These two conditions are often associated, but must be carefully distinguished. Thrombosis is the coagulation of blood within the vessels or heart. Embolism is the obstruction of a vessel by a plug brought from a distance. The coagulum which forms in thrombosis is a thrombus, the plug which obstructs in embolism is an embolus. A thrombus detached from its place becomes an embolus, and an embolus, whether consisting of coagulum or not, may grow by successive deposition of clot, that is to say, by thrombosis.