The Thrombus begins to be formed at the termination of the vessel, and from thence extends onwards in its axis to the point where the first lateral branch is given off; it possesses a conical shape from its commencement, and not being very thick, it does not entirely fill up the tube of the stump of the vessel, but simply projects into it with its base or middle, at which parts it is usually of a dark, blackish-red color, and of very inconsiderable consistence. The apex, however, is white, more dense and hard, and resembles coagulated fibrin. It occasionally acquires a more fusiform shape by the addition of supplementary new layers, consisting in such cases of concentric superficial strata in addition to the original coagulum in the centre. It occasionally lies free in the stump of the artery, but more commonly it adheres, although at first loosely, by its base. It is subsequently invested with an albuminous moisture, by means of which it adheres loosely to the wall of the artery, although its apex remains free. This adhesion is frequently effected by means of filamentous bridge-like attachments.

The inflammation, which is set up around the injured vessel, also implicates its cellular sheath. Plastic lymph exudes into the tissue of all the structures and into the cellular sheath of the vessel. We find also that a process of adhesive inflammation affects the spot at which the ligature is applied, and where the different folds of the lining membrane of the vessel come in contact with each other; and that there is an exudation of coagulable lymph, which causes a slight adhesion of the walls of the vessel, and of the thrombus at its base. The inflammation in the terminal part of the vessel is owing, not only to the irritation established by the operation, but also to the thrombus, which acts here as a foreign body; exudation being effused between the coats of the vessel as well as also on the free surface of the lining membrane.

The thrombus is always formed gradually; but in some cases it is found to be forming within half an hour or an hour after the operation, while, in other cases, there is no trace of it at the end of several hours. It is generally completely developed within twelve or eighteen hours after the closure of the vessel; it is more rapidly formed in small than in large vessels.

The further alterations include the metamorphosis of the developed thrombus, its coalescence with the wall of the vessel, and the final obliteration and atrophy of the artery.

The thrombus occasionally exhibits light-colored spots, both on its surface and in its interior towards its apex. Stilling observed fibrous or thread-like stripes on these spots, which he convinced himself by a lens were vessels.

In the course of time the thrombus acquires a porous structure, and becomes spongy and cavernous. Stilling found, in several experiments, that in addition to the numerous canals which he injected, and which traversed the thrombus in different directions, there was a central longitudinal canal opening into the cavity of the vessel. The periphery was especially injected in the more recent thrombi, whilst in those of older formation, the injection advanced more towards the centre or the axis. Where this so-called vascularization was present, the thrombus was always of a paler, flesh-like, faint rose-red color, turning to yellow, and finally to white, whilst the consistence was proportionally more considerable. - This metamorphosis of the thrombus is succeeded by its regressive formation.

In the meanwhile the thrombus becomes intimately adherent by its base, and very frequently by the whole of its body to the wall of the vessel, - in smaller vessels on the second or third day, and in larger ones on the fifth or sixth day; the former adhesion has now been converted into a firm coalescence. The greater part of the apex of the thrombus commonly, however, remains free during the period of its greatest vascularity.

The regressive formation of the thrombus consists in the diminution of the number of the so-called vessels within it, in its increased pallor and density, and in the fact, that "the whole mass of the thrombus still remaining at this period, merges, as it were, into the mass of the stump of the artery, forming with it one body." (Stilling).

This portion of the vessel gradually loses its proper texture; the exudation effused into the coats of the vessel becomes in part resorbed, and is in part metamorphosed into a cellular or fibroid tissue; the walls of the vessel gradually close around the shrivelling thrombus, and become obliterated into a cellulo-fibrous, ligamentous string, which in the course of time disappears still more, until it can no longer be recognized. This alteration is effected in smaller vessels in from about twenty to twenty-two days, and in larger ones in from thirty to forty-five days.

The ligatures by which the middle circular fibrous coat and the lining membrane have been originally divided, are loosened, and come away, in consequence of the suppuration of the cellular sheath at the spot where they have been applied, during the above-mentioned process. - This suppurative process not unfrequently gives rise to destruction of the coats of the vessel above the ligature, and of the thrombus, and hence induces hemorrhage.