The Circulation is established, after the application of the ligature, in the same manner as in obliteration of an artery generally, by the dilatation of the lateral branches and their anastomoses, - the so-called collateral circulation, which is developed, in cases of spontaneous and gradual obliterations, "with a rapidity proportional to the increasing contraction of the vessel, so that its final occlusion is imperceptibly effected.

When a main artery has been tied, the circulation is at first carried on by means of all the innumerable communications of the small ramifications; subsequently, however, some of these vessels and anastomoses dilate in a preponderating manner, while the others gradually return to their normal calibre. This dilatation is especially remarkable in the small branches, whilst the trunks and larger branches are relatively dilated to a very inconsiderable degree (Hodgson).

We would only add to this description the materials derived from a review of the facts and opinions which have been deduced from an investigation of tied arteries in man, and from the simultaneous consideration of highly important conditions analogous to thrombus.

We must, however, at once premise, that we do not regard this question as wholly settled, since this process in man presents numerous anomalies, independently of those cases in which the thrombus is not duly formed, in consequence of debility, cachexia, etc.

Our views in reference to the process, and the individual conditions on which it depends, are as follows:

1. We are of opinion, that the occlusion of a tied vessel may take place without the occurrence of thrombus (the inner plug), and that this is a mere incidental formation, and not by any means an inevitable and necessary condition of obliteration. There is very frequently no thrombus present, and occasionally its place is supplied by an adhering red gelatinous, in general irregularly thick, wrinkled, gland-like, shaggy coagulum, whose color is subsequently changed to a yellowish-red tinge; or the thrombus is inadequate to fill up the whole of the vessel, and hangs loosely in the stump of the artery, without actually adhering at any one point; although, notwithstanding this arrest of growth in the thrombus, it yet closes the extremity and a neighboring portion of the vessel. No trace of the previous existence of a thrombus can, however, be detected on cutting through this coalescence.

2. Although we would not wholly deny the point in reference to every case, we are yet of opinion that a true arteritis, with exudation on the inner surface of the vessel - constituting the so-called adhesive exudation, by which the thrombus is fixed and made to adhere to the wall of the vessel - is not an essentially necessary condition; since we have found it absent in numerous cases, in which all the requirements for occlusion were present, - that is to say where neither a change, induced by exudation in the tissue of the circular fibrous coat and in the lining membrane of the vessel, nor a free exudation on the latter could be distinctly recognized. We do not regard the albuminous or gelatinous layer, which invests the thrombus and the lining membrane of the vessel, and attaches the thrombus to the arterial wall, and which at first is transparent, but subsequently becomes white and opaque, as the product of arteritis, or as an exudation, but as a product of the blood - as a structure analogous to the lining membrane (see p. 204), which is produced in the stump of the vessel with a readiness proportional to the necessary mechanical conditions which are present. We may very often distinctly perceive how it encloses the thrombus, and extends, sometimes in a bridge-like form, from its basis towards the wall of the vessel. In other more advanced cases, these two lamellae are everywhere, or at some spots fused together; and, in the latter case, as the thrombus does not completely fill the vessel, it adheres by threadlike structures or bridges. The wrinkled, velvety coagulum investing the inner wall of the vessel, and which we have already described, has a similar significance; in like manner we believe, that the degeneration which attacks the circular fibrous coat, and gives rise to loosening, bleaching, and lacerability, is not to be regarded as the consequence of an exudation, and to be referred to its action on the tissue, but must be considered in the light of a regressive metamorphosis - an involution, - such as we meet with in atrophies of arteries, as, for instance, the obliteration of the foetal passages, etc, which are effected without the agency of any inflammatory process.

3. We hold that the occlusion and obliteration of tied arteries are essentially dependent on the same process that occurs in vessels which no longer receive an energetic current of blood, in consequence of the circulating fluid taking another course, and become unserviceable, as for instance, the umbilical arteries and the Ductus arteriosus. After the end of the tied artery next the ligature has become closed by the fusion of the opposite surfaces of the inner wall of the vessel, the further obliteration follows from the decrease of the vessel as the blood is turned into another course, and a collateral circulation established; and from its walls finally coalescing, either by means of the original lining membrane, or of a newly-deposited layer of that structure. The white mass, which we find as a central substance in the stump of the vessel, seems therefore to consist of this newly-deposited stratum.

When a thrombus is formed, which is far more commonly the case, the same process takes place - that is to say, the vessel contracts, and becomes occluded above it; it undergoes a metamorphosis into a fibroid string - a white fibrous mass.

4. Whether the thrombus, in certain cases, disappears by resorption into the mass of the blood in a state of minute disintegration or (as Remak expresses it) by solution, is a point which is certainly not at present established. There are, however, no facts positively opposed to this view, and it would be an occurrence in whose favor there are many analogies, - as, for instance, the fusion or resolution of coagula of blood in inflamed veins, the diminution and the final disappearance of vegetations on the valves of the heart, the disappearance of the ends of phlebo-lites, etc.

5. We have never observed the formation of vessels in a thrombus (its so-called vascularization). We do not, however, in the least doubt the accuracy of Stilling's observations, - that is to say, that the mass of the thrombus was porous, and capable of being injected; we cannot, however, participate in his view, that this condition depends on a true formation of vessels, and represents an organization of the thrombus. We prefer believing that this condition is the same as that with which we have become acquainted as channelling of the deposit (of the structure analogous to the lining membrane of the vessel, and formed in great excess, see p. 200), and which we regard as a very remarkable phenomena; that even this channelling sometimes occurs in other structures similar to thrombus, as, for instance, in the fibrinous coagula in the heart; and that it is of this, and nothing else, which observers speak, when they fancy they have injected polypi of the heart (Alex. Thomson, Vernois, see p. 166). We have recently had opportunities of observing this porosity, and the cavernous structure to which it gives rise, in vegetations within the cavities of the heart.

Holding this view of the case, we cannot regard the diminution and shrivelling of the thrombus, whereby its vessels - that is to say these canals - becoming obliterated, as a regressive formation in Stilling's sense.

6. Neither have we had an opportunity of observing a long central bloodvessel, either single or ramifying at its extremity, running through the stump of the obliterated vessel, as described by Lobstein and Blandin; nor have we ever observed the arborescent sprouting of vessels from a stump, as seen by Jones, Ebel, and others. According to our view, this phenomenon is intimately connected with the channelling of the thrombus, and the presence of these central vessels in the stump is exclusively owing to the persistence of longitudinal canals in the thrombus, such as have been frequently noticed by Stilling; and these arborescent vessels are nothing more than such persistent canals of the thrombus, which may perhaps, in the course of time, become longer and broader within the atrophying stump of the vessel. They most assuredly have no affinity with true vessels, however generally they may be regarded in that light.

We think it highly probable that Mayer's case of two arch-like lateral vessels, which connected the two extremities of the carotids after they had been tied, belongs to this class, although we are unable to give a definite opinion on the subject.

The formation of central canals in the coagula which obstruct inflamed veins - under which head we must include the case observed by Barth, of a central canal through an old plug obstructing the abdominal aorta, - may depend upon the same process of channelling, or upon another process, to which we shall refer, when we proceed to the consideration of the veins.