Another circumstance of importance is the Localization of aneurysms in different arteries. Nearly half the cases of aneurysm occur in the aorta, and the great majority of these in the thoracic portion. We have seen that atheroma is most frequent in this vessel, but, in addition to that, the aorta is most exposed to the excessive pressure of the blood when the heart is stimulated to unduly forcible action. Next to the aorta the popliteal artery is most frequently the seat of aneurysm. It has already been pointed out that this vessel is especially liable to injury from sudden flexures of the leg, especially when the middle coat is rendered brittle by calcareous infiltration. But besides that, the vessel is so situated as to be liable to localized increase of blood-pressure. As it issues from the popliteal space the artery passes between the two heads of the gastrocnemius, and is liable to constriction by the contraction of the muscle. On this principle has been explained the frequency of popliteal aneurysm in flunkies whose principal occupation is to exercise their gastrocnemii in standing. But apart from that, when a person makes a severe exertion in a standing posture, the gastrocnemii contract vigorously, thus producing a partial obstruction of the artery and an increase of pressure above the obstructed part, while the general blood-pressure is also increased.

Frequency Of Aneurysms

The preceding table from Crisp gives a statement of the frequency of aneurysms in different situations. It is to be observed, however, that it greatly underestimates the number of aneurysms of the cerebral arteries, and also those of the pulmonary artery in phthisis pulmonalis.

(B) The Coats Of The Artery In .aneurysms

An aneurysm begins as a localized dilatation or as a little pouching of a limited portion of the artery. In the former case we have the commencement of a fusiform, and in the latter case that of a sacculated aneurysm. The little pouch which forms the commencement of a sacculated aneurysm gradually enlarges, and while its opening remains small, it enlarges outwards in all directions into a distinct sac. The walls of the sac are sometimes folded back around the aperture so as to apply themselves to the external surface of the artery, and these may become mutually adherent. In that case the aperture has a sharp edge, and it appears as if the wall of the artery were folded over so as to form the wall of the aneurysm.

The Internal coat enters variously into the constitution of the aneurysmal wall. In the case of a fusiform aneurysm it is continuous over the internal surface, and probably presents very marked atheromatous changes. In the sacculated form it is usually to be traced some little distance from the edge of the aperture on the wall of the aneurysm, and even in the midst of the internal surface of the sac pieces of internal coat, greatly altered as a rule, may be discovered.

The Middle coat, as we have seen, undergoes atrophy in sacculated aneurysms. Even in fusiform aneurysms it is often difficult to trace the middle coat far from the beginning of the dilatation.

The External coat, on the other hand, may be regarded as forming the chief constituent of the sac. We have already seen that at the very commencement of a sacculated aneurysm inflammatory new-formation occurs in the wall of the vessel, the vascular tissue being chiefly derived from the external coat. As dilatation proceeds the new-formation of tissue goes on, so that the sac does not necessarily undergo thinning as it enlarges. The connective tissue of the external coat usually makes common cause with that of neighbouring structures, and so the aneurysm acquires adhesions, and the surrounding structures come to form, to a certain extent, constituents of the sac.

(C) Thrombi In Aneurysms

Blood-clots are of nearly constant occurrence in aneurysms, and they may almost be regarded as constituents of the sac, as they doubtless aid to a great extent in preventing rupture. Coagula are most frequent and most important in sacculated aneurysms. We meet with them in the form of firm, dry layers, which present a distinct stratification, generally parallel to the wall of the aneurysm. The coagula often form a kind of sac inside the proper sac, and after removal retain the shape of the aneurysm. The coagula are primarily White thrombi. The white blood-corpuscles adhere to the rough internal wall of the aneurysm, and after they have accumulated to some extent coagulation occurs and a thrombus is formed. This process is repeated, and the formation of fibrine is thus in successive layers. Not infrequently the layers become partially separated, and the blood insinuates itself between them. A layer of whole blood is thus formed, and when it coagulates we have a red thrombus mixed with the white. As time goes on the clots become firmer, dryer, and more stringy. The layers next the sac are often of a pearly whiteness, and may be taken for connective tissue. Under the microscope, however, they are seen to be devoid of definite structure, and acetic acid brings out no elongated nuclei as in the case of connective' tissue. There seems little tendency to the organization of these coagula unless the whole aneurysm becomes filled and its cavity obliterated by them. The continual distension of the cavity seems to interfere with the process of organization; but, if the cavity be obliterated by the formation of clots, then organization proceeds, as in the case of thrombosis in an artery, and by and by the aneurysm is converted into a connective tissue nodule which contracts more and more.

(d) The condition of branches given off at the seat of an aneurysm is a matter of great importance. These vessels are frequently obstructed, and there are various ways in which this may come about. The atheromatous process may occur to such an extent around the orifice of a branch as to narrow or even obliterate it. This is most frequent in small arteries like the intercostals, but is not uncommon in larger branches. Again the coagula may come to overlie the aperture, already narrowed, it may be, by atheroma. Further, it will be apparent that, as an aneurysm enlarges, especially a sacculated one, it will often drag on and contort vessels whose apertures are in or near its walls. The aperture may thus be reduced to a fissure, and the edge may be so placed as to valve the aperture. This is particularly the case in the sacculated aneurysms of the arch of the aorta. Sometimes by the enlargement of an aneurysm the aperture of the branch is to be found at the summit of the aneurysm. In that case the aperture may be obstructed in one of the ways already described, but it not infrequently remains at least partially free. The coagula may even be tunnelled in order to allow the current to flow into the branches (see Fig. 252). Another mode of closure is by the pressure of the aneurysm itself on the branch beyond its aperture. If a branch be closed in any of these ways it becomes the seat of a thrombus, and in the usual way becomes converted into a solid cord. In regard to the condition of branches it is to be observed that the aneurysm may, as it were, be continued into the branch, the first part of the latter being dilated along with the aneurysm.

Fig. 252. - Aneurysm of the abdominal aorta with clots tunnelled so as to allow the blood to reach the branches. The general lie of the stratification of the clots is shown. The coeliac axis and superior mesenteric artery are seen to be narrowed at their orifices. The posterior wall of the aneurysm is absent where it impinged against the vertebras. Half the natural size.