418 THE BABBIT. 



3. The Veins. 



The general relations of the veins in the rabbit, and the 

 changes which they undergo during development, are very 

 similar to those described in the next chapter as seen in the 

 human embryo, and will not be dealt with further in this 

 section (c/. pp. 578 to 583). 



4. The Course of the Circulation. 



It will be convenient to give here a brief account of the 

 course of the circulation during the latter half of intra-uterinc 

 life, when the placental circulation is in full swing ; and also 

 a summary of the changes which occur at the time of birth. 



As regards the heart, the ventricular septum is complete, as is 

 also the septum of the truncus arteriosus. The auricular septum 

 is, however, incomplete, the foramen ovale allowing blood to 

 pass across directly from the right to the left auricle. 



The blood is brought to the right auricle by the three vense 

 cavaa. Of these, the right and left anterior venas cavse — the 

 Cuvierian veins of the earlier stages — return to the heart venous 

 blood from the head and from part of the trunk of the embryo. 

 This is received into the right auricle and driven by it into the 

 right ventricle. 



The blood in the posterior vena cava is derived from many 

 sources. The main factors are the allantoic veins, which return 

 to the heart the blood from the placenta, blood which is arte- 

 rial both as regards nutritive matter and as regards its contained 

 gases. 



The other factors of the posterior vena cava are, the vitelline 

 veins from the yolk-sac, which are small and comparatively un- 

 important ; the mesenteric veins, which return venous blood 

 from the alimentary canal of the embryo, and which are of small 

 size ; and the posterior vena cava itself, which returns blood 

 from the kidneys and the hinder part of the body. Of these 

 factors, the allantoic veins are so much the largest that the blood 

 returned by the posterior vena cava to the heart may be rightly 

 spoken of as arterial. This arterial blood is discharged into the 

 right auricle, but never really enters the cavity of the auricle, 

 since it is directed at once, by the Eustachian valve, through the 

 foramen ovale into the left auricle, and driven thence into the 

 left ventricle. 



