B. _Organization._--This is the result of time, and means a metamorphosis into solid vascular connective tissue. Newly formed, minute, vascular loops project from the vasa vasorum into the thrombus, and it becomes thus vascularized, while the completion of the organization is due, in the main, to spindle-cell connective tissue, which is formed by wandering cells that penetrate into the thrombus from without. This gives the organized thrombus a certain resemblance to a sponge, and makes the original vein resemble a cranial sinus, since its interior is spanned by bands of connective tissue. Typical illustrations of this kind are seen, for instance, where the iliac veins join to form the inferior cava, by which a certain amount of obstruction to venous return is produced without its being total. The length of time required for these changes is indefinite. They begin, however, within a short time after ligature of a vein, and proceed with a rapidity varying according to circumstances. C. _Calcification._--Calcium salts are occasionally deposited in thrombi, usually not until they have undergone considerable contraction and alteration; as the result of which we have formation of small masses, essentially minute calculi, to which the name of _phleboliths_ has been given. These phleboliths are not infrequently found in more or less occluded and much distended varicose veins of the extremities, and they prohibit the occurrence of softening. D. _Softening._--This is the most serious termination of the thrombotic accident, and is usually due to the agency of infecting organisms. A non-infectious form is, however, recognized, by which there is a metamorphosis of original clot into an oily or pulpy fluid, usually dark colored, but in the white thrombi often yellowish white, reminding one crudely of pus. The discovery of such material under these circumstances has led in time past to the supposition that pus, as such, was found floating in the blood--a condition that does not exist except under extraordinary circumstances. It is with infection of thrombi and consequent softening, however, that surgeons have most to deal, and the paramount importance to them of such disturbances is emphasized in the article under Pyemia.