale is a flattened mass or plate in which no nucleus can be seen, and which consists not of the proteids and other constituents of ordinary cell substance, but almost exclusively of a material called keratin. This is a body, the exact nature of which has not yet been clearly made out, but which has the general percentage composition of proteids, from which it is a derivative, with the exception that it contains a considerable quantity of sulphur (the keratin of hair contains as much as five per cent); this sulphur appears to be somewhat loosely attached to the other elements of the keratin since it may be removed by boiling with alkalies.

The lowermost portions of the horny layer are composed of elements which may still be recognized as cells, in as much as each contains a nucleus, though this obviously undergoing change and on the way to disappear. Each cell is, however, flattened and plate-like, and its substance already consists largely of keratin. In passing upward from the lower to the more superficial pairs of the horny layer such an imperfect cell loses its nucleus, and becomes the wholly keratinous plate just described. The whole horny layer consists of strata of elements, horny to begin with, but becoming more completely so in the upper parts. Below, in contact with the moist Malpighian layer, the horny layer is moist but the superficial parts become dry by evaporation; and here the strata delaminate from each other, the outer ones, as we have said being shed in the form of flakes, which seen in the dry condition under the microscope have often the appearance of irregular fibres.

The karyomitosis seen in the cells of the Malpighian layer, not only in those of the vertical layer, but in the others as well, show, as we have said, that these multiply by division; we have no evidence of multiplication taking place elsewhere in the epidermis. The more superficial cells of the Malpighian layer, thrust upward by the newcomers, are transformed into the cells of the stratum granulosum; and although we do not as yet fully understand the exact nature of the transformation we may conclude that the peculiar granules of these cells are concerned in the manufacture of keratin. Changed by the consumption of their granules in this manufacture, the cells of the stratum granulosum become first the cells of the stratum lucidum, and then the cells of the distinctly horny layer, pushed upward through which, by the new formations continually succeeding below them, they pass to the surface and are eventually shed.

The papillae in the papillary part vary in size and aspect in different parts of the skin. They are pointed or thread-like about the fingers, and club-shaped or rounded over the general surface of the body. On the palm of the hand, about the nipple, and the sole of the foot, they are longest and largest, being often 66 to 1 m. They are shortest on the face. They are of two kinds-nervous, containing nerve fibres ending in tactile corpuscles, and vascular, containing blood-vessels in the form of a loop.

There are, in addition, spindle-shaped cells, or cells anastomozing by processes, amongst the connective tissue bundles in the substance of the corium and around the vessels.

The thickness of the corium various. It is thinnest about the eyelids and prepuce; on the face, scrotum, and body, generally, it is thicker. On the sole of the foot and palm of hand it is thickest, being from 2.25- 2.28 of a millimetre.

The corium is well supplied with vessels, lymphatics, and nerves. The vessels come up from the connective tissue below, give off branches to the fat and the glands, then constitute a network which sends off more or less obliquely twigs to form a longitudinal plexus along the base of the papillae, from whence finally capillary loops are supplied to most of the papillae. The lymphatics have much the same arrangement as the blood-vessels. The plexuses, however, are situated beneath those formed by the capillaries, but no lymphatics are supplied to the papillae. The spaces normally seen between the connective tissue fibres are supposed to be lymphatic spaces; they have no proper boundaries. The blood-vessels and lymphatics are thought to communicate, though this has not been proved, by means of peri-vascular spaces where the blood-vessels and lymphatics run together, the walls of these spaces being formed by the connective tissue around.

Nerves accompany the blood-vessels coming up from below, and are of two kinds, medullated and non-medullated; the former go with the tactile and Pacinian corpuscles, and the latter form, it is thought, a fine network below the site, in connection with the capillary plexus, and are in communication with those found in the site. Non-medullated fibres also run with the capillaries, which supply the vascular papillae.

The subcutaneous cellular tissue is nothing but a lax network of connective tissue. The bundles of fibres, “usually cylindrical, exhibit constrictions at various points like those of the arachnoid membrane, and consist of a number of sinuous fibres of connective tissue, between which lie numerous fusiform and connective-tissue corpuscles of various forms and dimensions.” It is this part in which the fat cells are collected in the meshes of the fibres, the only seats destitute of fat being the eyelids, penis, scrotum, and ears. The fat I need not describe. The fusiform cells that lie between the bundles and fibres of the connective tissue are connected with these fibres by means of processes given off one at either end, with other minor ones elsewhere. Other fusiform cells, without processes (the migrating cells), and small cells like blood-cells, are observed, the latter being contiguous to the vessels. There is no line of demarcation between the corium above and the subcutaneous tissue below.

The Sweat Glands.-A sweat-gland, like other glands, consists of a secreting portion and a conducting portion. The secreting portion is a long tubular alveolus coiled up in a knot and placed in the subcutaneous connective tissue at some distance from the epidermis. Generally the gland is formed of one such tubule only, but sometimes two tubules unite into a common duct. The duct beginning in the knot, in the convolutions of which it shares, runs a somewhat wavy but otherwise straight course vertically toward the surface of the skin onto which its lumen opens.

Through the epidermis the duct is nothing more than a tubular passage excavated out of the epidermis with a remarkable corkscrew course, the turns of the screw becoming more open and the canal wider in the upper part as it approaches the surface. In the Malpighian layer the cells bordering on the passage are flattened and inclined downwards so as to afford a more or less definite lining; there is a similar arrangement, but not so well seen, in the corneous layer. Reaching the dermis in a valley between papillae, the passage becomes a regular duct, with an independent epithelium of its own, a distinct basement membrane continuous with the upper surface of the dermis, and an outer coat of connective tissue strengthened, in the case of some of the larger glands, such as those of a the axilla, with plain muscular fibres. The epithelium consists of two or three layers of small rounded cells, each with a relatively large but absolutely small nucleus, generally staining deeply. The cells leave a narrow tubular thread-like lumen which is lined with a very characteristic distinct cuticle.

The duct continues to possess these characters after it has entered the knot and begun to pursue a twisted course, but soon changes suddenly into the secreting tubule. This may be distinguished from the duct by being wider, and by being lined by a single layer of cubical or columnar cells larger than those of the duct, bearing larger nuclei, and behaving differently toward various staining reagents. The lumen though fairly distinct is not lined by any cuticle as in the duct. Lying between the basement membrane and the epithelial cells, or rather imbedded in the basement membrane, are seen a number of plain muscular fibres disposed longitudinally or in an elongated spiral, and often forming a distinct coat beneath the epithelium.

As in the case of other glands, we are unable to make any statement as to the work carried on by the epithelium lining the duct, but we may probably assume that the sweat is mainly secreted by the larger cells of the terminal coiled part of the tubule. These cells, therefore, like other secreting cells, are probably “loaded” and “discharged”, but as yet no structural changes in the cells corresponding to these phases have been satisfactorily ascertained, though after the administration of pilocarpine, which causes sweating, the cells of glands hardened in alcohol stain more deeply than usual with carmine. It must be remembered, however, that the sweat contains normally neither mucus nor proteid substances, and we should, therefore, not expect to observe granules in the cells.

Melford Eugene Douglass
M.E.Douglass, MD, was a Lecturer of Dermatology in the Southern Homeopathic Medical College of Baltimore. He was the author of - Skin Diseases: Their Description, Etiology, Diagnosis and Treatment; Repertory of Tongue Symptoms; Characteristics of the Homoeopathic Materia Medica.