202 The Alligator and Its Allies 



In the diagrams the outlines of the more impor- 

 tant organs are accurately shown by dotted lines, 

 and the relative diameters of the blood-vessels are 

 shown as accurately as possible by the solid black 

 lines. 



The Heart 



In the Crocodilia, as is well known, the heart is 

 four-chambered and has about the same general 

 shape as in the higher vertebrates, Fig. 58. 



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 dor- 

 sal surface of the heart to enter the sinus venosus. 



The arterial blood is brought from the lungs by 



