756 



ANATOMY OF VERTEBRATES. 



588 



of the heart, h' s receives not only the blood which has circulated 

 through the body of the foetus, but also that which has passed 

 through the placenta, consequently a mixture of venous and ar- 

 terial blood; — the blood in the precaval, v*, being entirely 

 venous, that in the post-caval, v, being mixed. A part of this 

 blood so accumulated in the right auricle 

 passes into the left auricle, h, by the f fora- 

 men ovale,' /, in the septum auricularum, 

 and it is chiefly the blood from the post- 

 caval which takes that course. The rest of 

 the blood entering the right auricle passes 

 into the right ventricle, h', and thence into 

 the pulmonary artery : but very little blood 

 is sent to the collapsed lungs, for a passage 

 of communication continues from the pul- 

 monary artery into the descending aorta by 

 retention of part of the third primitive arch, 

 fig. 420, forming the ' ductus arteriosus,' 

 fig. 588, d; thus the greater mass of the 

 blood, which in the adult would have pro- 

 ceeded to the lungs, is in the fcetus imme- 

 diately transmitted to the aorta, A. This, 

 after its origin from the left ventricle, 

 delivers almost all the blood expelled by the 

 contraction of that cavity into the carotid 

 and subclavian arteries, while the ductus 

 arteriosus passing between the trunk of the 

 pulmonary artery and the descending aorta 

 directs the blood which passes through the 

 right ventricle to the lower regions of the body. In this manner 

 the upper regions are supplied with the most arterialised part of 

 the blood from the left side of the heart and aorta, while the 

 purely venous blood is propelled from the right ventricle through 

 the pulmonary artery and ductus arteriosus into the descending 

 aorta, and consequently into the lower part of the body, and by 

 the allantoic arteries to the placenta. The circulation in the 

 fcetal Mammal thus offers a close and interesting analogy to that 

 in adult Crocodilian Reptiles (vol. i. p. 512). 



The foramen ovale in the septum of the auricles, the ductus 

 arteriosus passing from the pulmonary artery to the aorta, the 

 ductus venosus leading from the allantoic vein to the post-caval, 

 and the allantoic (umbilical) vein and arteries, are the structural 

 peculiarities of the mammalian fcetal circulating organs. These 



Foetal circulation seen from 

 behind. 



