THE COUESE OF THE CIRCULATION. 313 



head and fore-limbs. The pulmonary trunk, arising from the 

 right ventricle, leads to the fifth pair of aortic arches, which are 

 directly continuous with the dorsal aorta of the body of the 

 embryo, and from which also the small pulmonary arteries arise. 

 From the aorta a vitelline artery carries blood to the yolk-sac ; 

 and a still larger allantoic artery runs from the aorta to the 

 allantois. 



The blood is brought back to the heart by three great veins : — 

 the right and left anterior venae cavse, and the posterior vena cava. 

 The right and left anterior venae cav« return venous blood from 

 the head and fore-limbs of the embryo. The posterior vena cava 

 returns blood from the hinder part of the body, the hind limbs, 

 and the kidneys ; just before reaching the heart it is joined by 

 the ductus venosus, which returns the blood from the yolk-sac, 

 from the allantois, and from the alimentary canal of the embryo, 

 by the vitelline, allantoic, and mesenteric veins respectively. The 

 blood in the vitelline vein is arterial as regards nutrient matter ; 

 the blood in the allantoic vein is arterial as regards its gaseous 

 components ; and the blood in the mesenteric vein is venous. 

 The blood in the posterior vena cava is venous as regards 

 nutriment, and as regards gaseous components, but, having just 

 passed through the kidneys, is arterial as regards freedom from 

 nitrogenous excretory matters. 



The blood brought to the heart by the posterior vena cava 

 may therefore be spoken of as arterial, and stands in this 

 respect in marked contrast to the venous blood brought to the 

 heart by the right and left anterior venae cav^. 



All three venee cavee open into the right auricle of the 

 heart ; but, owing to the position and direction of the open- 

 ing, and to the Eustachian valve, the arterial blood from the 

 posterior vena cava is directed at once thi'ough the foramen 

 ovale into the left auricle, while the venous blood from the right 

 and left anterior venae cavas remains in the right auricle. The 

 right auricle is thus filled with venous blood, and the left auricle 

 with arterial blood. 



On contraction of the auricles, the blood they contain is 

 driven into the ventricles, so that the right ventricle will be 

 filled with venous, and the left with arterial blood. 



The left ventricle drives its arterial blood along the systemic 

 trunk, and through the third and fourth pairs of aortic arches 



