302 THE SIXTH DAY. [CHAP. 



and head is more aerated than that in any other part 

 of the body. 



From the anterior extremities the blood is to a 

 great extent returned by the left superior cava, and 

 goes into the right auricle, whence, by the right ven- 

 tricle, it is distributed through the fifth pair of arches 

 over the body, after joining the more aerated blood 

 passing through the fourth pair of arches. 



The blood from the lungs is brought back by two 

 small veins into the left auricle. 



The characteristics of the circulation at this time are 

 that the blood is aerated by the allantois, and that there 

 is a partial double circulation. (Vide Chap. vm. p. 263.) 



As soon as respiration commences the canals 

 leading to the dorsal aorta from the fifth pair of arches, 

 which communicate only with the right ventricle, be- 

 come closed. The blood passing along the fifth arch 

 now flows only into the lungs, through the pulmonary 

 arteries. The blood from the left ventricle owing to 

 the cessation of the circulation of the yolk-sac and of the 

 allantois is distributed exclusively to the body of the 

 chick, from whence it is all brought back into the right 

 auricle by the three now independently opening vena3 

 cavse. 



The portal veins henceforward receive blood from 

 the intestines only, and the ductus venosus is ob- 

 literated, so that all the blood of the portal vein passes 

 through the capillaries of the liver. 



The partition between the auricles is rendered com- 

 plete by the closure of the foramen ovale; into the 

 right auricle the veins of the body enter, and into the 

 left the pulmonary veins. 



