There are two types of valves in the heart: the bicuspid {mitral') and tricuspid between the atria (auricles) and ventricles, and the two sets of semilunar valves at the entrance of the pulmonary artery and aorta. (See Fig. 220).

The bicuspid valve is the most important and is the deepest seated. It lies at the edge of the left border of the sternum opposite the fourth costal cartilage. It separates the left atrium and ventricle and lies nearly transversely.

The tricuspid valve lies in the middle of the sternum opposite the fourth intercostal space. It runs obliquely downward and to the right from the third left intercostal space to the fifth right costal cartilage. It separates the right atrium and ventricle.

The pulmonary semilunar valve lies opposite the sternal end of the third left costal cartilage. It is the most superficial valve and the one highest up on the sternum. It prevents regurgitation of the blood into the right ventricle from the lungs.

The aortic semilunar valve lies under the left side of the sternum about level with the lower edge of the third costal cartilage. It is just below and to the right of the pulmonary valve, and above and to the left of the bicuspid valve.