(D) Psoriasis

The nature of the irritant is in this case quite unknown, but it is probably to be found in some special condition of the blood. We know, at least, that remedies introduced into the blood (arsenic, and, according to Napier, chrysophanic acid administered internally) frequently cure the disease.

The lesion is mainly of the epidermis, and especially of its Mal-pighian layer. The papillary layer of the cutis is hypersemic and partly infiltrated with leucocytes; the papillae are also described as enlarged, but these alterations are comparatively trifling, as, after death, very little indication of them is to be found. During life the hyperemia gives the corium a red colour. In the middle regions of the Malpighian layer of the epidermis there is an abundant new-formation of epidermic cells. The new-formation is so great that there is no time for the cells to become horny before they pass to the surface. Hence the epidermis on the patch is entirely composed of Malpighian layer, and these cells, being soft, as they dry adhere much more readily into considerable scales than do the horny cells. For a similar reason, as they dry they shrink much more than the horny cells, and air insinuates itself into interstices between them. It is the finely divided air which gives the scales the peculiar silvery appearance characteristic of this disease (Rindfleisch). When the scales are removed the papillary layer of the skin is seen to be red and bleeds readily.

(E) Pityriasis Rubra

In this disease we have a more or less general affection, and it is not improbable that it depends on a lesion of the trophic centres of the skin. It is characterized by an excessive discharge of scales consisting of masses of epidermic cells. These scales pass off from slightly raised and reddened surfaces. In the earlier periods the cutis presents little more than hyperemia with excess of leucocytes in its papillary layer, but when the disease has lasted for some time there is thinning of it with flattening of the papillae. The sebaceous glands and hairs, as well as the sweat glands, are usually atrophied. There is often a very excessive discharge of epidermis; a small basket-full may sometimes be gathered daily from the bed of the patient. There is a form of this disease designated Pityriasis rubra pilaris in which the excessive production of epidermic cells is concentrated in and around the hair follicles. There is here also hypersemia and cellular proliferation in the cutis. (See case by Morton, with microscopic examination by author, in Brit. Jour, of Dermal, VIII., 1896).

(F) Lichen Ruber

This is a diffused papular eruption, depending like pityriasis rubra on some obscure general cause. There are individual red papules on a red basis. As the disease goes on the skin gets infiltrated and thickened, so that the movements of the joints are hindered, and fissures are formed at the folds of the skin. The nails are usually thickened. The disease seems to consist in an inflammatory infiltration of the papillary layer of the cutis and the Malpighian layer of the epidermis. The cells of the rete Malpighii are enlarged and increased in number, and the interpapillary processes are enlarged. The papillae show abundant accumulation of round cells, especially round the vessels. There is often accumulation of epidermic cells in the hair follicles.

(G) Prurigo

This also is a diffused affection of the skin, which somewhat resembles urticaria, except that it is more permanent. Like urticaria it seems to depend on some condition of the nervous system, induced especially among children by neglect of sanitary conditions and defective nutrition. Numerous small papules appear on the skin and they are intensely itchy. The papules are due to an inflammatory thickening of the papillae and rete Malpighii. There are round cells and a serous fluid exuded, so that when the head of the papule is removed by scratching a drop of fluid appears. The papules are often induced to bleed by scratching, and it is probably from this that they frequently leave a brown stain. The irritation produced by scratching may also lead to eczema.

(H) Pemphigus

This condition is characterized by the formation of large vesicles or bullae, filled with fluid, which is at first clear, but usually becomes more or less purulent. As the bullae usually occur at intervals over a considerable surface, the disease is to be ascribed to some general cause, and in some acute cases the existence of an irritant in the blood is suspected. There is not here, as in small-pox, an exudation among the cells of the Malpighian layer, but for the most part the epidermis is raised as a whole; and the bleb is not divided by septa into loculi. If the bleb remain unruptured a layer of epidermis forms on the surface of the cutis. But if it burst, then the exposed cutis discharges for a time; by and by, however, a crust forms, under which the epidermis grows. In some cases the blebs are imperfectly formed and neighbouring ones coalesce, so that a considerable surface is affected. When these blebs burst there is little tendency to new formation of epidermis, and the skin continues discharging fluid as well as crusts produced by the fluid drying in. This form is designated Pemphigus foliaceus.

(I) Acne

This name is applied to inflammation of the sebaceoum glands and their, neighbourhood, chiefly in connection with accumulation of the secretion in the gland. There are two conditions, named respectively comedo and milium, in which sebum accumulates in the glands; it is chiefly in connection with the former that acne occurs.

In Comedo a sebaceous follicle is filled with secretion, which forms a solid plug in the gland. The end of the plug presents at the surface, and has a black or dark blue colour. The plug consists of shed epidermis and sebum, with occasionally a small parasite,' the acarus folliculorum.

In Milium there is an accumulation of the secretion with closure of the orifice of the gland. The accumulated secretion forms a rounded solid globe, which can be felt in the substance of the skin, but over which the epidermis passes unbroken. It is, in fact, a small retention cyst, and it contains the secretion of the gland mixed with epidermis, so that on cutting into it a whitish globular body is discharged. The cause of the closure of the duct is obscure.

In Acne there is a more or less acute inflammation of the sebaceous glands, usually in connection with comedo. There is redness and swelling around, and pus is usually formed, and mingles with the substance of the comedo. Sometimes there is not a proper suppuration, but the substance of the plug is softened by the inflammatory exudation. The inflammation is due to the irritation of the plug, especially when by long stagnation it has become foul.

The name Acne mentagra, or Sycosis, is given to cases where there is inflammation of the sebaceous glands in connection with the hairs of the beard. There is a sycosis of parasitic origin, but in the simple form there is suppuration in and around the hair follicles of the nature of acne.

A more chronic inflammation of the sebaceous glands of the face, with special hypersemia of the vessels around, has received the name Acne rosacea. There is very little tendency to suppuration, but great thickening of the skin occurs, so that sometimes there are large lobulated and red protuberances, especially on the nose, on the surface of which many comedones are seen.