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.
The consequences of embolism vary with the character of the embolus and its situation. A simple plug of fibrine in a vessel only produces changes in the circulation, but a plug from a gangrenous thrombus has a tendency to excite suppurative inflammation wherever it lodges.
The size of the artery thus obstructed, and the character of its collateral circulation also lead to a variety of results. If a large artery is stopped, the part becomes empty; if a small one the part may become extravasated with blood from the collateral circulation. Thus, if the large arteries supplying the extremities are obstructed, paralysis of the muscles, and finally dry gangrene, ensue.
Emboli of the pulmonary arteries produce sudden death; of the coronary arteries, sudden death; of the cerebral arteries, softening of the brain; of the retina, sudden blindness; of the mesenteric arteries, congestion and gangrene of the intestines.
Abscesses are also produced. This takes place when the embolus is of an irritative and gangrenous nature. Large abscesses are only found in the lungs and liver, therefore the mechanical results of an embolus produce the following condition of affairs: A set of vessels are first emptied of their blood, and the nutrition of their walls thus impaired; after a time these vessels are filled up from the veins, their abnormal walls can no longer resist the blood pressure, and hemorrhage takes place.