It is now necessary to explain the nature and composition of both the thrombus and embolus, and how, through their mechanical agency, the process of embalming may become seriously impaired, if not altogether arrested. A thrombus is a clot, composed principally of the fibrine of the blood, formed during life in the cavities of the heart, the arteries, the veins, or the capillaries. An embolus is a solid body, usually a portion of a thrombus, carried by the blood current into some artery or capillary, and becomes fixed there. A thrombus may entirely fill the cavity of a vessel, or it may only form a layer on its wall, or it may project from a smaller vessel into a larger one. An embolus may completely or only partly plug up a vessel; it may remain alone or a thrombus may be formed around it. A thrombus may merely become harder and whiter in time, or it may become organized, or it may degenerate, soften, and change into a mass of puriform fluid and _gangrenous looking fibrine_, or it may calcify. The production of thrombi is due to various causes. (1.) Spontaneous thrombi are produced by any cause which seriously weakens the whole system, as old age and chronic exhaustive diseases. They are usually formed in the veins of the lower limbs and pelvis, and in the sinuses of the dura mater. (2.) Compression of the vessels by ligatures, tumors, dislocated bones, and inflammatory exudation. _Pneumonia_ may produce _thrombosis_ of the _pulmonary veins_. _Phthisis_ may produce _thrombosis_ of the _pulmonary arteries_. (3.) Thrombi may form into capillaries or small veins, and increase until they project into larger veins. (4.) It may also form when there is inflammation, especially of a gangrenous and suppurative nature, in the parts around a vessel. The causes of embolism are also various. (1.) Fragments of thrombi in the veins are the most frequent material of emboli. (2.) Thrombi of the heart, vegetations on the valves and on the walls of the aorta. (3.) Portions of the inner-coat of the aorta may become detached and form an embolus; also portions of tumors which project in the veins, and fat may form emboli. In a considerable number of cases, it is impossible to find any source for embolus. It must be remembered, however, in emboli, in connection with wounds, that not only the veins of the wounded region are to be examined, but also those of the pelvis. The consequences of thrombosis vary with the situation of the clot; thrombi in the arteries produce either a lack of circulation in the region supplied by the artery, or else intense congestion from the collateral circulation. Later there may be inflammatory changes, softening, or gangrene. Thrombi in the veins produce inflammation in and around the vein, thickening of the skin and connective tissue, gangrene and hemorrhage.