Aneurisms very frequently open into canals, as the trachea, the bronchial tubes and their large branches, and oesophagus, and more rarely into the intestinal canal and the cavities of the urinary passages. They however very commonly open into other bloodvessels, either arteries or veins, and even into the cavities of the heart, more especially the auricles. Such openings very frequently occur in aortic aneurisms into the trunk of the pulmonary artery and its branches, and into the ascending or descending Vena cava.

Aneurisms that are imbedded in parenchymatous structures do not often open; the hemorrhage here takes place into the parenchyma, and after the latter has been extensively displaced or perforated in the form of a canal, the blood flows freely into the adjacent serous cavity.

We have seen one instance in which an aneurism of the aorta opened into a tuberculous pulmonary cavity having healed, consolidated walls.

Finally, aneurisms may sooner or later penetrate to the general investments, and open externally.

The manner in which aneurisms open is not the same in all.

Aneurisms which project into a serous cavity burst at that part which, having coalesced with the serous membrane, and become extremely thin from a deficiency of the surrounding tissue adapted to strengthen and protect it, offers the slightest degree of resistance in consequence of the excessive attenuation of its walls. The opening generally occurs at one of the most saccular portions of this wall, and is either in the form of a fissure, or more frequently of a roundish aperture having a fringed margin. The latter appearance induced Hasse to believe that the opening was preceded by a pre-existing and self-induced process of softening, but we have never been able to detect its presence in the numerous observations in which we have been engaged.

In those cases in which the aneurism bursts through the walls of the canals on which it is seated, and opens into their cavities, the process is more complicated. Thus aneurisms open into the trachea, the bronchus, and the oesophagus, when the fibro-cartilaginous and muscular elements, together with the adhering wall of the aneurism, are destroyed by detritus, in consequence of the mucous membrane becoming the seat of inflammation, and tearing in that condition with the aneurismal wall. In other cases, as for instance at the oesophagus, a gangrenous eschar is developed in the mucous membrane over the encroaching aneurism, and, by extending over the whole of the aneurismal wall, usually gives rise to extensive opening of the aneurism.

The opening of the aneurism into the cavity of the neighboring bloodvessels is brought about in various ways. In some cases the aneurismal wall coalesces with the cellular sheath of the adjacent artery in such a manner as to deprive the circular fibrous coat of the latter of its proper support. As the aneurism exerts a stronger impulse on the vessel, the cellular sheath becomes completely separated from the artery, and, consequently, the aneurism and its circular fibrous coat at length burst. The rent is in general large, and presents an angular form in this coat of the artery; it is usually complicated with detachment of the cellular sheath over varying extents of surface from the fissure. (See Dissecting Aneurism.) In other cases, the cellular sheath of an adjacent artery coalesces not only with the aneurismal wall, but also with the circular fibrous membrane of the affected artery, in consequence of a very chronic process of inflammation, and the slow development of the aneurism. The circular fibrous coat directly coalescing with the aneurism is thus rendered thinner, whilst its fibres separate from one another, and at last wholly disappear at different points. At the point which corresponds with the most marked protrusion of the aneurism into the artery, the aneurism bursts together with the layers of the lining membrane of the artery coalescing with its wall. The rent, as in aneurisms that open towards a serous cavity, is small, fissure-like, or resembles a roundish hole. - The opening of aneurisms into a contiguous vein is effected in the same manner. (Spontaneous varicose aneurism).

We occasionally find in aneurisms imbedded in parenchymatous organs and cellular accumulations, that there is an acute inflammatory process, which hinders the development of a protecting and strengthening callus, and by predisposing the tissue to softening and laceration above the pulsating spot, occasions laceration. In other cases the tissues are separated by the pressure, without the concurrence of any such inflammatory process, and thus give rise to the rupture of the aneurism.

When an aneurism opens outwards on the surface of the body the process depends, as in aneurisms that open into the mucous canals, on a high degree of inflammation in the true skin, occasioning a separation or laceration of the tissue, or on a gangrenous eschar implicating the general investments.

Such openings are very often rapidly formed and single, although occasionally we observe several small perforations, so that there is at first only a gradual and recurring oozing of blood, until the opening acquires a very considerable size.

Finally, the wall of a cylindrical, spindle-formed, or saccular (true) aneurism, frequently exhibits perforations which are owing to a laceration and detachment of the diseased inner coats from the cellular sheath of the vessel. But this is a subject to which we shall revert when we pass to the consideration of the spontaneous lacerations of arteries.

The above forms of aneurismal ruptures in ordinary cases produce death by hemorrhage, externally, or into one of the large serous cavities, or into the trachea, the alimentary canal, etc. When the aneurism opens into other vessels, such, for instance, as the arteries in the vicinity of the heart, or into any of the cavities of the heart, the result is in general speedy death, in consequence of the obstruction in the circulation. There are, however, exceptions to this rule; and we find that in some instances, small perforations of the latter kind may exist for a prolonged time without causing death, in which case the aperture through which the communication is maintained, acquires a smoothed, healed appearance, from its margin being invested with a recently formed lining membrane. This is more especially the case when the aneurism opens into a vein, and thus constitutes a basis for the formation of a so-called spontaneous varicose aneurism (Thurnam).