The plugging of a small blood vessel by an EMBOLUS which has been carried through the larger vessels by the bloodstream. It is due usually to fragments of a clot which has formed in some vessel, or to small portions carried off from the edge of a heart-valve when this organ is diseased. However, the plug may also be a small mass of bacteria, or a fragment of a tumour, or even a mass of air bubbles sucked into the veins during operations on the neck. The result is usually more or less destruction of the organ or part of an organ supplied by the obstructed vessel. This is particularly the case in the BRAIN, where softening of the brain, with APHASIA or a STROKE, may be the result. If the plug is a fragment of malignant tumour, a new growth develops at the spot; if it is a mass of bacteria, an ABSCESS forms there. Air embolism occasionally causes sudden death in the case of wounds in the neck, the air bubbles completely stopping the flow of blood. Fat embolism is a condition which has been known to cause death – masses of fat, in consequence of such an injury as a fractured bone, finding their way into the circulation and stopping the blood in its passage through the lungs. (See also PULMONARY EMBOLISM.)