Historical notes on the mechanism of closure of the atrioventricular valves.

"The exact mechanism for closure of the mitral valve is unknown." This was stated in a textbook on The heart, published in 1978.1 Ifthis problem has not yet been clarified it is not through lack of effort, the work of many authors having produced various ideas since Galen. Galen's clear appreciation of the function of the heart valves is evident from the following passage: "The common purpose of them all [the valves] is to prevent things from moving backwards again. But each kind has its own special purposes. In the cases ofthose which direct things out ofthe heart it is to prevent backflow into that organ; in the cases of those which direct things into the heart, it is to prevent any outflow from it As there are altogether four openings, two in each ventricle, one ofeach pair leads in and the other out."2 The function of the cardiac valves to allow the flow of blood in only one direction while preventing movement in the opposite one was one of the features on which Harvey,3 who quoted the above passage of Galen, based his concept of the circulation of the blood. In the introduction of his De motu cordis he wrote: "If the three tricuspid valves at the entry into the right ventricle hinder return of blood into the vena cava, and if the three semilunar ones at the opening into the artery-like vein [pulmonary artery] have been made to hinder return ofblood; how, when a similar arrangement holds in the left ventricle, can we say that these valves have not been made similarly to hinder forward or backward movement of the blood?" And, associating himself with Galen, ". . . of the four kinds of valve present, two subserve entry and two exit of the blood".4 These quotations refer to all four valves of the heart. The present paper deals only with the atrioventricular (a-v) valves, i.e. those between atria and ventricles, and structures directly involved in their position and movements, viz. the papillary muscles and their tendinous connexions to the valves, the chordae tendineae. It is proposed to preface the discussion of the mechanism of a-v valve closure by remarks on two closely related anatomical features, and then to treat the functional aspects by discussing (1) passive closure; (2) active closure; (3) combination ofthe two; (4) role of atrial contraction; (5) some additional aspects.