(From Aneurisma 686 to dilate much,) called also hematocele arteriosum, abscessus spirituosus, emborysma. See Abscess.

The aneurism is a tumour arising from the dilatation or rupture of the coats of an artery. Arteries only are the seat of this disorder; and any artery, in any part of the body, may be thus affected, as any vein may be the seat of a varix. It is defined a soft pulsating tumour upon an artery.

Dr. Hunter divides aneurisms into four kinds; viz. the true, the false, the mixed, and the varicose.

First, of the true Aneurism.

The true aneurism is formed by a dilatation of the artery. It may happen in any part of the body, but most frequently is found in the curvature of the aorta, which is subject to this disorder from the extraordinary impulse of the blood; from the curvature it runs upwards along the carotids, or the subclavians, generally increasing, till by its great distention it is ruptured, and the patient dies.

The degrees of the dilatation of the aorta, in cases of this kind, are various; in some the curvature of this artery hath been so enlarged as nearly to fill the upper part of the breast. It is singular, that the part of the vessel which is the weakest, and where the disease begins, is apt to be stretched more in proportion than other arteries; and to form particular cells, where they meet with firm resistance, more than where their support is soft and yielding.

The sac formed by the distention of the artery is not a distention of a particular coat, but of the whole substance of the vessel; but the thickness of the coats of these sacs continues only to a certain period; for when the vessels of the coats can no longer yield, the circulation grows languid, the sac becomes thinner at its apex, and soon after bursts. As the aneurismal tumour also increases in size, it meets with resistance from the neighbouring parts; and as the coats will be more or less affected, according to the degree of the resistance, in some places they will be simply distended, in others absolutely destroyed. Where the aneurism presses against the diaphragm, it will be thinner than where it suffers no pressure; it is still thinner where it presses against the tendinous part of this muscle; and where it presses the spine, it is the soonest destroyed. A proof that all pressure must be avoided in such cases.

The blood that fills these tumours is always fluid, by being constantly renewed; but, notwithstanding this blood is fluid, its passage in the tumour is retarded; and this remissness in its motion, which is more or less considerable according to the size of the aneurism, occasions some of the fibrin of the blood to separate from the red part; and adhering to the internal coat of the aneurism, it there forms fibrous strata, which may easily be taken for real membranes by those not accustomed to observe them. These fibrous strata cannot be dispersed by any means, either external or interna!; and pressure cannot be used, because it will destroy the coats of the artery.

Secondly, the False Aneurism, called Ecchymoma Arteriosum,

Is formed by a rupture or wound in the coat of the artery, and is of two kinds, viz. the diffused and the circumscribed.

The diffused is that in which the extravasated blood runs through the cellular membrane, in the interstices of firmer parts: this generally makes a rapid progress, may extend itself to a great distance, and hath little or no pulsation except very near the aperture of the artery; but these circumstances will somewhat vary, according to the size of the opened artery and the strength of the circulation. This species of false aneurism is analogous to the emphysema, and is the highest species of ecchymosis.

The circumscribed tumour beats, and sinks under pressure, like the true aneurism; and indeed cannot be distinguished from it except by the knowledge of its cause, or by a careful dissection of the part: it appears soon after the accident which gave rise to it, and is commonly slow and gradual in its progress. It happens when the orifice in the artery is very small, so that the blood flows but slowly, and finds the adjacent membranes so firmly united as to keep it within a certain channel. It consists of one bag with a smooth inside, and communicates by an aperture with the cavity of the artery. This species of aneurism is, perhaps, the most common among those that happen in the arm after bleeding, especially when a considerable pressure hath been made immediately after the accident.

Thirdly, the Mixed Aneurism,

Is formed partly by a wound or rupture in the artery, and partly by a dilatation. It cannot easily be distinguished from the circumscribed species of the false aneurism; and will often so emulate the true one, as not to be distinguished from it but by a careful dissection.

Fourthly, the Varicose Aneurism, or the Aneurismal Varix,

Occurs when there is an anastomosis, or an immediate communication between the artery and the vein of the part where the patient hath been bled, in consequence of the artery being wounded through the vein, so that blood passes immediately from the trunk of the artery into that of the vein, and so hack to the heart.

This species differs from the common spurious aneu-rism in one circumstance only, viz. the wound remaining open in the side of the vein as well as in the side of the artery. But this circumstance will occasion a great difference in the symptoms, the tendency of the complaint, and in the proper method of treating it.

Mr. Bell, in his System of Surgery, divides the aneurism into two species, viz. the encysted, and the diffused. The encysted includes all those instances in which, the coats of the artery being only dilated, the blood is confined in its proper coat: of this kind he reckons the varicose aneurism. The diffused includes all those in which, from an aperture in the artery, the blood is spread in the cellular membrane out of its proper course.