OCR Text |
Show 1895.] ON THE HEART OF THE ALLIGATOR. 345 arrangement markedly recalling that of the Frog. Anteriorly and to the left side the large left anterior cava opens into it; the much smaller right anterior cava opens opposite to the latter on the right side. The postcaval veiu enters the sinus in the middle line posteriorly and is of enormously large size. A large coronary vein leaves the line of junction between the ventricles and enters the postcaval after a short free course. The two auricles are free from the surface of the ventricles, thus differing from birds; they are attached to the dorsal side of the heart, and their free ends are partially wrapped round but do not meet on the ventral side. The right auricle is markedly longer than the left and its free extremity is forked. The line of junction between the ventricles is plainly marked exteriorly ; and it passes down immediately to the right of the gubernaculum, which is thus attached to the left ventricle only. The great vessels which arise from the ventricles are closely attached to each other and form an enormous bulging expansion anterior to the heart, and showing externally no trace of a division into the separate vessels. The accompanying drawings are illustrative of sections through this bulbus arteriosus at different levels. The pulmonary veins enter the dorsal side of the heart towards the left hand; the right pulmonary vein being exactly in the middle line, and the left entering at right angles to it and attached by membrane to the left precaval. Cavities of the Heart.-The small cavity of the sinus venosus leads directly into the right auricle; the valve between them (atrio-auricular) is bicuspid, being composed of two large thin muscular flaps, each semicircular and like an eyelid in shape ; the dorsal valve is slightly overlapped by the ventral at the sides, and the two do not join. The cavity of the right auricle is twice as large as that of the left; the septum between the two is complete and lies in the middle dorsal line of the heart. The' interior of both auricles is richly sculptured. The most striking point, of course, about the interior of the ventricles is the enormous thickness of the spongy walls and the very small amount of free cavity. W h e n the apex was cut off, a well-marked line, concave towards the left ventricle, and situated in the middle of the spongy tissue, showed the boundary of the inter-ventricular septum. The spongy cavities ran closely up to this line, but in no case was it actually penetrated by them. There is in short an absolute separation between the two ventricles. The right auriculo-ventricular valve consists of two separate valvular flaps equal in size; the septal, or inner flap is chiefly muscular, but a triangular piece near the upper free extremity is chiefly membranous, as is shown in the drawing (woodcut fig. 2, c). The strand of muscle running along the valve arises by a column with several roots from the septal wall of the heart posteriorly. The right, or outer, valve is entirely muscular (fig. 2, cl) • its upper surface is sculptured, chiefly in vertical lines, and bound to the |