. The elasmobranch fishes . Fig. 159. The heart opened to show valves. (From Garman.) A. Isuriis. B. CeplialoscyUium. C. Mobida. OIL, auricle (atrium) ; , conus arteriosus; , sinus venosus; vn., ventricle. projecting into the sinus venosus. They are so arranged as to permit the free passage of blood into the auricle, but a flow in the opposite direction is pre- vented by their closure. The auricle (atrium) in the Elasmobranchs in general is thin-walled and lies over the ventricle. The walls of the sac, however, may be folded and may even give the appearance of possessing two rooms. Int
. The elasmobranch fishes . Fig. 159. The heart opened to show valves. (From Garman.) A. Isuriis. B. CeplialoscyUium. C. Mobida. OIL, auricle (atrium) ; , conus arteriosus; , sinus venosus; vn., ventricle. projecting into the sinus venosus. They are so arranged as to permit the free passage of blood into the auricle, but a flow in the opposite direction is pre- vented by their closure. The auricle (atrium) in the Elasmobranchs in general is thin-walled and lies over the ventricle. The walls of the sac, however, may be folded and may even give the appearance of possessing two rooms. Internally the auricle in practically all Elasmobranchs is smooth, that is, it rarely possesses tendinous supporting cords which pass across the cavity from one wall to the other. The auriculoventricular opening may be shifted sharply to the left, so that the communication between the auricle and ventricle is visible in ventral view, ^riie auriculoventricular valve consists of two pocket-like flaps, the concavities of which are directed toward the ventricle. The ventricle is relatively small in all the Elasmol)ranchs, although in the rays it may be relatively thick. It may be described as a pyramid with the base posterior, two faces directed ventrally and outward, and the other dorsalty in position. A section through the ventricle shows its greatly thickened walls (vn., fig. 159). The lining, unlike that of the auricle, is often exceedingly rough and irregular. The tendinous cords (chordae tendineae) present in the ven- tricle of the Elasmobranchs are muscular at one end and drawn out into longer or shorter tendons at the other. The ends are attached to opposite walls and prevent the vessel from spreading beyond its capacity.
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