COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 241 



the renal portal and abdominal veins as shown in Figure 57-6. We note further that the ventral 

 abdominal vein (which is homologous with the lateral abdominal vein of elasmobranchs) has 

 shifted its anterior connections; in elasmobranchs it returns to the common cardinal vein 

 as in Figure 57^!, but in Amphibia and reptiles it passes to the liver where it joins the hepatic 

 portal vein, as in Figure 57^. It thus happens that in Amphibia and reptiles the blood from 

 the hind limbs and tail can pass either into the renal portal system or into the ventral abdominal 

 veins. This arrangement appears to be an attempt to prevent the stagnation of the blood 

 from the posterior part of the body in the kidneys. It will be noted that the ventral abdomi- 

 nal veins act as connections between the renal portal and hepatic portal systems. The renal 

 portal vein passes into a capillary system in the kidneys from which the blood is re-collected 

 into the postcaval. There is, however, some evidence that already in the turtle there exist 

 direct channels through kidneys by which the renal portal veins empty directly into the post- 

 caval. This is the beginning of the retrogression of the renal portal system as a portal system. 

 The postcaval vein with the loss of the anterior parts of the posterior cardinal veins 

 becomes the chief vein of the posterior part of the body. Its mode of origin in Amphibia and 

 reptiles is shown in Figure 57$ and C. Its anterior part arises from the hepatic veins which 

 are the proximal portions of the vitelline veins. Its posterior part between the kidneys con- 

 sists of the two subcardinal veins, which in elasmobranchs are continuous with the posterior 

 cardinals. The middle part of the postcaval between these two regions is formed by an out- 

 growth from the hepatic portion. In some urodeles the posterior cardinals persist, and they, 

 together with the postcaval, connect with the subcardinals, as in Figure 57C, but in reptiles 

 these parts of the posterior cardinals vanish, leaving only the postcaval to collect from the 

 kidneys (Fig. 57 D). 



E. THE CIRCULATORY SYSTEM OF THE PIGEON 



In case a fresh specimen is provided for this work it should be opened as 

 before by deflecting the pectoral muscles from either side of the keel of the 

 sternum, then cutting through the sternum on each side of the keel and removing 

 a median portion of the sternum including the keel. The peritoneal cavity is 

 to be opened as before by a longitudinal incision. The specimen should have 

 been injected through the pectoral artery. 



i. The chambers of the heart. The heart is relatively large and more com- 

 pact than in the forms previously studied. The chambers are more closely knit 

 together than in the lower vertebrates. The major portion of the heart is formed 

 of the right and left ventricles, which together constitute a muscular thick-walled 

 cone, having a pointed apex directed posteriorly and a broad base directed 

 anteriorly. The two ventricles are completely separated from each other, but 

 the division between them is indistinct externally. This division passes obliquely 

 from the left side of the base to about the middle of the right side of the heart; 

 the left ventricle is therefore much the larger of the two and includes the whole 

 of the apex of the heart. Anterior to the ventricles are the two much smaller 

 auricles, thin-walled chambers. The division between auricles and ventricles 

 is generally concealed by a line of fat which should be removed. From the 

 anterior end of the heart between the auricles the great arteries spring without 

 the intervention of a conus arteriosus. On raising the ventricles the dorsal 

 oortions of the auricles become visible. There is no sinus venosus, the great 



