The term applied to any semi-solid mass of blood, lymph or other body fluid. Clotting in the blood is due to the formation of strings of fibrin produced by the action of a ferment. Milk clots in a similar manner in the stomach when exposed to the action of the enzyme rennin. Clotting occurs naturally when blood is shed and comes into contact with tissues outside the blood vessels. It occurs also at times in diseased vessels (thrombosis), producing serious effects upon the tissues supplied or drained by these vessels. Clots sometimes form in the heart when the circulation is failing. (See COAGULATION; EMBOLISM; THROMBOSIS.)