THE CIRCULATION. 



153 



seen on the exterior along the partition which divides the 

 right and left ventricles. 



5. The Valves of the Heart. In order that by means 

 of the contractions the blood may flow in the proper direc- 

 tion, the heart is provided with a series of valves, four in 

 number, two between the auricles and the ventricles, one at 

 the beginning of the pulmonary artery, the fourth at the 

 beginning of the aorta. The valves below the auricles 

 are called auriculo- ventricular valves. The valve between 

 the right and left auricle consists of three flaps so arranged 



Fig. 72. THE HEART WITH SIGHT AURICLE AND RIGHT VENTRICLE EXPOSED. 

 1, descending vena cava; 2. ascending vena cava; 2', hepatic arteries; 3, 3', 3", walls 

 of auricle; 4, wall of ventricle; 4', papillary muscle; 5,5', 5", tricuspid valves; 6, semi- 

 lunar valves; 7, pulmonary artery; 8, aorta; 9, branches of aorta; 10, left auricle; 11, left 

 ventricle. 



that when they are brought together they close the opening 

 leading to the auricle. This is the tricuspid valve. The 



