These are aneurisms which patients refer to some traumatic influence, such as a contusion, shock, or some unusual muscular effort, etc, and which the physician, in the absence of all disease in the coats of the vessel, must regard as having such an origin. Aneurisms of this character especially occur in the arteries of the extremities, and, as we have already observed, they are too commonly included without further inquiry under spontaneous aneurism, when they are undoubtedly of traumatic origin. To this class belong a certain number of aneurisms of the femoral, popliteal, and brachial arteries.

We are here led to inquire what disturbance is set up in the wall of the vessel by the traumatic influence, which can give origin to the formation of the aneurism.

This question is very difficult of solution; for on the one hand, we rarely or never have an opportunity of examining the artery immediately after the accident, while on the other an examination of an aneurism, when already developed, does not afford absolutely valid grounds for judging of the original disturbance. It is, however, very probable that this disturbance may be based upon some traumatic influence inducing paralysis of the circular fibrous coat at the affected spot; destroying its contractility, and causing a separation of its fibres; and occasioning a partial laceration of the coat, not affecting either the integrity of the lining membrane or of the cellular sheath of the vessel. We are led to this opinion from a consideration of the following facts:

1. We cannot believe that traumatic aneurism can be produced by spontaneous laceration of the lining and of the circular fibrous coat of the artery, and therefore be owing to the dilatation of the cellular sheath at the spot where the rent occurs. We have never, for instance, seen an aneurism arise from a separation of the continuity of the lining and circular fibrous coats when it appears either as a spontaneous or a mechanically induced rent; but, on the other hand, there is always, in these cases, a more or less violent and extended detachment of the cellular sheath, whether the arterial coats be healthy or diseased, constituting a secondary laceration of the sheath, and effusion of blood over the vessel. (See Dissecting Aneurism).

2. In consequence of the great toughness and power of resistance of the cellular sheath of the artery, a separation of continuity can only be effected in all the coats of the vessel when the shock or contusion has been such as necessarily to produce extensive and repeated laceration of both coats. The consequence of this would at all events be to produce effusion of blood from the vessel over a considerable extent, giving rise to an evident false diffused aneurism which would also be subsequently apparent in the consecutive condition of false circumscribed aneurism.

3. The dilatation developed at the affected spot manifests itself originally as a circumscribed and gradually enlarging tumor which is slowly developed, and shows both by its form and construction the probability of the view we have advanced in reference to the disturbance on which traumatic aneurism is based. This dilatation exhibits either the form of a saccular expansion, or of a pedicled aneurism according to the degree of depression of vitality and loosening of continuity produced by external influences in the circular fibrous coat. Its walls principally consist of the lining membrane of the vessel and of the cellular sheath; in the first form we find remains of the circular fibrous coat between the other membranes, while in the second form the fibres of this coat are separated through external agencies; the lining coat of the vessel adhering within the interstices to the cellular sheath, and gradually protruding through it. The attenuation which the lining membrane of the vessel must necessarily undergo, cannot be directly observed, in consequence of the new membrane which has been simultaneously formed in the aneurism. This second form of traumatic aneurism appears therefore to be a hernial aneurism, according to the signification we shall attach to it in the following remarks.