The mode of action of the flaps of the tricuspid and mitral valves is like that of a lateen sail of a boat, if we substitute the blood stream for the air current; the tendinous cords acting as the "sheet" or rope which restrains the sail when filled with wind.

The curtains of the valves may at first be considered as lying close to the ventricular wall. As the ventricle gradually becomes filled, the flaccid muscular wall is moved away from the valves, which are held in the midst of the fluid by the tendinous cords coming from the elastic papillary muscles. When the auricle contracts, a column of blood is driven into the ventricle, which, though not distended, is already filling with blood. This sudden central inflow gives rise to lateral back eddies, which get behind the flaps of the valves and carry them toward the auricle. By the time the auricle has emptied itself into the ventricle, the flaps of the valves are in contact with each other and the orifice is closed. When the ventricle begins to contract upon its contained blood, the pressure makes the valves tense and the fluid bellies out the sail-like flaps toward the auricles, so that their convex sides come into still closer apposition with one another. Their free margins are held firmly in position by the papillary muscles contracting and tightening the cords. The flaps are kept at much the same tension by the papillary muscles shortening in proportion as the ventricle empties itself and the cavity diminishes in size. By this mechanism the valves are prevented from bulging too much into the auricles, or allowing the blood to pass back into them.

The Orifices of the Heart seen from below, the whole of the ventricles being cut away.

Fig. 117. The Orifices of the Heart seen from below, the whole of the ventricles being cut away, and the curtains of the aunculo-ventricular valves drawn down by threads attached to the chordae tendineae. {Huxley).

Rav

Right auriculo-ventricular opening surrounded by the flaps of tricuspid.

La V

Lett auriculo-ventricular opening and attached mitral valve.

Pa

Pulmonary valves closed. AO. Aortic valves clcsed.