550 



VERTEBRATE LIFE AND ORGANIZATION 





1 Arm. 

 Duclrus a.rteriosuS 



PulTnonai'y cLrlery 



Pulmonajry vein. 

 Fors-merL ovale 



DorsaJ aorta. 

 ^ Kidney 



I — Posterior vena-Ca-v^a. 



ommon iha.c 



a.rtery 



Umbil ic aJ ar tc ly 



Figure 27.5. Circulation in a fetal mammal. The shading gives some indication 

 of the mixing of the blood, though there is more mixing than can be indicated 

 diagrannnatically. The lightest shading represents blood with the highest oxygen 

 content; the darkest shading, blood with the lowest oxygen content. (Modified after 

 Patten.) 



through the body, requires certain differences in the fetal circulation 

 (Fig. 27.5). Blood rich in oxygen returns from the placenta in an um- 

 bilical vein that enters the posterior vena cava, where it is mixed with 

 blood returning from the posterior half of the fetus. The posterior vena 

 cava empties into the right atrium, which also receives venous blood 

 from the head by way of the anterior vena cava. 



The lungs cannot handle all of this blood and are largely by-passed 

 in one of two ways. The entrance of the posterior vena cava is directed 

 toward an oj^ening, the foramen ovale, in the partition separating the 

 two atria. Most of the blood from the posterior vena cava tends to go 

 through this into the left atrium, thence to the left ventricle and out 

 to the body through the arch of the aorta. The rest of the blood from 

 the posterior vena cava enters the right ventricle along with the blood 

 from the anterior vena cava, and starts out the pulmonary artery toward 

 the lungs. However, only a fraction of this blood passes through the 



