The venous blood is emptied into a thin-walled sinus venosus on the dorsal side of the heart by three large vessels and one small one. The largest of these, the postcava, empties into the posterior side of the sinus venosus and brings blood from the posterior regions of the body; it is quite wide, but is exposed for a very short distance between the liver and the heart. Two large hepatic veins empty into the postcava so near the sinus venosus that they practically have openings into the sinus, as is shown in a somewhat exaggerated way in Fig. 59. Near the postcaval and hepatic openings is the distinct coronary vein, lying in a slight depression between the right and left ventricles. From the anterior regions of the body the blood is brought back through two fairly wide but very thin-walled precaval veins which pass across the dorsal surface of the heart to enter the sinus venosus. [Illustration: FIG. 58. HEART OF A. LUCIUS. (Dorsal View.) (From Bronn, after Fritsch.) A_o_, _d_, _Ao_, _s_, right and left aortæ; _At_, _d_, _At_, _s_, right and left atria; _Au_, _d_, _Au_, _s_, right and left auricles; _Ca_, _pr_, primary carotid; _P_, _d_, _P_, _s_, right and left pulmonary arteries; _Sc_, _d_, _Sc_, _s_, right and left subclavians; _Sv_, sinus venosus; _V_, _cc_, coronary vein; _V_, _c_, _d_, _V_, _c_, _i_, right _precava_, and inferior cava; _Ven_, _d_, _Ven_, _s_, right and left ventricles; _V_, _h_, hepatic vein; _V_, _p_, _d_, _V_, _p_, _s_, right and left pulmonary veins.] The arterial blood is brought from the lungs by two wide, thin-walled pulmonary veins, Fig. 58, V.p.s., V.p.d. They leave the lungs somewhat caudad to their middle region, near the point of entrance of the bronchii and the pulmonary arteries, pass mediad in a direction almost at right angles to the long axis of the body, and enter the left auricle at the same point. Blood leaves the heart through five large vessels: (1) the pulmonary artery, (2) the two aortic arches, (3) the right subclavian, (4) the primary carotid. The pulmonary leaves the small right ventricle as a single stem, which soon branches into two arteries that pass cephalad and laterad to the lungs, along with and close to the main bronchi. The other arteries that carry blood into the systemic circulation are fused at their base to form a sort of conus arteriosus which may be distended in injected specimens until it is larger than the two ventricles together. When opened this conus is found to contain two chambers that lead into the left ventricle; the larger chamber gives origin to the right systemic arch, the right subclavian, and the primary carotid; the smaller chamber is the basal part of the left systemic arch. The two systemic vessels, Fig. 58, Ao.s, Ao.d, pass, in the usual manner, as two arches to the dorsal region, just posterior to the ventricles, where they form the dorsal aorta in the manner to be described in connection with the arterial system.