NEED OF VALVES IN THE HEART.--As the auricles do not need to contract with much force simply to empty their contents into the ventricles below them, there is no demand for any special contrivance to prevent the blood from setting back the wrong way. Indeed, it would naturally run down into the ventricle, which is at that moment open to receive it. But, when the strong ventricles contract, especially the left one, which must drive the blood to the extremities, some arrangement is necessary to prevent it from returning into the auricle. Besides, when they expand, the "suction power" would tend to draw back again from the arteries all the blood just forced out. This difficulty is obviated by means of little doors, or valves, which will not let it go the wrong way. [Footnote: The heart of an ox or a sheep may be used to show the chambers and valves. The aorta should be cut as far as possible from the heart, and then by pumping in water the perfection of these valves will be finely exhibited. Cutting the heart across near the middle will show the greater thickness of the left ventricle.] THE TRICUSPID AND BICUSPID VALVES.--At the opening into the right ventricle, is a valve consisting of three folds or flaps of membrane, whence it is called the _tricuspid_ valve (_tri_, three; and _cuspides_, points), and in the left ventricle, one containing two flaps, and named the _bicuspid_ valve. These hang so loosely as to oppose no resistance to the passage of the blood into the ventricles; but, if any attempts to go the other way, it gets between the flaps and the walls of the heart, and, driving them outward, closes the orifice. FIG. 39. [Illustration: _Diagram showing the peculiar Fibrous Structure of the Heart and the Shape of the Valves._ A, _tricuspid valve,_ B, _bicuspid valve;_ C, _semilunar valves of the aorta;_ D, _semilunar valves of the pulmonary artery._] THESE FLAPS ARE STRENGTHENED like sails by slender cords, which prevent their being pressed back through the opening. If the cords were attached directly to the walls of the heart, they would be loosened in the systole, and so become useless when most needed. They are, therefore, fastened to little muscular pillars projecting from the sides of the ventricle; when that contracts, the pillars contract also, and thus the cords are held tight. THE SEMILUNAR VALVES.--In the passages outward from the ventricles, are valves, called from their peculiar half-moon shape _semilunar_ valves (_semi_, half; _Luna_, Moon). Each consists of three little pocket-shaped folds of membrane, with their openings in the direction which the blood is to take. When it sets back, they fill, and, swelling out, close the passage (Fig. 40).