^q6 structure of vertebrates 



so that two streams of blood, one oxygenated and one not, 

 enter the heart. The pulmonary veins of Polyptems lead into the 

 sinus venosus, but those of the Dipnoi go straight to the left side 

 of the auricle, which is partially divided by a septum into right 

 and left halves. This condition, which presumably also occurred 

 in the ancestors of the tetrapods, made possible the next stage, 

 which we find in most Amphibia, where there are two separate 

 auricles in parallel. The right receives only deoxygenated blood 

 from the sinus venosus, while the left receives oxygenated blood 



from the two pulmonary veins. In both 

 Dipnoi and Amphibia there is only a single 

 ventricle (though in the former there is a 

 slight septum), and it has generally been 

 assumed that mixing of the two streams 

 of blood is prevented by the shape of the 

 cavity and the valves in the conus. It is 

 now known, however, that in the frog 

 there is almost complete mixing. In the 

 reptiles the arrangement of the auricles is 

 similar to that in the frog except that the 

 sinus venosus has disappeared, but there 

 is an almost complete range of sub- 

 division of the ventricle into right and 

 left halves and the conus is lost. It would 

 be unwise to speculate on the degree of 

 mixing of the two streams in those 

 groups, such as lizards, where the ven- 

 tricular septum is incomplete, but we can 

 be reasonably confident that in crocodiles, where the connection 

 between the two ventricles is only by a small hole, the foramen 

 of Panizza, there is hardly any mixing at all. As we shall see later, 

 however, there is no separation in the systemic system, so that 

 the body receives both oxygenated and deoxygenated blood 

 from the heart. In mammals and birds there are two separate 

 ventricles, and the aorta arises from one only of them, while 

 the other feeds the pulmonary arteries. The body thus receives 

 only oxygenated blood. This result is arrived at by different 

 means in the two groups. In both the conus is lost. 



The conus arteriosus leads forwards into the ventral aorta, 

 which sooner or later divides into right and left halves, which 

 represent the anterior portions of the original paired vitelline 



V-c.s. 



Fig. 461. — A semi-dia- 

 grammatic ventral view 

 of the heart and neigh- 

 bouring blood vessels of 

 a cod. 



a.b.a., Afferent branchial arteries ; 

 a.c.s., anterior cardinal sinus ; 

 au., auricle ; b.a., bulbus 

 arteriosus ; d.C, ductus 

 Cuvieri ; p.c.s., posterior car- 

 dinal sinus ; v., ventricle ; 

 v.ao., ventral aorta. 



