The processes known as thrombosis and embolism are so intimately associated with the diseases of blood vessels that it is convenient to define these terms in the first instance.
Thrombosis.—The term thrombus is applied to a clot of blood formed in the interior of the heart or of a blood vessel, and the process by which such a clot forms is known as thrombosis. It would appear that slowing or stagnation of the blood-stream, and interference with the integrity of the lining membrane of the vessel wall, are the most important factors determining the formation of the clot. Alterations in the blood itself, such as occur, for example, in certain toxśmias, also favour coagulation. When the thrombus is formed slowly, it consists of white blood cells with a small proportion of fibrin, and, being deposited in successive layers, has a distinctly laminated appearance on section. It is known as a white thrombus or laminated clot, and is often met with in the sac of an aneurysm (Fig.†72). When rapidly formed in a vessel in which the blood is almost stagnant—as, for example, in a pouched varicose vein—the blood coagulates en masse, and the clot consists of all the elements of the blood, constituting a red thrombus (Fig.†66). Sometimes the thrombus is mixed—a red thrombus being deposited on a white one, it may be in alternate layers.
When aseptic, a thrombus may become detached and be carried off in the blood-stream as an embolus; it may become organised; or it may degenerate and undergo calcification. Occasionally a small thrombus situated behind a valve in a varicose vein or in the terminal end of a dilated vein—for example in a pile—undergoes calcification, and is then spoken of as a phlebolith; it gives a shadow with the X-rays.
When infected with pyogenic bacteria, the thrombus becomes converted into pus and a localised abscess forms; or portions of the thrombus may be carried as emboli in the circulation to distant parts, where they give rise to secondary foci of suppuration—pyśmic abscesses.
Embolism.—The term embolus is applied to any body carried along in the circulation and ultimately becoming impacted in a blood vessel. This occurrence is known as embolism. The commonest forms of embolus are portions of thrombi or of fibrinous formations on the valves of the heart, the latter being usually infected with micro-organisms.
Embolism plays an important part in determining one form of gangrene, as has already been described. Infective emboli are the direct cause of the secondary abscesses that occur in pyśmia; and they are sometimes responsible for the formation of aneurysm.
Portions of malignant tumours also may form emboli, and their impaction in the vessels may lead to the development of secondary growths in distant parts of the body.
Fat and air embolism have already been referred to.