. Animal physiology. Physiology, Comparative; Physiology, Comparative. VALVES OF THE HEART. 211 the two membranous folds, which are kept in their places by the tendinous cords, d. Now when the blood is passing from a to 5, these folds yield to the current; but when the cavity b is ,... /"|j /f/^..... / filled and begins to contract, the blood presses against their under sides, so as to make them close against each other, so far as they are permitted to do by the tendinous cords. In this manner, the aperture is completely shut,' and no blood can flow back. A i r,v , . i • , . .j « .-, Fig.
. Animal physiology. Physiology, Comparative; Physiology, Comparative. VALVES OF THE HEART. 211 the two membranous folds, which are kept in their places by the tendinous cords, d. Now when the blood is passing from a to 5, these folds yield to the current; but when the cavity b is ,... /"|j /f/^..... / filled and begins to contract, the blood presses against their under sides, so as to make them close against each other, so far as they are permitted to do by the tendinous cords. In this manner, the aperture is completely shut,' and no blood can flow back. A i r,v , . i • , . .j « .-, Fig. 121.—SECTroN of one valve of this kind exists on eacn side of the SIDB OF THE heart. heart; but there is a slight difference be- tween the forms of the two, whence they have received different names. That on the right side has three pointed divisions, to which the tendinous cords are attached; and it is hence called the tricuspid valve; whilst that on the left side has only two, so as to bear some resemblance to a bishop's mitre, whence it is called the mitral valve. 273. The aorta and pulmonary artery are in like manner furnished with valves, which prevent the blood that has been forced into them by the contraction of the ventricles, from return- ing into those cavities when they begin to dilate again. These valves, however, are formed upon a different plan, and more resemble those of the veins, which will be presently described. They consist of three little pocket-shaped folds of the lining membrane of these arteries (similar to those at b b, Fig. 122), which are pressed flat against the walls of those tubes, when the blood is forced into them ; but as soon as they are filled, and the ventricles begin to dilate, so that the blood has a tendency to return, it presses upon the upper side of these pockets, and fills them out against one another, in such a manner as completely to close the entrance into the ventricle. The three little pocket- shaped folds, however, would no
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