[Illustration: Fig. 41. 1. The descending vena cava. 2. The ascending vena cava. 3. The right auricle. 4. The opening between the right auricle and the right ventricle. 5. The right ventricle. 6. The tricuspid valves. 7. The pulmonary artery. 8, 8. The branches of the pulmonary artery which pass to the right and the left lung. 9. The semilunar valves of the pulmonary artery. 10. The septum between the two ventricles of the heart. 11, 11. The pulmonary veins. 12. The left auricle. 13. The opening between the left auricle and ventricle. 14. The left ventricle. 15. The mitral valves. 16, 16. The aorta. 17. The semilunar valves of the aorta.] The valves are so arranged that they present no obstacle to the free flow of blood from the auricles into the ventricles, but if any is forced the other way, it gets between the valve and the wall of the heart, and drives the valve backwards and upwards, thus forming a transverse partition between the auricle and ventricle, through which no fluid can pass. At the base of the heart are given off two large arteries, one on the right side, which conveys the blood to the lungs, called the _pulmonary artery_, and one on the left side, which conveys the blood to the system in general, called the _aorta_. At the junction of each of these great vessels with its corresponding ventricle, is another valvular apparatus, consisting of three pouch-like valves, called the _semilunar valves_, from their resemblance, in shape, to a half-moon. Being placed on a level and meeting in the middle line, they entirely prevent the passage of any fluid which may be forced along the artery towards the heart, but, flapping back, they offer no obstruction to the free flow of blood from the ventricles into the arteries. [Illustration: Fig. 42. A representation of the venous and arterial circulation of the blood.]