An embolus is a foreign body in a blood-vessel, usually too large to pass through the smallest capillaries, and the disturbances resulting from its presence are included under the term embolism. Although most emboli are detached portions of thrombi, any foreign body of suitable size may become an embolus. Such are tissues, as the pulmonary elastic fibres, fragments of diseased valves of the heart and of the intima of arteries, or portions of tumors growing into vascular canals. Others are globules of oil entering the torn veins when fat-tissue becomes crushed, or air-bubbles admitted through veins either wounded by instruments or opened after parturition by the dislodgment of their obstructing thrombi. Still others are granules of pigment derived from the coloring-matter of the blood, as in melanæmia, or introduced from without, as india-ink and cinnabar. The echinococcus has been found as an embolus, and it is highly probable that the cysticercus, the trichina, and other animal parasites may be disseminated as emboli over the body. Vegetable parasites, like the bacterium and aspergillus, have also been included in the list, although the disturbances resulting from their presence are less due to mechanical obstruction than to colonization and growth. The experimenter uses the most various objects as emboli--bits {63} of wood, rubber, and glass, globules of mercury, fragments of tissue, etc. Emboli are to be regarded as of arterial or venous origin. The arterial emboli are carried toward the capillaries, while venous emboli are carried toward the heart. The effect of both is partly or wholly mechanical, and partly due to the specific properties of the constituents. The mechanical effect of an embolus is manifested by the obstruction it offers to the circulation, and the degree of the obstruction depends upon the size, shape, and density of the embolus and the nature and size of the vessel obstructed. An embolus may be so large as to be unable to pass through the valvular orifices of the heart. A long and narrow embolus might pass through a vessel which would not admit one which was short and thick. A jagged and dense embolus, by repeated blows or prolonged and forcible contact, might cause a weakening or rupture of the wall of a vessel, and thus produce an aneurism. Certain vessels (the terminal arteries of Cohnheim) furnish the sole supply of arterial blood to a district, and when they are obstructed, the results, to be mentioned later, differ widely from those taking place where free vascular anastomoses exist. When a trunk bifurcates, the larger branch usually receives the embolus.