SKIN AND SKIN DISEASES. The skin is the group of organs which form the outer covering of the body. It may be regarded as an extensive, complex, uninterrupted mem brane, actively functionating, and merging into the mucous membranes at the natural orifices of the body. Beneath it stands in the most intimate structural relationship to the under lying tissues; externally it is exposed to varied and innumerable contacts with the outer world. By reason of this unique position as the boundary of the living organism, the skin has important functions to perform, all of which are contributory to the well-being df the in dividnal, while some are essential to his con tinued existence.
To the touch the skin in general is approxi mately smooth, soft, supple and movable upon the underlying tissues. In certain localities those subject to special contacts, as the palms and soles — it is more dense and less pliable than elsewhere. Inspection discloses the pres ence of various surface markings in the form of minute pores and intersecting lines, furrows, and ridges. There is also seen an extensive but variable growth of hair, and at the end of the dorsal portions of the fingers and toes are the horny structures known as the nails. The pores represent the combined openings of the sebaceous glands and the hair-follicles, and as such give exit to the hair-shafts. The lines and furrows of the skin are primarily the product of its elasticity. They are fine, intermediate and coarse. The last type is best seen about the joints, or upon the palms, where by reason of great latitude of movement, the skin is often thrown into folds; the other types occur upon those parts less subject to wide range of mo tion. The ridges are best observed upon the palms and soles, appearing in curved and straight, more or less parallel formations, said to be characteristic in each individual. Hair is quite generally distributed over the body surface, though certain parts are free; the amount and quality vary greatly with the loca tion. The nails, though unlike the skin in texture and appearance, are entirely derived from the dermal tissues.
Anatomy of the Technically in de scription the term skin includes only the cellu lar and connective-tissue layers, with their blood-vessels and nerves. The highly differen tiated structures which by derivation, function, and position are a part of the skin, as the se baceous and sweat glands, hairs, hair-follicles, and nails, are termed appendages of the skin.
From within outward the skin proper is made up of three layers: the subcutaneous tissue or hypoderm; the corium or true skin, called also the cutis or derma; and the epidermis, cuticle, or scarfskin. The hypoderm can scarely be considered a distinct layer; it is more prop erly a connecting bond between the body tissues internally and the corium externally. It is loose in texture, and is composed of inter lacing fibrils and bundles of connective tissue, more or less obliquely directed, within the wide meshes of which. are deposited masses of fat. This fatty tissue in the hypoderm forms the panniculus adiposus, and to It is due the rounded contour of the well-nourished body. Its dis appearance by absorption following prolonged illness, and in old age, produces angularity of feature and wrinkling of the skin. Besides blood-vessels, lymphatics, and nerves, this layer contains sweat-glands, deep hair-follicles, and special nerve-endings. The corium, cutis, or derma, is made up in general of a more or less dense network of fibrous and elastic tissue, carrying within it vessels, nerves, glands, hair, and hair-follicles. It is divided into two layers; the pars- reticularis and the pars papillaris; but ---tbe-dividing line between the two is not clear-cut; The -pars reticularis is the layer in contact with the subcutaneous tissue. It is well. -described by its -name—a. reticulum of loofa connective tissue. The pars papillaris is relatively much thinner, but more dense than the pars reticularis, and lies external to the latter. It derives its name from the fact that its outermost portion presents innumer able papilla-like projections, which interdigitate with corresponding downward prolongations from the innermost layer of the epidermis. The total number of papilla upon the body surface is enormous; an estimation of approximately 64,000 to the square inch has been made. The curved and parallel ridges previously mentioned as seen upon the are due to an arrange ment of papilla in rows. Each papilla is provided with a vascular loop or a special nerve-ending; rarely both are present. The proportion of the vascular to the nervous variety is about four to one; hence it is that the especially sensitive areas of the skin con tain the greater number of papilla.