The Phlebitis depending on coagulation of the blood differs from the form of acute phlebitis hitherto treated of, inasmuch as the coagulation within the vessel is the primary phenomenon, whilst the inflammation of the coats of the veins - phlebitis - is associated with it merely as a secondary affection. The coagulation is therefore not occasioned by the inflammatory product of the coats of the veins - that is to say, by the absorption of the exudation into the blood from the inner surface of the vein, but is the result of a disease of the blood, which is either spontaneous or occasioned by the absorption of different products of stasis or inflammation, deposited either within or external to the vascular system. This disease reaches so high a degree of development at different points, and in the second case at different distances from the centres of infection, that the column of the blood coagulates more or less rapidly, with a more or less complete separation of the fibrin. When the coagulum is once formed, inflammation of the coats of the veins, if not invariably and very rapidly developed, is at all events of very common occurrence.

The existence of this process, when developed in the manner above described, proves the undoubted occurrence of coagulation of the blood in the various portions of the vascular system, from the centre to the capillaries, even where there is no trace of inflammation in the vessel; but it does not prove that where inflammation of the vein is present, its intensity and development have been sufficient to cause the coagulation by the deposition of exudation on the inner surface of the vessel.

The indications of this phlebitis are, in general, identical with those observed in the inflammation of the veins which we have already considered, but it is nevertheless of the greatest importance to notice the following special points:

1. We very frequently observe the above indicated want of relation between the nature and metamorphosis of the coagulation and the degree of intensity we should expect to meet with at the beginning of true inflammation of the coats of the veins. The disease generally, however, exhibits a very slight intensity, while the lining membrane of the vessel is not in a condition which would seem to indicate the immediate pre-occurrence of an exudation into its tissue, extending by means of the latter to the inner surface of the vessel. This is the most remarkable, since, as we shall see by the following facts, it is usually owing to a purulent exudation that the process is developed in its subsequent course.

2. In general it is an ordinary pyaemia which occasions the coagulation of the blood in vessels of considerable calibre. In accordance with this view, the coagulum commonly undergoes a purulent metamorphosis, a fusion into a more or less organized pus or ichor. According to the circumstances of the case, the vein finally contains a chocolate-brown, grayish-red, or yeast-yellow purulent fluid, mixed with partially dissolved fragments of the plug; or a dirty-brown, brownish-green, foetid, ichorous fluid, or even a very discolored, stinking, gangrenous ichor (Phlebitis septica). The contents are here not the product of the inflamed venous wall, but proceed from the metamorphosis of the coagulum.

3. In consequence of this metamorphosis - that is to say, of the contact of the inner coat of the vessel with the deleterious substance and its subsequent imbibition, the inflammation of the coats of the veins rapidly attains a high degree of development, and gives rise to corresponding purulent and ichorous exudations, which are added to the above described contents of the vein.

4. The diseased condition of the blood is in a high degree the controlling cause, while the so-called metastatic processes, to which it gives rise, are distinguished by their number and intensity.

5. An isolating or sequestrating coagulation cannot, as is obvious, be in any way conducive to this process.

It must be remarked, in reference to the terminations of this phlebitis, that -

1. The ordinary termination, in case death be not sooner induced by the general disease, is an acute ulcerous fusion, a gangrenous and ichorous destruction of the vein, arising from the process already considered under 2.

2. It is very rare, in accordance with the facts above referred to, for the disease to terminate in permanent occlusion, or in complete or incomplete obliteration. When the pre-existing inflammation of the coats of the veins has attained so high a degree of development as to cause a plastic exudation to be deposited on the inner surface, it will give rise to the adhesion of the obstructing plug; this may also be effected by the direct coalescence of the lining coat of the vessel with the plug.

To this form of phlebitis belong also those processes in the capillary system of the different tissues which have been commonly designated lobular processes, metastasis, and capillary phlebitis by the French. They are, in fact, the same process which we have already considered as that form of phlebitis which is induced by coagulation of the blood. We shall revert to this subject when we enter upon the consideration of the diseases of the smaller vessels, and of the true capillaries.

Although we think we need hardly enter upon any special discussion of the differential diagnosis of these two forms of acute phlebitis, after having considered them with every possible attention under the heads A and B, we would yet draw attention to the following additional remarks: