tution the periosteum of the tooth is subject to rheumatic inflammation, followed by thickening of the membrane and continued irritation. Most of the examples of thickening of the peri-dental membrane are, according to my experience, the result of rheumatic inflammation.

Diseases of the Alveolus.

The bony structure which encloses a tooth, and which is called the alveolus or socket process, is subject to the same classes of diseases as the jaw-bones themselves, of which, in fact, it forms a part. It is liable to suffer from inflammation, caries, necrosis, exostosis, absorption, and cysts. The alveolus is sometimes fractured in the operation of extraction of a tooth.

ERRORS IN DENTITION.

In the process of cutting the teeth, and in the development of the teeth,--whether the temporary or the permanent sets,—there are certain errors or irregularities which are noticed by our standard authority as forms of disease. These irregularities are: (a) An irregularity in the time of eruption of the temporary or permanent teeth, the irregularity consisting, usually, in delay in the time of eruption: (b) irregularity in the position of the temporary or permanent teeth, as when one tooth overlaps or displaces another (c) irregularity in the number of the temporary or permanent teeth: (d) irregularity in the form of the teeth of either set: (e) development of unnatural structures in the enamel, the dentine, or the cementum.

In addition to these irregularities there are also added changes in the jaws and teeth. (a) Unnatural development of the sockets of the teeth in relation to size or form: (b) defective growth of the lower jaw: (e) mechanical injuries of the sockets and periosteum, including fracture and hemorrhage: (d) mechanical injuries to the teeth themselves, such as fracture, dislocation, and, a not uncommon cause of caries, friction from pressure.

CHAPTER X.

DISEASES OF THE SKIN AND MEMBRANOUS SYSTEM.

THE membranous system includes, as we have already learned: (a) the covering of the body called the skin; (b) the lining of the external cavities of the bronchial surface, of the mouth, of the whole length of the alimentary canal, and of the bladder, called the mucous membrane; (c) the membranes which envelop the vital organs, as the brain, the heart, the lungs, the intestines, called fibrous and serous membranes; (d) the strong membranes which envelop the bones and line the joints, called periosteal and synovial membranes; (e) the connective or sponge-like web of membrane which connects all the organs of the body together, called connective or areolar membrane, or, more commonly, cellular or connective tissue.

We have studied the diseases affecting the mucous membranes, the serous membranes, the periosteal and the synovial membranes, in the descriptions of the diseases of the different organs and parts with which those membranes are connected. have only, therefore, now to study the diseases of the skin and of the cellular or connective tissue.

DISEASES OF THE SKIN.

We

The older medical authorities have been accustomed to divide diseases affecting the skin into nine orders, according to the external appearances presented to observation. By this method they described the following forms of cutaneous disease:

(a) The Papulæ or pimples.

(b) The Squamæ, or scaly diseases.

(c) The Exanthems, flower-like or eruptive diseases.

(d) The Bullæ, blisters or blebs.

(e) The Pustulæ, or pustular diseases, in which pus or matter is thrown out in points.

(f) The Vesiculæ, or vesicular diseases, in which small vesicles or water bladders are formed.

(9) The Tuberculæ, or tubercular diseases, in which hard swellings or tubercles are formed.

(h) The Macula or spots, spotted diseases of the skin.

(2) The Ulcerous diseases, in which there is break of continuity of the skin with loss of structure.

The authorities of the Royal College avoid this classification, and very properly throw out of their list the exanthematous eruptions, like those of measles or scarlet fever, which are merely signs of general diseases. I shall, as usual, follow the College order, without, however, ignoring the classification given above, which, in many respects, is practical, useful, and distinctive.

Erythema.

Under the term erythema is described redness of the skin of an acute form. It is commonly defined as "a nearly continuous redness of a portion of the skin, attended with some disorder of the constitution, but not contagious." Sir Erasmus Wilson gives another definition. He defines the especial characteristic of erythematous affections as "redness without exfoliation or desquamation (scaling), or other secondary change."

There are six varieties of erythema recorded in the official list. 1. Erythema læve, redness of a dark character, surrounding an old ulcer or large vein, or spreading over a limb affected with anasarca or dropsy. 2. Erythema fugax, the redness that suddenly suffuses the face and neck of nervous hysterical subjects. 3. Erythema marginatum, redness marginated or edged. 4. Erythema papulatum, redness very bright and in patches, with many red points or pimples, appearing on the limbs or the breast. The papulæ or pimples are usually small, and dispersed on the red surface, though distinguishable to the touch as well as the sight; but sometimes they are in clusters, and sometimes they themselves extend or increase from their edges. The affection is commonly acute and transient, lasting only a few days, but it may become chronic. 5. Erythema tuberculatum, a variety in which, instead of mere pimples, small raised swellings or tumors are present. Persons affected with tubercular disease of the lungs. are supposed to be most liable to this erythema, which generally comes on with some fever and constitutional disturbance. 6. Ery

thema nodosum, erythema in raised patches of round or oblong form, occurring on the inner side of the limbs, and looking like a spotted eruption. This variety of the disease is always attended, according to my experience of it, by dyspepsia and slight fever. It often recurs in the same person, and may last a week or ten days.

Intertrigo.

The term intertrigo is applied to define the red cutaneous eruption and soreness, resembling erythema, and by some classified with it, which is brought on by the rubbing of two surfaces of the skin together in their folds. The affection is apt to occur in persons who are very corpulent, and in fat infants, especially when they are not kept wholesomely dry and clean. The redness may pass into actual abrasion of the skin from the friction, followed by watery exudation.

Roseola.

Roseola is a word used to define a rose-colored rash on the skin which is evanescent in character and attended with slight fever. The rash may be in patches or general, and may resemble that of measles or scarlet fever, for both of which it is often mistaken by the unskilled. The disease is not accompanied by soreness of throat, is not contagious, and only lasts a few days. It commonly comes on from indigestion, and in children is often connected with derangement of the stomach from errors in diet, such, for instance, as are incident to children's parties. It may be acute or chronic, and it is said to present four varieties:1. Roseola æstiva, summer roseola; 2. Roseola autumnalis, autumnal roseola; 3. Roseola symptomatica, indicative or symptomatic; 4. Roscola annulata, annular like, or in form of a ring.

Urticaria. Nettle-rash.

The disease urticaria, though commonly called a skin disease, is in truth an affection of a general kind, attended with eruption on the skin as one particular symptom. The disease may be acute or chronic, and it is prone to recur in certain persons under conditions favorable to its development. The eruption on the skin is erythematous, with raised surfaces in wheals, or in round white points resembling closely the sting of the nettle. The skin

is often raised around the white point. The eruption is attended with much heat and irritation. When it attacks the palms of the hands or soles of the feet the burning sensation may be of the acutest kind, and, in the worst forms of the affection, excruciating. There is always some constitutional disturbance and dyspepsia, but there may be no fever, and the malady is certainly not contagious.

Urticaria is stated to appear under six different varieties:1. Urticaria febrilis, in which case it is accompanied with fever. 2. Urticaria evanida, a short or evanescent form. 3. Urticaria perstans, a lingering or persistent variety. 4. Urticaria conferta, in which the points of eruption are close or crowded together, confluent. 5. Urticaria subcutanea, where the skin is much inflamed and raised from beneath. 6. Urticaria tuberculata, where the erythematous surface is affected with small hard swellings or tubercles; a rare form of the disease.

Urticaria seems to be almost invariably connected with derangement of the digestive system. It is often accompanied with much disturbance in the stomach and bowels, and is relieved most readily and determinately by purgation. It often follows the taking of certain articles of food or drink, and is therefore fairly considered as due to intestinal irritation in most instances, and to peculiar action of certain foods on particular constitutions. In persons liable to it I have seen it follow the taking of lobster, mullet, mussels, pork, oranges, and whitebait. I have known one death occur during urticaria. It was caused by the mucous membrane of the larynx becoming implicated in the eruption, with suffocation as the result.

Pellagra.

The term pellagra is employed to describe a disease of southern Europe which is also known under other names, as the leprosy of the Asturias and as the elephantiasis of the Asturias. It is met with in Italy, in Spain, and in some parts of France. It commences as a slow cutaneous inflammation, not unlike erysipelas, and ends in hardening of the skin and accumulation of scales. The attack commonly comes on in the spring, goes through a distinct series of stages, and dies away in the following winter. It returns in a worse form the next spring, and still worse in the spring following. At last the skin is more or less covered with a