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 
