This is the process by which sores or ulcers are produced in animal bodies. In this operation, the lymphatics are commonly believed to be at least as active as the bloodvessels; an ulcer being, according to the doctrines most prevalent, a chasm formed in some surface of the body by the removal of parts back into the system by the action of the former vessels. At first it may be difficult to conceive how a part of the body can be removed by itself; but to the renowned Dr. Hunter there was no more difficulty in conceiving this, than how the body could form itself - both facts seemed to him equally well confirmed. When it becomes necessary that some whole living parts should be removed, it is evident (says he) that nature in order to effect this object must not only confer a new activity on the absorbents, but must throw the part to be absorbed into a state which yields to this operation. The absorption of whole parts in disease are referable to five causes; namely, pressure, irritation of stimulating substances, weakness, inutility of parts, and death of them.

Ulceration takes place more readily in the skin, mucous, cellular, and adipose tissues than in muscles, tendons, ligaments, nerves, and bloodvessels. Hence in the progress of pus to the surface of the body, in consequence of the intervention of textures backward to ulcerate, ulceration often takes a circuitous course for the purpose of bringing the matter to the skin.

Again, parts at a considerable distance from the source of circulation are generally more disposed to ulcerate than others situated nearer the heart, hence one reason of the greater number of ulcers on the lower extremities. Here, however, another cause is likewise concerned, and that is the retardation and stagnant state to which the blood in the veins of the lower extremities is so frequently subjected. In this instance the probability is that ulceration does not take place till the venous blood, accumulated largely in the capillary vessels, excites by its presence an irritation like that resulting from the presence of a foreign body. Now, there are three modes in which texture seems to modify the nature and course of this process; firstly, as it occurs in highly vascular structures; secondly, in parts possessing a somewhat lower degree of vascularity; and thirdly, as it is observed to take place in parts endowed with the least degree of organisation.

The remarkable disposition to ulceration in those textures that are well supplied with blood must be obvious, if not familiar, to all interested, more especially in the mucous membranes. These parts abound in vessels of large size, and are liable under moderate degrees of inflammation to pass into the ulcerative state. The mucous lining of the intestines, holding a first rank among vascular structures, quickly ulcerates under some forms of inflammation; that of the trachea (or windpipe) being somewhat less vascular, is less prone to ulceration. The lining membrane of the mouth speedily exhibits an aphthous surface (this being well defined in epizootic aphtha - foot-and-mouth disease), or even a deeper extent of ulceration from trivial causes; and the gums, disposed as they are to ulcerate, have this disposition still further increased when they become spongy and more vascular. Now, when ulceration takes place in consequence of the death of an external part, it occurs first on the outer edge between the dead and living substance.

A tumour, when it makes equal pressure in every direction around, will only make its way in an external course, because interstitial absorption happens in no other direction.

The parts situated between an abscess or any extraneous substance and the nearest surface are those which are most susceptible of ulceration. This is one of the most carious phenomena connected with the process under consideration. It shows that there is a principle in the body by which parts are always prone to free themselves from disease. Slight pressure from without will often produce a thickening of parts; consequently there appears to be a corresponding backwardness to admit disease, because, if the lining membrane of the nose becomes thickened, it forms a barrier against the progress of the disease inwards.

Again, that pressure is a frequent cause of ulceration, is also evinced by the occasional effects of chains and harness on horses. But whatever be the lesion preceding ulceration, it must not be forgotten that its production, though the work of acute or chronic stimulation, does not depend upon such stimulation alone, for by varying the degrees of the latter a texture cannot always be made to ulcerate at option.

The causes of ulceration are special conditions, which consist neither in the intensity nor in the duration of the irritation by which the process is constantly either preceded or accompanied. Frequently an extensive ulceration originates from a very slight, transient, and scarcely appreciable irritation; while in other instances a most intense stimulation, such as is produced by the passage of concentrated acid into the stomach, or a stimulation of very long continuance, such as exists in the alimentary canal when a patient is afflicted with a diarrhoea of long standing, does not give rise to any ulceration. There are, moreover, cases in which ulceration cannot be regarded as the simple result of a local affection; and, like many other lesions of the circulation, nutrition, or secretion, it is only one of the modes in which a general morbid state of the system is manifested, the existence of which is revealed by local lesions of the most different kinds in respect to their seat and apparent nature.

Absorption with suppuration, or, in other words, ulceration, either happens in consequence of suppuration already begun, in which event the pus acts as pressure, or else absorption attacks external surfaces from particular irritations or weakness, in which case suppuration must follow.

Again, the production of ulceration requires much greater pressure from without than from within. The process is always disposed to take place more quickly when the cause is near the surface of the body, and its progress becomes accelerated in proportion as it arrives near the skin.

The adhesive inflammation precedes the suppurative, and prevents the pus from becoming diffused as soon as it is secreted; and when the cyst afterwards ulcerates, in order to let the matter approach the skin, the adhesive inflammation still continues to go before the ulcerative process, and thus prevents the matter from insinuating itself into the interstices of the cellular substance.

Now, the pain of ulceration is in some degree proportioned to its quickness. When ulceration begins on a surface, or. takes place for the purpose of bringing matter to the skin, the pain is always considerable; but when ulceration takes place in order to separate a dead part, as in sloughing, exfoliations, etc., there is seldom any great degree of pain.

The ulcerating sore always exhibits little cavities, while the edge of the skin is scalloped and thin, at the same time turning a little out and overhanging more or less the ulcerated surface. The face of the sore appears foul, and the discharge is very thin. When ulceration stops, the edges of the skin become regular, smooth, a little rounded or turned in, and of a purple colour, covered with a semi-transparent white.

Again, ulceration may be looked upon as a degeneration of tissue, a change in the affinities existing between its component parts, by which it becomes changed from a solid organised texture to a fluid inorganic mass. It differs from gangrene in being a vital action, while gangrene, by at once producing death in a part, prevents any such change taking place. In gangrene the supply of blood to the part ceases altogether, while the integrity of tissue is preserved; under ulceration the circulation in the vessels continues during the action, and the part still belongs to the living mass, and remains under the influence of vital action until its separation is completed.

Ulcers of all kinds, if closely watched in their formation, clearly exhibit the breaking up of the tissue, and its gradual conversion into pus. In the earliest stage of an ulcer, before the vesicle has burst or the skin given way, the extent of substance lost is compensated for by the amount of fluid formed. In its progress, when rapid, and when a large portion of structure is quickly destroyed by ulceration, a corresponding quantity of pus may be always seen to occupy its place; and when the action is more chronic, as in fibrous structures, the debris of the tissue can be seen mixed with the purulent fluid. These observations of the process by which a dead is separated from a living part, will prove that this, like others that have been improperly attributed to absorption, is an act in which the absorbents do not take a part.