The Coagulation is effected in a simple manner, above and below the inflamed vein, and around the coagulum which originally filled it. The blood is coagulated below the inflamed vein (at its circumference) in all the branches where it is retained by the coagulum obstructing the vessel; above this point, towards the centre, the blood is arrested as far as the next considerable-sized venous branch that opens into the diseased vein. The coagulation is thus dependent on the coagulum originally obstructing the inflamed vein, and is essentially a thrombus.

Seeing that this form of coagulation can only be effected after the formation of the original coagulum, there are two points to be considered in reference to its significance.

A. A General Infection

A General Infection usually occurs, as has been already stated, before the development of the coagulum which originates in the absorption of the exudation, and consequently still longer before the formation of the sequestrating clot.

B. The Original Coagulum

The Original Coagulum is rapidly formed after the deposition of the exudation, and it hinders the general infection by entirely filling up the tube of the vessel, and absorbing the whole of the exudation. The sequestrating plug does not appear, in either case, to be of any essential service.

We here, however, draw the following inferences in reference to the possibility of infection arising from the metamorphosis of the original coagulum:

1. If the phlebitis had deposited a so-called plastic exudation, and belonged to the form which terminates in disintegration or obliteration (see the modes of termination of Phlebitis), the sequestrating plug would be of no obvious utility in either of the cases considered under a and b.

2. If the phlebitis had deposited a purulent ichorous exudation, there would necessarily have existed one or other of the following conditions: a. A general infection of the blood must have been induced previously to the development of the original coagulum, which may either have filled the diseased vessel throughout its entire length, or may have been limited to the margins of an accumulation of pus in the vein, if that fluid were exuded in large quantity. The sequestrating plug cannot, in such a case, hinder the pyaemia, in the course of whose existence it has, in fact, been developed; while it is itself, moreover, subject to purulent fusion from a prolonged continuance of the disease.

b. Or the original coagulum may have been rapidly formed by coming in contact with the pus, in which case general infection of the blood could not possibly have been effected at that moment, but inasmuch as this coagulum undergoes a more or less rapid purulent fusion, there is a possibility of the blood becoming secondarily infected by the disintegrated admixture of the coagulum. This coagulum may be permanently retained through a subsequent coagulation of the blood that has continued unaffected. It is only under these conditions that the coagulation which occurs at the limits of the inflamed vein is of essential use - that is to say, it is only the pus proceeding from the metamorphosis of a coagulum established in the vein by the absorption of a purulent exudation produced at the spot, that can in the true sense of the word be sequestrated.

In addition to the signs and consequences of phlebitis to which we have already referred, there are certain associated and consecutive phenomena, which still require notice; namely, accumulation of blood in the small veins and capillaries beneath the inflamed vein, a cyanotic tint, and oedema around this portion of the vascular system; these are consequences of the occlusion of the inflamed vein. As the inflammatory process extends, we have inflammation of the cellular sheath of the vein and of the surrounding cellular tissue, with sero-fibrinous, sero-purulent, hemorrhagic exudations; we further have inflammation of the skin in the form of erysipelatous redness, which, from the beginning, accompanies the inflammation of the subcutaneous veins in the form of red streaks along the course of these veins; and, as a final result, we have moist gangrene, caused by the stasis established in the capillaries by the extensive occlusion of the inflamed vein.