2. The true aneurism is when all the coats of an artery, in one particular part of its cir cumference, are so far deprived of their elastic properties as to yield, become distended, and form a pouch, in which the contained blood is similarly circumstanced.
3. It is not difficult to conceive that the sac of a true aneurism, as just described, will not long endure its state of unnatural distension before its internal and fibrous coats either ulce rate or rupture, and then an aneurismal sac is formed, consisting in one part of all the coats of the dilated vessel, and in the other of the cellular tunic alone. This is obviously a mixed form of aneurism.
4. When there is a wound, rupture, or ulceration of all the coats of an artery, in such wise as to permit the escape of the blood into the adjacent cellular tissue, a diffused aneurism is formed. This, for reasons that need not explanation, will be most frequently observed to succeed to wounds or punctures of vessels, but it may also be the consequence of an acci dental rupture of the sac of a circumscribed aneurism allowing the blood to pass through it, and spread itself (as it generally does) in every direction throughout the loose cellular tissue of the entire limb.
5. A direct and immediate communication may be established between an artery and a vein lying close upon it, as by the passage of a lancet transfixing one vessel and entering the other. This is the aneurismal varix, obviously occurring with greater frequency as the result of a wound, but nevertheless occasionally seen as the product of disease.
6. A. small circumscribed aneurismal sac has been found situated between an artery and vein so transfixed, communicating with both, and permitting a transmission of blood from one vessel into the other. This variety has been named the varicose aneurism.
7. A portion of blood may be contained within a new and diseased formation of cellu lar structure, the precise nature of which is not understood. The trunks of the arteries in the neighbourhood are neither distended nor ruptured, and the blood within it passes through the general circulation, and of course does not coagulate. It is difficult to class this disease with aneurism in any form, yet is it termed the aneurism by anastomosis.
No part of the natural history of any disease can be more interesting than that which has reference to its causes, whether predisposing and remote, or immediately exciting. Cer tainly, when an aneurism has been formed, a knowledge of the circumstances that occa sioned it will not be very useful in contributing to its removal, although it may often assist in forming a prognosis as to the result or an operation : yet if it can be made available in the prevention of the disease, it must prove of no inconsiderable value. It is admitted that aneurism frequently appears suddenly as the result of a blow, a strain, or some violent exer tion, the patient being conscious of something having torn or• given way within him. With
still greater frequency it occurs without any such consciousness on the part of the sufferer, and persons have borne this formidable disease about them for months, and even for years, not only without being themselves aware of its existence, but, if situated internally, without its being recognized by their professional at tendants;'' and it often happens that a patient complains of the crookening of the fingers or the numbness of the foot, unmindful of the tumour under the clavicle or in the popliteal space. Without denying that an artery, in a perfectly healthy condition, can become the seat of aneurism, because there are too many facts apparently in support of such an opinion, it may be remarked that if such was generally or even frequently the case, the disease ought to be much more common amongst the labouring poor, and also that it should prevail amongst some particular trades. These considerations lead to a belief, that previous to the occurrence of spontaneous aneurism, the artery has under gone some change predisposing to it, although it may not be so easy to point out the nature of that change, or the causes that lead to its production.
It is observed that aneurism is of far less frequent occurrence in woman than in man ; a comparison between the numbers of internal cases proving this fact in a remarkable manner, and in cases of external aneurism still more so. It is very rare to meet with a popliteal aneu rism in a female. Certainly, the more labo rious habits and constant exposure to accident in the one sex may in some respects serve to account for this circumstance, but to those who know that in many places women are obliged to undergo at least as much hardship and fatigue, the explanation will be far from satisfactory. Again, it has been stated that certain pursuits of life predispose to aneurism, inasmuch as it prevails amongst coachmen and postilions, but there never has been even a plausible reason offered to explain this greater liability of particular callings. It cannot be the bent positions of the limbs of such per sons, because many other classes, studious persons for instance, maintain similar postures for a longer time and with greater frequency, yet is not aneurism common amongst them. Neither will the sudden stretching of the limb by pressing the foot against the stirrup or foot board in managing the horses throw any light upon the subject, for it is found by experiment that no force will rupture a healthy artery short of what would also tear asunder the ligaments of the adjacent joints. Allowing, therefore, the accuracy and truth of these observations, their explanation is still to be sought for.