Saccular Aneurisms are commonly of a round form, although they are occasionally oval or conical in shape, even from their commencement; more frequently the form loses its roundness in consequence of excessive disease of the coats of the vessel at different points, when the dilatation preponderates more or less in one direction.

Finally, secondary pouches in the form of roundish or conical elevations, or protuberances - Cruveilhier's A. sous l'aspect d'ampoules a bosse-lures - not uncommonly occur in the aneurisms already described (viz., in cylindroid, fusiform, and sacciform aneurisms), when the disease of the coats of the vessel preponderates at several generally inconsiderable, although somewhat sharply-defined points. These protuberances may in their turn give rise to excrescences of a secondary form (a tertiary aneurismal formation), which project in various ways over each other, giving the vessel the appearance of an irregularly lobulated sac.

When a cylindrical or fusiform dilatation attacks a vessel irregularly over a considerable portion of its surface, and preponderates at different alternate points of the periphery, causing the vessel to extend in a longitudinal direction, by which its course becomes twisted, and as it were distorted round its axis, we again have the form of aneurism termed cirsoid.

A study of the development of the above forms yields the following facts in relation to the construction of the walls of these aneurisms.

When the dilatation has not exceeded a certain degree, the walls of all these aneurisms consist of the whole of the diseased coats, and hence constitute true aneurism. The internal stratum is formed by the deposition in its different conditions of opacity, fibroid metamorphosis, atheromatous process or ossification, and in a state of actual channelling. It represents the lining membrane of the vessel. Next follows the decolorized and cleft circular fibrous coat in the act of being metamorphosed into fat; and, lastly, the elastic and cellular coat - the cellular sheath - whose fibres are entwined with one another, and with those of the circular fibrous coat in a hyperaemic and vascular condition, and which exhibits either a bluish red coloration, or pallor and sclerosis.

When the dilatation is very considerable, the tissue of the circular fibrous coat is found to be much separated, while the stratum seen through it is thinner than usual. When the dilatation has extended beyond a certain limit, the fibres of the circular fibrous coat are not only completely separated, and even wanting at several inconsiderable portions of the aneurism, but this coat gradually disappears at the borders of the highest elevation of the fusiform or saccular aneurism, and at the margins of the superposed secondary pouches; the wall of the aneurism consisting here only of the diseased lining membrane of the vessel and of the cellular sheath. At these pouches the dilatation constitutes the so-called internal mixed or hernial aneurism, which we purpose considering more in detail.

There is a form of aneurism presenting considerable interest in many points of view, which Cruveilhier has designated A. sous l' aspect de poches a collet, and which may be termed pedicled aneurism. It differs very decidedly in its marked external form from the saccular variety already described, and is separated from it by many stages of transition.

This aneurism resembles a round sac, which is in general attached to the diseased vessel by means of a neck-like base or contraction. This base corresponds to an opening into the vessel, which is equal in circumference to the contraction, and is either round or oval in form, and surrounded by a projecting margin. It constitutes the channel of communication between the vessel and the aneurism.

No form of aneurism presents such striking variations in magnitude as this; since, without reference to the calibre of the vessel, it may exhibit every possible size, from that of a pea to that of the fist, or even of a man's head. The most common size is from that of a walnut to that of a middling-sized apple. These aneurisms are, moreover, distinguished by their tendency to burst when still of very inconsiderable dimensions, as for instance, when they are not larger than a pea or a bean.

An examination of the walls of these aneurisms yields the following facts:

1. In most cases, the wall of the sac at its base near the opening into the artery, consists of the wall of the artery that has been everted by the aneurism and of all the diseased coats of the artery, whilst the circular margin surrounding the opening is formed by a duplication, as it were, of the entire wall of the vessel. At different distances from this point, however, the circular fibrous coat, after having become gradually thinner, entirely ceases, and the wall of the aneurism then consists almost wholly of the deposition (the inner coat of the vessel) and the cellular sheath. The margins of the opening are smooth and covered by the deposit; the aperture is roundish. - These appearances present themselves in very small as well as in large aneurisms of this nature. This form of aneurism almost invariably occurs as a secondary formation, being seated on a cylindroid or fusiform aneurism.

2. In some few cases the circular fibrous coat terminates sharply at the margin of the aperture in the artery. The aperture is generally irregular and angularly contracted, whilst the wall of the saccular pouch above it consists of the cellular sheath and of a deposit, which projects from the contiguous inner surface of the vessel over the margin of the circular fibrous coat in the form of bridge-like plates and strings, and adheres loosely to the cellular sheath in the cavity of the expanded portion of the vessel. This appearance is observed only in small aneurisms before they exceed the size of a bean or a hazel-nut, and they then commonly prove fatal by bursting. They usually occur as primary aneurisms, and in general in arteries that are only slightly and locally diseased. The aperture in the circular fibrous coat is obviously the result of loss of substance.

3. In cases of similarly rare occurrence, we meet with a sharply-defined bulging of the artery, filled with the atheromatous mass resulting from the disintegration of the deposit and the circular fibrous coat. The wall is here composed of the cellular sheath.

On considering the above relations, we arrive at the following conclusions in reference to the origin and form of these aneurisms.

Although nothing positive can be determined in reference to the question, whether the appearances considered under the first head are the result of the further development of the appearances included under the second head, it is however very probable that such is not the case. The aneurism considered under the first head appears to be the result of excessive disease of the coats of the artery at a circumscribed spot. The artery bulges, and its wall then bends at the margin of the diseased tissue towards the tube of the vessel, with which it forms, as it were, a duplicature of the wall of the artery. Finally, the circular fibrous coat gradually separates at the top of the bulging, when the deposit, consisting of the diseased lining membrane of the vessel, is brought in contact with the cellular sheath with which it coalesces within this cavity. The further enlargement is now especially exhibited at this point, until the wall of the aneurism finally consists, for the most part, merely of the deposit and the cellular sheath.

This is undoubtedly the form that has been named external mixed aneurism (Scarpa's Aneurysma spurium), and is believed by some observers to consist merely of the cellular sheath of the artery, in consequence of their having regarded its investment as unessential and as a recent formation, and from their inability to trace the whole of the layers passing into the aneurism. Since we are disposed from our view of the subject to regard the investment as originally formed by the diseased lining coat of the vessel (although certainly in a condition of expansion and attenuation) which has coalesced with the cellular sheath in the aperture formed in the circular fibrous coat, we may regard this aneurism as the same which has been named by other observers hernial aneurism.

The appearances considered under the second head, have undoubtedly been developed from those noticed under the third head; at all events we are unable to discover in what manner this loss of substance has taken place within the wall of the vessel, unless by the atheromatous process described under the third head. We find that the deposit and the circular fibrous coat are affected throughout and destroyed. The atheromatous mass is gradually lost by being absorbed into the blood, and hence the bulging at this point consists of the cellular sheath. - This aneurism, when considered in accordance with the above-described mode of origin, is, strictly speaking, an external mixed aneurism; but in this form it probably never constitutes the subject of anatomical investigation. Thus, for instance, an inner coat of the vessel is produced in the form of a recent deposit, which renders it difficult, or indeed almost impossible, to recognize and distinguish this aneurism from others, especially when it has existed for a prolonged period. This aneurism does not appear, however, to be of long continuance, but generally bursts while of inconsiderable size. It is, moreover, of very rare occurrence when compared with other aneurisms, and cannot therefore have been the sole means of giving rise to the theory in reference to external mixed aneurism, or of originating the opinion of the frequency of its occurrence.

We take the present opportunity of answering the question, Whether a rent in both the inner coats of the artery can give rise to the formation of an aneurism of this class? The belief in this mode of origin has met with almost universal accordance, although, as far as we know, the correctness of the opinion has never been proved by any one. The cavity in the neck or pedicle of these last-named aneurisms has commonly been regarded as a fissure. Yet, as far as we are aware, no such rent has ever been detected, nor have we ever found that a fissure in the inner coats of the artery afforded a basis for the formation of an aneurism. (Compare Laceration of the Arteries).

We believe that the above observations comprise all the most important points in reference to the form of spontaneous aneurisms and the construction of their walls, however much they may seem at variance with a sharply-defined classification of aneurisms in accordance with any fixed principle. By way of completing our observations, we will only remark, that the deposit covering the inner surface of the last described aneurism which is attached by a neck, is also found to be affected by the different conditions of opacity, atheromatous disintegration, and ossification.

There still remain several other appearances in the aneurism which demand our attention.