Introduction

THE diseases of the skin are exceedingly manifold, and the names applied to them somewhat complicated. In this section an endeavour is made to summarize the pathological conditions and group together the various diseases according to the nature of the lesion.

Normal Structure

The Corium or True skin is a very vascular dense membrane composed of interlacing fibres of connective tissue, with numerous elastic fibres. It is richly supplied with nerves, and possesses bundles of smooth muscle. It has also a rich system of lymphatic vessels. In the corium we may distinguish a superficial or Papillary layer and a deeper one. Many of the diseases affect the papillary layer especially.

The Epidermis on the surface of the corium is in several layers (Fig. 474). Most superficial is the horny layer (a) composed of flat cells, which are little more than scales, and have lost their nuclei. Next comes the stratum lucidum (b) composed of flat transparent cells. Most deeply situated is the stratum mucosum or rete Malpighii, composed of cells which, in the deepest layer, are cylindrical, but towards the surface become polygonal and flattened. In the deeper layers the cells are serrated at the margins, so as to give the appearance of prickles by which the cells fit into each other. The Malpighian layer extends between the papillae of the corium, forming interpapillary processes.

Fig. 474. - Section of epidermic layer of skin with a hair and its follicle, a, horny layer; 6, stratum lucidum; c, Malpighian layer with (d) the deepest layer of cylindrical cells; e, hair, whose sheath presents two layers, one continuous with the horny and the other with the Malpighian layer, x 350.

The Sebaceous glands may be regarded as modified prolongations of the Malpighian layer. They are mostly connected with hair follicles into which they open, but sometimes large glands are connected with small hairs and small glands with large hairs, while there are glands not connected with hairs at all. The Sudoriparous glands are usually situated beneath the skin, their ducts passing through corium and epidermis, having a spiral course in the latter. The Hairs are composed of horny epidermis, and are placed in follicles. In the latter, two layers of epidermis can be distinguished (Fig. 474), the outer root-sheath corresponding with the Malpighian Jayer, and the inner root-sheath corresponding with the horny layer. At the bottom of the follicle is a papilla continuous with the corium, and on this is set the bulb of the hair. The Nails are composed of compressed horny epidermis. Beneath the nail are still two layers of epidermis, a horny layer of loose cells, and a Malpighian layer covering well-formed papillae.

The exposed position of the skin renders it very liable to the influence of agents acting from without. It is also liable to be influenced by irritants circulating in the blood, in which case the skin affection will probably be an insignificant part of a general condition. The skin again is liable to be affected by states of the nervous system. From these remarks it will be inferred that the inflammations of the skin are its most important morbid conditions, and will call most largely for description.

The skin, like other structures, and more obviously from its visibility, presents great differences in detail of structure in different individuals. These are variations in pigmentation, fineness of texture, vascularity, elasticity, etc., which form parts of the individual constitution of the person. One of the most striking variations is an extreme elasticity of the skin presented by some individuals, so that it can be laid hold of and drawn outwards from the underlying parts. Of similar constitutional significance are the varying susceptibilities to morbid influences which different individuals present. This is exemplified in many forms of disease of the skin.

Literature

Oeneral ivories - Erasmus Wilson, Dis. of skin, 1867; Neumann, (Pathological anatomy is very good) Lehrb. d. Hautkrank., 1880; Hebra and Kaposi, Dis. of skin (Syd. Soc), 1866-1880; Duhrino, (literature very complete) Dis. of skin, 1882; Ziemssen, Handbook of Dis. of skin, 1885; Kaposi, Haut-krankheiten, 1893; M'Call Anderson, Dis. of skin, 1894; Path, very fully in TJnna, Die Histopathologie der Hautkrankheiten (part of Orth's Path. Anat.), 1894; also in Atlas by Leloir and Vidal, the plates of which give histology of skin diseases with great accuracy. Elastic skin - Ohmann-Dumesnil, Internat. Med. Mag., i., 1892.