The coronary sinus is the main vein of the heart and is about 3 cm. long. It lies in the coronary sulcus at the posterior surface of  the heart in the posterior atrio-ventricular groove. The coronary sinus opens into the right atrium. Its tributaries are the great cardiac vein, the middle cardiac vein, the small cardiac vein, the posterior vein of the left ventricle and the oblique vein of the left atrium. In addition to the above some anterior cardiac veins lying on the left ventricle open into the right atrium. A number of  venae cordis minimae drain directly into the chambers of the heart.