The existence of a hernial aneurism, or of an internal mixed aneurism, has formed the subject of numerous investigations, from the time of Haller to our own day.

We have already become acquainted with aneurisms, which, as we incidentally observed, must be regarded as hernial aneurisms, in consequence of the anatomical disturbance to which they owe their origin, and in consequence also of the construction of their walls. It now remains for us more closely to define the sense in which we are led from experience to admit the existence of a hernial aneurism.

If it be requisite for the establishment of a hernial aneurism that it should exhibit a dilatation of the lining membrane of the vessel in the form of a hernia through an opening in the middle and outer coat of the artery, we must wholly deny the existence of such a form of aneurism.

1. The direct experiments of J. Hunter and E. Home showed that the removal of the external coat of an artery did not give rise to the protrusion of the lining membrane in the form of an aneurism. Whether the external coat alone, or that and the middle one, were both loosened and detached, the result was simply inflammation and cure without any alteration in the calibre of the injured artery.

However limited may be the application to be extended for various reasons to the results of these experiments, they are still highly interesting, and must excite our surprise from the opposition in which they stand to the result we should have been led to expect. We are not astonished merely at the circumstances that, after the removal of the outer and the middle coat, the lining membrane did not protrude, but still more that, considering its slight power of resistance, it did not at once give way. The circumstance that the middle and lining membranes were not lacerated after the removal of the outer coat, is very probably owing to the elastic sheath having been left on the vessel, and not removed with the outer coat.

2. To the results of these experiments we must add those yielded by observations on human arteries.

Detachment of the sheath of an artery, which consists of an elastic and a cellular layer, is not attended, as we learn from observations of the so-called dissecting aneurism, by a saccular expansion of the exposed yellow and lining membranes, but by its immediate laceration, both in those cases in which it is owing to external influences, and those in which it has resulted spontaneously from a morbid process. When, moreover, this occurrence is met with in cases where the middle and lining membranes were observed to be healthy, we are the more led to conclude that it would exist where there is disease of these membranes.

In such ulcerous perforations of the arterial wall from without inwards, as we noticed in the femoral arteries (see p. 198), notwithstanding the probably gradual and stratified separation of the different layers of the tube of the artery, we perceive no trace of aneurismal formation at the affected spot.

There is, on the other hand, a form of aneurism very frequently met with, which, when considered in the following sense, represents hernial aneurism.

a. In spontaneous, spindle-shaped, saccular aneurism, the diseased circular fibrous coat gradually yields at spots which vary in number according to the size of the aneurism. The lining membrane of the vessel (the deposition) coalesces in the interstices thus produced with the cellular sheath, and wherever these portions are excessively dilated, the secondary aneurismal formation deposited on a cylindroid, fusiform, or sacciform aneurism, gives rise to aneurisms of a secondary form, commonly known as hernial aneurism.

b. Aneurisms that are attached by a neck, and that are composed for the most part merely of lining membrane and cellular sheath, essentially constitute hernial aneurism, in as far as they are produced in the same manner as the above-named secondary dilatations.

c. Finally, traumatic aneurism, in accordance with the process from which it arises, and which has already been described, is a hernial aneurism.

Dubois, Dupuytren, Breschet, and others have undoubtedly taken a similar view of the question, when they maintained the existence of a hernial aneurism. We would, moreover, specially remark, although the circumstance seems sufficiently evident from the foregoing observations, that the inner coat of our hernial aneurism is by no means composed of the original lining membrane of the vessel, but consists almost entirely of newly-deposited strata.

We do not, for obvious reasons, regard the establishment of hernial aneurism as a separate class to be essential, nor do we think it possible, in all cases, to separate it strictly from spontaneous aneurism.