Abscess (Lat. abscedere, to separate), a collection of pus in a circumscribed cavity. (See Pus.) This cavity is usually of new formation, produced by the separation and destruction of the parts by the matter effused, the wall consisting of a layer of thickened tissue. The name is given, however, to collections of pus in some of the naturally existing cavities; e. g., that in the upper jaw. An abscess may be acute or chronic according to the character of the inflammation which produces it. The formation of an acute abscess is indicated by pain, often of a throbbing character; redness, if it be not too deeply seated; swelling; and an increase in the temperature of the part. The patient also suffers from fever. When matter has formed and is sufficiently near the surface, its presence is made known by the feeling of "fluctuation," produced by placing the fingers of one hand upon one side of the swelling and with those of the other giving a smart tap upon the opposite side. The pus usually tends toward the surface, which it reaches by a gradual thinning of the intervening tissues; but if these be very resisting, it may travel in other directions.

It is in these latter cases that an early incision by the surgeon is especially called for, before "pointing" has taken place. - If an abscess be threatened, its formation may sometimes be prevented by appropriate treatment, such as rest, the local abstraction of blood, and cold or warm applications, as the case may demand. Should these means fail, poultices must be used to promote suppuration. The matter may be evacuated by incision, or in certain cases be allowed to make an exit for itself. In a chronic abscess the pain, redness, increased temperature, and fever are often absent, and hence it is also knovn as cold abscess. In most cases its progress is slow, and it may remain for a long time without increase in size, or any tendency to open through the skin. Indeed, in some instances it may disappear by a process of absorption. Usually, however, it is necessary to open it, which may be done by cutting directly into the cavity, or by what is known as subcutaneous incision, the knife being passed for some distance beneath the skin before it enters the abscess. This latter procedure is made use of in order to prevent the admission of air, which in some cases excites an amount of suppuration sufficient to exhaust the patient.

Besides acute and chronic, abscesses are spoken of as being " by congestion " when the matter, usually dependent on caries, makes its appearance at some distance from the diseased part; as "idiopathic," when the cause is not known, etc.