Few subjects are of greater practical importance than the mode of action of the valves of the heart, inasmuch as these structures are so frequently the seat of pathological changes which produce serious disturbances throughout the whole circulatory system.

On the general principles of the mode of action of the aortic and pulmonary valves all investigators seem to be agreed. When we turn to the auriculo-ventricular valves we find that, while in some of the best known text-books their action is considered as so thoroughly investigated and completely understood as to merit no discussion, and to require only the briefest description, in others the mechanism is admitted to be imperfectly comprehended, and is described in the most obscure and indefinite manner.