embolus (ĕmˈbələs) [key], foreign matter circulating in and obstructing a blood vessel. It may be a portion of a clot that has separated from the wall of a vessel (see thrombosis), a bubble of gas or air (known as an air embolus), a globule of fat, a clump of bacterial matter, or a clump of tumor cells. It circulates freely through the vessels until it reaches one so small that it cannot go further. An embolus in one of the vessels leading to the lungs, brain, or heart, if large enough, can be fatal; in an arm or leg it may lead to gangrene and, ultimately, the need for amputation. Emergency surgical removal is usually the treatment of choice for a solid embolus. Otherwise, drugs that dilate the vessels and anticoagulants are indicated.