The alligator and its allies

by A. M. Reese

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.

Fig. 58. Heart of A. Lucius. (Dorsal View.)
(From Bronn, after Fritsch.)

Ao, 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.

The further course of the primary carotid and of the right subclavian will also be described in connection with the arterial system.

The auricles are very large in proportion to the ventricles, though their relative sizes will, of course, vary with the amount of contained blood.