The Cavity Of The Aneurism very frequently contains fibrinous coagula, which usually form very distinctly stratified masses. The external and older layers consist of a whitish fibrinous substance, generally more or less deprived of color, and of a faded appearance. They are dense, compact, tough like leather, and dry. The inner layers constantly become looser, more moist, and colored, until at length the innermost - those of most recent formation - resemble a recent coagulum of blood. The fibrinous layers frequently exhibit many other conditions of great interest. Thus, for instance, the external, denser layers become, in some cases, converted into a whitish, callous texture, which coalesces with the wall of the aneurism, and very considerably strengthens it. In some cases they present an ossification similar to that which appears in the fibroid exudations, as, for instance, on the serous membranes; while, at other times, they are observed to be softening into a yeast-like yellow or whitish pulp, or a cream-like fluid.

As an important phenomenon which is often presented, we may notice that a recently formed layer of the lining coat of the vessel is inserted at different points between the strata of the fibrinous coagulum, giving the whole mass the appearance of being invested with such a membrane, which is then prolonged into the deposit investing the interior of the vessel. We here discover the means employed for restoring and maintaining the continuity of the vessel by closing the cavity of the aneurism with a new layer of the lining membrane of the vessel.

The fibrinous layers in the aneurism fulfil, therefore, no other purpose than that of assisting mechanically to maintain the coagulation of the blood and of its fibrin. They are not the product of an inflammatory process in the wall of the aneurism, nor do they exhibit the character of a malignant growth.

These coagula are not present in every aneurism. As a general rule it may be assumed that, without reference to the size of the aneurism, they will be present in large quantities, in proportion to its distance from the axis of the blood-current, and to the smallness of the communication between the cavity and the calibre in the vessel, when compared with the size of the aneurism. Hence we see the reason why fibrinous coagula are so much more readily and extensively deposited in aneurism of the pedicled form, which presents these two requirements in the most highly developed degree, and why their formation becomes the less easy when the aneurism differs from the above and approximates to the spindle-shaped or cylindrical variety. In these and saccular aneurisms, the formation of the coagula depends only on the extent to which the wall of the pouch recedes from the axis of the blood-current. It is, moreover, natural that fibrinous coagula should occur in larger quantity in large than in small aneurisms of the same form.

These fibrinous coagula derive importance from the obstruction they oppose to the rapid increase of the aneurism and to its early bursting, and in consequence of their causing a wasting of the aneurism, and thus inducing its spontaneous healing.

2. These aneurisms differ very considerably in their dimensions, as has been already observed, varying from the size of a pea or bean to that of a man's head, and thus occasionally filling up the greater part of one of the large cavities of the body. In general, the largest aneurisms occur on the large arteries, more especially on the trunk of the aorta; but there is no invariable proportion observed between the size of the aneurism and the calibre of the vessel, for aneurisms fully equal in size to those which occur on the trunk of the aorta, are occasionally met with in vessels of inferior calibre, as, for instance, the femoral and popliteal arteries.

Large aneurisms experience a very extensive alteration in the construction of their walls, to which sufficient attention has not been paid. Until the aneurism has acquired a certain degree of enlargement, it retains its primary wall, whose composition we have already described; but when the aneurism exceeds these limits, and the wall is no longer equal to the expansion, its place becomes supplied by adventitious tissues and structures, either over the whole extent, or at more or less sharply-defined spots, corresponding to the direction of the increase in volume. These are the structures with which the aneurism is in contact during its increase, and with which it gradually coalesces. This circumstance explains the reason why aneurisms, which only increase very slowly in volume, and therefore are only gradually brought in contact with structures able to compensate for the loss of substance of their walls, may attain so great a size, whilst those aneurisms which are rapidly formed and enlarged, and are, therefore, not brought in contact with many of these structures, speedily burst. These adventitious products are accumulations of cellular substance, serous and fibrous membranes, muscular expansions, etc, together with parenchyma, as, for instance, that of the lungs.

We must distinguish between the manner in which aneurisms of great size lose their primary wall, and the loss arising from the result of detritus - the absorption occasioned by pressure where the aneurism is in contact with bone. Thus we find, that where aneurisms, even of very inconsiderable dimensions, are in contact with bone, the aneurismal wall, together with the periosteum, is partly destroyed and the bone exposed.

3. We have already considered all the essential points in reference to the form of these aneurisms. It will be evident that the vicinity of resisting structures may, in various ways, modify the form of the aneurism during its growth. Thus aneurisms on the descending aorta occasionally assume a bilobar form posteriorly, in consequence of the resistance offered by the vertebral column, which causes it to separate into two sacs lying on either side.

4. We find great diversity in the number of aneurisms which may be simultaneously present. In some cases, several aneurisms are present together, either on different arteries or in close vicinity to each other on the same artery, so that the tube of the vessel exhibits a row of adjoining and even confluent aneurisms. Large aortic aneurisms are usually isolated, which may be explained, at least in part, by the weakening of the mechanical force through the carrying off of a large quantity of blood towards the aneurismal sac.

5. The greatest interest and the most important results arise from the relations exhibited by the branches passing from an artery affected with aneurism; they consist in narrowing or entire closing and displacement of the mouths, and the consequent atrophy of the vessel. These results are produced by various and often intimately connected means.

a. A highly developed degree of deposition (see p. 206), very commonly gives rise to the important conditions of contraction, and, finally, complete closure of the mouths of vessels opening into the artery affected with aneurism. It more especially affects the mouths of small vessels branching off from the diseased trunk, either at a right or an obtuse angle, as, for instance, the mouths of the intercostal arteries, and of other vessels branching off from the diseased thoracic aorta; although it also not unfrequently implicates the mouths of vessels of larger calibre, as, for instance, those of the carotids, the subclavians, etc.

b. Secondly, the mouths of the branches of the vessels are also rendered insufficient and are displaced by means of the fibrinous coagula deposited on the wall of the aneurism. They have commonly been already contracted by the deposition, or have been rendered insufficient by means of the fissure-like opening, which we shall shortly notice. This imperviousness of the mouths is more especially limited to those vessels which branch off from the diseased trunk, either at right or obtuse angles.

In consequence of the displacement and closure of the mouth, the blood which reaches the branch of the vessel through the collateral circulation coagulates, and the vessel is then obliterated from above the plug to the point where the next branch is given off.

c. Thirdly, the branches passing off from a diseased trunk are rendered insufficient by the round form of the mouth being contracted and altered into a cleft-like opening, which is frequently rendered still more impervious by the projection of a valve-like margin which inclines backwards in the direction of the heart. This is more especially found to occur in the branches of the arch of the aorta, when the latter is the seat of large saccular dilatations.

d. Finally, there is a mode of obliteration that occurs in the vessels branching off from an aneurism, either independently, or complicated with the above-described forms. This mode of obliteration is the result of inflammation with exudation upon the inner surface of the vessel, and of the subsequent coagulation of the blood. It appears only in vessels having thin walls, and which are, therefore, liable to this form of inflammation.

The effect of the aneurism on neighboring parts is to displace and press upon them, in proportion to their inability to offer any resistance to this pressure. By these means the functions of the injured organs are either partially or entirely obstructed. Thus aneurisms of considerable size may variously contract the space of the cavities of the body, and either diminish the apertures of different passages, such as the trachea, the bronchial tubes, the oesophagus, the arteries and veins, etc, or compress them so powerfully as to render them entirely impervious.

Pressure gives rise in different structures to various alterations which are proportional to the degree of pressure and the capacity for resistance presented by the tissue. Moderate pressure generally occasions inflammation in the contiguous structures, which gives rise to condensation and thickening - increase of bulk. When the pressure exceeds a certain limit, it results in atrophy. Both of these results are, however, frequently combined, being found simultaneously present in different parts of the tissue; thus, for instance, the parts in the immediate vicinity of the aneurism may be atrophied, whilst the more remote tissue exhibits a new formation of cellular substance and of fibroid tissue. We very frequently observe that bones which have been exposed to the action of an aneurism exhibit atrophy (detritus), whilst various osseous formations - osteophytes - occur at detached points surrounding the aneurism, and even sclerosis may be present in the contiguous bony layers.