THE ELASMOBRANCH FISHES 



171 



Tlie sinus voiiosus may in general be described as a delta-shaped collecting 

 sac, the apex of which leads to the auricle and the base of which is posterior in 

 position (see }). 203. fig. 188). Dorsally the sinus venosus is fused to the pos- 

 terior part of the roof of the pericardial cavity; laterally each angle of the 

 delta extends to the right or left as the duct of Cuvier. The principal change in 

 the form of the sinus venosus from that just described is found in some of the 

 rays (see p. 209. fig. 194b) in which the lateral angles are drawn out into the 

 elongated ducts of Cuvier. 



Connecting the sinus venosus with the auricle is the sinu-aurieular aperture 

 which, as in Hepta7ich us, is guarded by the sinu-auricular valves. These valves 

 are nothing more than double folds of the endothelial lining of the auricle 



Fig. 159. The heart opened to show valves. (From Garman.) A. Isuriis. B. CeplialoscyUium. 

 C. Mobida. 



OIL, auricle (atrium) ; c.a., conus arteriosus; s.v., 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. 



