The semilunar valves are mere membranous pockets, and have no tendinous cords attached to them; but on account of the extent of their convex attachment, when their free margin is made tense by the pocket being filled from the artery, the valves can only pass a given distance from the wall of the vessel and are thus held firmly in position. The force of the blood leaving the ventricle distends the vessel and pushes its wall away from the less elastic valve. When the force begins to diminish, the blood passes behind the semilunar flaps and raises them from the wall of the distended artery. The moment the current from the ventricle has ceased to flow, the pockets are forced back by the aortic blood pressure and bulge into the lumen of the vessel, so that the convex surface of the lunated portions of each valve is pressed against corresponding parts of its neighbors. Their union, which is accomplished by their overlapping to some extent, forms three straight radiating lines, and is a perfectly impervious barrier to any backward flow of blood (Fig. 118, PA and Ad).