This is of very frequent occurrence in veins. The full description of this process in a previous section applies especially to the veins. As the blood in the veins is normally at a lew pressure and flows slowly, it is readily brought to a standstill, especially in the pouches of the valves. Hence, any cause which produces prolonged passive hyperemia or weakness of the circulation is apt to induce thrombosis. Thrombosis is also a result of inflammation of veins, such inflammation being mostly septic.

Thrombosis induces an Inflammation of the wall of the vein, whether the thrombus be specially irritating or not. In the case of a simple thrombus its presence induces a chronic inflammation which extends through the wall of the vein to its adventitia and surrounding connective tissue. There are thus Adhesions formed between the vein and its sheath, and these may extend to the sheath of the neighbouring artery, causing considerable matting around. A vein containing an old thrombus is therefore, as a rule, somewhat difficult to dissect clean from the surrounding tissue. Thrombi in veins not infrequently become organized, and there may be a partial restoration of the calibre in the manner described and illustrated at pp. 100-102. (See also Fig. 257).

Fig. 257. - Two veins with organized thrombi in them. The thrombi contain larire channels by which the circulation is restored. X 8.