COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 271 



the subcardinal veins which establish connections with the anterior portions of the posterior 

 cardinals. 



11. The venous system of fishes is in essentially the stage reached to this point, with a 

 renal portal and a hepatic portal system, and the cardinal veins as the chief somatic veins. In 

 fishes there is also an inferior jugular vein in the head region. 



12. The arterial system of fishes is also in a primitive condition. There is a ventral aorta 

 connected by aortic arches with a dorsal aorta which supplies the body below the heart. Of 

 the original six pairs of aortic arches the first is missing, the second imperfect, and only the 

 last four are retained entire. In fishes the central part of each aortic arch is secondarily broken 

 up into capillaries in the gill region; the ventral halves of the arches then become the afferent 

 branchial arteries, the dorsal halves the efferent branchial arteries. 



13. The primitive vertebrate heart is a bent tube consisting of four chambers, named 

 from the posterior end forward: sinus venosus, auricle, ventricle, and conus arteriosus. There 

 is a single circulation, composed of venous blood, through the heart; the blood enters the 

 sinus and passes through the chambers of the heart in the order named. This condition is 

 permanent in fishes (except Dipnoi). 



14. The anterior cardinal veins persist in all vertebrates as the internal jugular veins. 

 These unite with other veins of the anterior part of the body to form large trunks, the precaval 

 veins. The bases of the precavals are the common cardinal veins. In some mammals the 

 left precaval joins the right precaval in front of the heart, forming a single trunk. In such 

 cases the reduced base of the left precaval (left common cardinal) remains as the coronary 

 sinus, a vein of the heart wall. 



15. The anterior portions of the posterior cardinal veins lose their importance in land 

 vertebrates and disappear above urodeles, except that in mammals the proximal portion of 

 the right one contributes to the azygos. The posterior portions of the posterior cardinals 

 become the renal portal veins in all vertebrates. In fishes they collect only from the tail, 

 while above fishes they also usurp the veins from the leg which at first open into the abdominal 

 veins. The renal portal veins in fishes, Amphibia, and reptiles flow into a capillary system in 

 the kidneys, from which the blood is re-collected into the subcardinal veins which run between 

 the two kidneys. 



1 6. Simultaneously with these changes in the posterior cardinals there appears a new 

 vein, the postcaval vein. It is found in a few fishes and is present continuously from the 

 Amphibia onward. It is formed by the union of a hepatic vein in the liver (which is in turn 

 derived from the vitellines) with the subcardinal veins, chiefly the right subcardinal. The 

 subcardinals then become parts of the postcaval vein. 



17. Simultaneously with these changes the abdominal veins, which become connected 

 with the renal portal veins when the latter usurp the veins from the legs, change their anterior 

 connections. Whereas they originally entered the common cardinal veins, they now enter 

 the hepatic portal vein. The abdominal veins thus come to constitute a connection between 

 the renal portal and hepatic portal systems. 



18. In Amphibia and reptiles the postcaval vein extends only to the posterior end of the 

 kidneys. 



19. In birds the postcaval vein establishes direct connections with the renal portal veins; 

 the renal portal circulation is thus greatly reduced. In mammals the connection is completed 

 so that the veins from the legs, tail, and adjacent regions pass directly into the postcaval. 

 The renal portal circulation then vanishes. 



20. With the changes outlined in paragraph 18 the abdominal vein loses its function. It 

 is probably present in birds but has established different connections and is lacking in mammals 

 except monotremes. In embryonic stages, however, this vein (or veins) is of great importance 

 as the veins of the allantois or embryonic respiratory organ. 



