The tumours which are formed by a preternatural dilation of a part of an artery, as well as those swellings which are occasioned by a collection of arterial blood effused in the cellular tissue in consequence of a rupture or wound of the coats of the artery, but having no outlet in the integuments, receive the name of aneurisms. According to these opinions, then, aneurisms are of two kinds, the first being termed true, the second spurious or false.

Some writers reckon another form of aneurism, which is said to happen when the external coats of an artery being weakened by mechanical injury or disease, the internal coat protrudes through the breach in the outer coat, so as to form a tumour distended with blood.

An aneurism may then be defined to be a tumour filled with blood, either in a fluid or solid state, usually attended with pulsation, and the sac of which has an opening in it by which it communicates with the artery from which the blood is transmitted into it. Another definition is, that it is a tumour filled by arterial blood and communicating with an artery, and divides all aneurisms into traumatic and spontaneous, according as they happen to be produced by a wound or disease of the coats of the artery.

Now when any part of an artery is dilated (attended with particular circumstances marking its difference from another form of dilation which, as I shall explain, perhaps ought not to be set down as aneurismal), the swelling is commonly named a true or genuine aneurism. In such cases the artery is either enlarged at only a small part of its track, and the tumour has a determinate border, or the vessel is dilated for a considerable length, in which circumstances the swelling is oblong, and loses itself so gradually in the surrounding parts, that its margin cannot be exactly ascertained. The first case, which is the most common, is termed the circumscribed true aneurism, the last the diffused true aneurism.

Again, when blood escapes from a wound or rupture of an artery into the adjoining cellular substance, the opening in the skin having closed, the swelling is denominated a spurious or false aneurism. In this instance, the blood either collects in one mass, distends the cellular substance, and condenses it into a cyst, so as to form a distinctly circumscribed tumour, or it is injected into all the adjacent cellular substance, and extends along the course of the great vessels, from one end of the leg to the other, thus producing an irregular oblong swelling.

Now, in true aneurism, the coats of the artery are not always in the same state, the kind of changes observed depending upon the progress of the tumour. In the early stage of the disease, either the whole cylinder of the vessel or only a part of the circumference is dilated, but this period is generally of short duration, especially in arteries of middling size, because their middle coat is capable of less resistance than that of the larger arteries. At length, in consequence of the increasing distension, some of the coats of the artery possessing the least elasticity give way, and these are found to be the internal and middle coats, while the external one still makes resistance, and continues to be more and more dilated by the lateral impulse of the blood. A false aneurism is always attended with at least a rupture, or giving way of the inner coat of the vessel, and usually with breach in both this and the muscular coat, the outer elastic tissue forming the pouch in which the blood collects. But after the swelling has attained a certain size, this coat also bursts, and then the blood either becomes diffused, or a large circumscribed space is formed for it by the condensation of the surrounding cellular membrane. False aneurisms, when produced by a wound or puncture, are of course from the first attended with a division of all the coats of the vessel This form is often seen in horses and cattle that are bled.

The Treatment Of Aneurisms

Now, the grand means most to be depended upon for curing aneurisms, is tying the artery above the tumour. This more certainly prevents the great ingress of blood into the sac, and, what is quite as important, more certainly excites adhesive inflammation within the tied part of the vessel, and by holding the opposite sides of it steadily in contact, brings about their union, and an obliteration of the tube of the vessel with tolerable regularity. The chief current of blood into the sac is thus stopped, the contents of the aneurism are afterwards gradually absorbed, and the tumour dwindles away in proportion. The natural course of the blood being now permanently interrupted in the arterial trunk, it passes more copiously into the collateral branches, and these enlarging and anastomising with others which originate from the large arteries beyond the obstruction, the necessary circulation is carried on. It appears then, from these facts, that the obliteration of the artery, for a certain extent above and below the tumour, forms the primary indication in the radical cure of aneurism, whether compression or the ligature be employed; all other means are only auxiliary.

Internal remedies may be useful, inasmuch as they tend to moderate the determination of the blood towards the place where the artery has been tied or compressed.