185. The Valves of the Heart. The heart is a valvular pump, which works on mechanical principles, the motive power being supplied by the contraction of its muscular fibers. Regarding the heart as a pump, its valves assume great importance. They consist of thin, but strong, triangular folds of tough membrane which hang down from the edges of the passages into the ventricles. They may be compared to swinging curtains which, by opening only one way, allow the blood to flow from the auricles to the ventricles, but by instantly folding back prevent its return. Illustration: Fig. 70.—Lateral Section of the Right Chest. (Showing the relative position of the heart and its great vessels, the œsophagus and trachea.) A, inferior constrictor muscle (aids in conveying food down the œsophagus); B, œsophagus; C, section of the right bronchus; D, two right pulmonary veins; E, great azygos vein crossing œsophagus and right bronchus to empty into the superior vena cava; F, thoracic duct; H, thoracic aorta; K, lower portion of œsophagus passing through the diaphragm; L, diaphragm as it appears in sectional view, enveloping the heart; M, inferior vena cava passing through diaphragm and emptying into auricle; N, right auricle; O, section of right branch of the pulmonary artery; P, aorta; R, superior vena cava; S, trachea. The valve on the right side is called the tricuspid, because it consists of three little folds which fall over the opening and close it, being kept from falling too far by a number of slender threads called chordæ tendinæ. The valve on the left side, called the mitral, from its fancied resemblance to a bishop’s mitre, consists of two folds which close together as do those of the tricuspid valve. The slender cords which regulate the valves are only just long enough to allow the folds to close together, and no force of the blood pushing against the valves can send them farther back, as the cords will not stretch The harder the blood in the ventricles pushes back against the valves, the tighter the cords become and the closer the folds are brought together, until the way is completely closed. From the right ventricle a large vessel called the pulmonary artery passes to the lungs, and from the left ventricle a large vessel called the aorta arches out to the general circulation of the body. The openings from the ventricles into these vessels are guarded by the semilunar valves. Each valve has three folds, each half-moon-shaped, hence the name semilunar. These valves, when shut, prevent any backward flow of the blood on the right side between the pulmonary artery and the right ventricle, and on the left side between the aorta and the left ventricle. Illustration: Fig. 71.—Right Cavities of the Heart.