. A text-book of human physiology . her as in By this means closure is established over a greater surface and is securedalso by the fact that the bent portions of the edges dovetail into each other bytoothlike folds, so that the valves are able to sustain the great pressure. Thecircumference of the base of the heart, and at the same time that of the atrio-ventricular opening, becomes very much smaller in ventricular systole: the mus-cles surrounding the opening therefore must be credited with a share in theclosure of the passage. The role of the papillary muscles in the closing of the
. A text-book of human physiology . her as in By this means closure is established over a greater surface and is securedalso by the fact that the bent portions of the edges dovetail into each other bytoothlike folds, so that the valves are able to sustain the great pressure. Thecircumference of the base of the heart, and at the same time that of the atrio-ventricular opening, becomes very much smaller in ventricular systole: the mus-cles surrounding the opening therefore must be credited with a share in theclosure of the passage. The role of the papillary muscles in the closing of the atrio-ventricular valveshas been conceived in very different ways. The most probable view is, that theyprevent swinging of the valves into the auricles, the approach of the heart basetoward the apex being compensated by their contraction and consequent pull onthe chordae tendinae. When the auricles contract, regurgitation of blood into the great veins isprevented by contraction of the circular musculature passing around the Fig. 54.—Position of the atrio-ventricular valves when closed,schematic drawing, after Krehl. THE HEART SOUNDS 167 their openings being in this way either narrowed considerably or actuallyclosed. B. THE SEMILUNAR VALVES Since the blood cannot flow back into the auricle during the ventricularsystole, it must pass into the great arteries. The mouths of the latter areclosed by means of valves, each consisting of three pouchlike flaps. These flaps are semicircular membranes, fastened by their curved edges tothe wall of the vessel, so that they stand out with their straight edges from thewall and present concave surfaces toward the arteries. In this way pouchesare formed in which the blood is caught and dammed back, while at the sametime the wall of the pocket turned toward the lumen of the vessel is put onthe stretch. In both the aorta and the pulmonary artery the wall bulges outward directlyabove the attached edges of the valve, forming in each three
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