1895.] ON THE HEAET OF THE ALLIGATOR. 345 



arrangement markedly recalling that o£ the Frog. Anteriorly and 

 to the left side the lai'ge left anterior cava opens into it ; the 

 much smaller right anterior cava opens opposite to the latter on 

 the right side. The postcaval vein enters the sinus in the middle 

 line posteriorly and is of enormously large size. A large coronary 

 vein leaves the hne 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 illus- 

 trative 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 semicircidar 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 sculptm-ed. 



The most striking point, of course, about the interior of the ven- 

 tricles is the enormous thickness of the spongy walls and the very 

 small amount of free cavity. When 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-ven- 

 tricular 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 Saps 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 fi'om 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 Unes, and bound to the 



