The heart is supplied by the coronary circulation and is enclosed in a double-membraned sac–the pericardium. This attaches to the mediastinum, providing anchorage for the heart.[13] The back surface of the heart lies near to the vertebral column, and the front surface sits deep to the sternum and costal cartilages.[7] Two of the great veins – the venae cavae, and the great arteries, the aorta and pulmonary artery, are attached to the upper part of the heart, called the base, which is located at the level of the third costal cartilage.[7] The lower tip of the heart, the apex, lies just to the left of the sternum (8 to 9 cm from the midsternal line) between the junction of the fourth and fifth ribs near their articulation with the costal cartilages.[7] The right side of the heart is deflected forwards, and the left deflected to the back.[7]