The causes of aneurisms are various. A natural weakness in a part of an artery is the immediate cause of the true aneurism. The internal causes are, a fulness of the arteries concurring with some violent motion or concussion ; an internal tumour pressing on some part of an artery ; or violent action, sudden anger, vomiting, etc. which propel the blood too forcibly to some particular part. By stretching the artery, a true, or by bursting it, a false aneurism, or the mixed one, will be thus formed: convulsions, and other violent spasmodic symptoms, with other concurring causes, may be ranked in the number of internal causes. In some instances the coats of the artery are unusually and irregularly weak. We then find aneurisms through the whole arterial system; and the predisposition to this complaint is styled diathesis aneurismatica. Externally, strains, blows, and punctures, are the most frequent causes: pressure used on a true aneurism, by bursting the coats of the artery, produces a false one; suspending the breath, as in lifting great burthens, wrestling, etc. may occasion either.

It hath been said that a polypus, existing internally, sometimes occasions an aneurism; but Dr. Hunter observes, that it rarely or never happens that a polypus is formed till the last moments of life, when the heart's power having nearly ceased, the whole blood cannot be propelled from the heart; which, when found after death, has been supposed to have pre-existed, and to have been the cause of what it was only the effect.

There is no certain criterion by which to ascertain the existence of internal aneurisms before they approach to the surface of the body; whatever symptoms they occasion before they form a tumour externally, as they may be produced by other causes, are but equivocal signs. The pathognomonic sign of all the species of aneurism is, a perceptible pulsation in some part of the tumour, more or less manifest, as the artery is seated superficially or deep; yet we shall find, that any other tumour receiving pulsation from an adjoining artery will sometimes resemble it; and when the effused blood is in large quantity, it will obscure the pulsation. The true aneurism is generally of an oblong figure, and hath a strong pulsation in it ;.it subsides on depression : if i' is an aneurism of the aorta, a strong pulsation is perceived against the sternum and ribs on every systole of the heart; and when it extends above the sternum, there is a tumour, with pulsation. These tumours are without pain or discoloration in the skin, except on the point of bursting; they subside by pressure while the blood is fluid, but when it is coagulated they yield in a very slight degree; if the sac has a narrow basis, the blood re-enters the artery with.a hissing noise when the tumour is pressed. Sometimes there is a redness from the expansion of the parts beyond their capacity, or a dark colour from the putrefaction of the blood, in which case a fever and fainting also occur. The common appearances of an aneurism from the wound of a lancet are a discharge of blood through the orifice of the skin, by jerks, instead of an uniform stream; and on stopping the orifice the blood spreads between the

Q muscles of the shoulder and arm, constituting the dif-fused aneurism: in this case the arm becomes livid from the ecchymosis, and the blood coagulating, obscures any sensible pulsation.

In the false kinds of aneurism, the cyst is probably formed of a portion of the aponeurosis that runs over the vessel, which, from extravasated blood underneath, is thickened and expanded: that this membrane is the cyst, seems to be confirmed by our so readily discovering the puncture in the artery upon opening the tumour; or it may be formed of the cellular membrane, which admits both of thickening and expansion.

In the varicose aneurism, the vein that was punctured will become varicous, and will have a pulsatile jarring motion, on account of the stream from the artery; there will be a hissing noise, which will be found to correspond with the pulse; and the blood in the tumour will be almost entirely fluid, because it is kept in constant motion: it is soon formed to its largest size, and so remains, if not disturbed by imprudent management: no considerable inconveniences arise from it. This sort of aneurism may be further known by placing a finger over the orifice in the artery, where the stream of blood propelled into the vein at every pulsation is felt: by applying the ear to the tumefied vein, a tremulous motion and noise are perceived; by pressing the corresponding artery, this motion and noise cease; and on the removal of this pressure, the motions, etc. return; the artery becomes larger in the arm and smaller in the wrist; the vein being emptied by pressure, instantly fills again on taking the pressure off; the pulse at the wrist grows weaker as the artery above enlarges.

The beginning aneurism, in the aorta should be distinguished from a palpitation of the heart; from hysterics, in which symptoms of suffocation sometimes attend; from fever with fainting, both which are sometimes the consequence of false aneurism; from varices of the veins and their effects; from an emphysema; from an ecchymosis; from encysted swellings in the neck, in which is often perceived a strong pulsation from the stroke of the adjacent artery; and from tumour formed in consequence of ruptured veins.

The aneurism of the aorta may prove fatal by injuring the general health, as it continues to increase in its size; it may be supported during many years, but no cure can be attempted, nor other palliatives used than what consist in composure of mind and quietude of the body. All aneurisms arc incurable that lie too low for theoperation; and, if unadvisedly opened, the patient's is in immediate danger; for bandages, which are the only palliatives in such cases, are but uncertain aids. The diffused aneurism is not only subject to haemorrhages, but also to mortification.