It is of little consequence, then, whether the aneurismal sac possesses an elastic covering proper to itself or not, the resistance of the external structures being sufficient to explain the phenomenon of pulsation, and the impor tance of the integrity of these structures in the progress and termination of the case is ex tremely interesting. If even a small quantity of blood was thrown at each pulsation of the heart into a yielding, unresisting bag, it must of necessity remain there, and in a very short space of time the accumulation would be enormous ; but if there is a re-acting force capable of returning a portion of this blood and restoring it to the circulation, the accumula tion and consequent growth of the tumour will be measured by the quantity of blood thus left behind. The volume of blood sent into an aneurismal sac must be proportioned to the aperture through which it has to pass, while the actual quantity lost must depend not so much on this as on the non-resistance of it and its coverings, and their incapability of return ing the fluid back into the circulation. Hence the growth of external aneurisms is in general rapid or slow according as they have existed a greater or less length of time ; for in old aneu risms the aperture into the sac is generally large, and the elasticity of the external coverings is weakened by over-distension.
The pathology of aneurism arranges itself under two distinct orders, one having relation to the open and bleeding artery, the other con sequent on the haemorrhage being internal. This latter circumstance is interesting to the surgeon, because the presence of the blood in the limb, the position it occupies, and the pressure exercised by it on the adjacent structures and organs, very often form the most prominent and important features of the disease, and nearly as frequently cause the destruction of the patient as the bursting and bleeding of the tumour. But the consideration of this part of the sub ject does not immediately belong to the patho logy of the arterial system, to which these re marks are more particularly directed. To re turn, then, to the open or ruptured artery. The condition of the vessel is scarcely different from that of one wounded by a knife. It is a bleeding artery, and the same principle that is applicable to haemorrhage under any other circumstances is also available here, for if a wound of this species of vessel cannot heal whilst its calibre remains open, neither can an aneurism be cured until the artery from which it springs is completely obliterated at the spot where the aperture into the sac exists. The complete closure of the vessel is to be accom plished by placing its opposite walls in contact and under the influence of such pressure as will occasion inflammation and the effusion of coagulating lymph,—a pressure that can be applied either ab externo by means of com press and bandage, or from within, by placing the blood in the sac in a condition that will admit of its perfect and complete coagulation.
• Pressure on the tumour, if it could be ex actly applied and firmly maintained, ought to succeed, and, in truth, has often been success ful, particularly when the disease is consequent on a wound ; but there are so many difficulties to be surmounted and dangers to be encoun tered in its use, that few entertain much confi dence in it, and perhaps it never would be resorted to but from a dread of consecutive hemorrhage after a ligature. A bandage, if applied with sufficient firmness to operate with rapidity, occasions such excruciating pain that it can scarcely be endured ; and if loosely, it is liable to slip ; and if even it does finally work a cure, the progress of the case is so protracted that many patients become wearied with the trial. Again, the large trunks of arteries throughout the extremities are generally accom panied by nerves and veins in such close apposition with them, that a compress can scarcely be applied to one without interfering with the other ; and instances have occurred of dreadful mischief having been occasioned by interruption of the venous circulation in such cases, even in the course of one night. Finally, pressure has very frequently caused the rupture of the sac, and the aneurism, from being cir cumscribed, has suddenly become diffused ; and if there was no other source of apprehension but the possibility of this latter occurrence, it should make a surgeon pause before lie adopted so hazardous a mode of treatment.
Pressure from within is effected by re moving the impulse of the heart from the blood within the sac for a sufficient time to permit of the sac becoming perfectly filled with blood, and for that blood to become coagulated. This object will be accomplished by interrupt ing the flow of blood under the impulse of the heart through the leading trunk of the vessel for a given time : in cases of small aneurisms forty-eight hours being sufficient, but the larger and older requiring a longer period. A ligature placed around the vessel between the tumour and the heart effects this purpose ; but it does more than is requisite, for it divides its in. ternal and middle coats, occasions the effusion of lymph and the obliteration of the artery there, and involves the risk of consecutive haemorrhage afterwards on its final separation. To avoid these inconveniences, the presse artere of Deschamps and a number of other con trivances for arresting the flow of blood through an artery, and admitting of easy removal after the object has been accomplished, have been proposed and tried, but success has not been so great as to warrant their general adoption, and the operation by ligature is still very generally preferred. It may be applied either at the cardiac side of the tumour, when it acts in the manlier above stated, or between the aneurism and the capillary circulation, in which case the principle of its operation is somewhat different.