VIII.] THE ADULT CIRCULATION. 223 



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 ventricle, 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. VII. § 9.) 



As soon as respiration commences, the canals leading to 

 the dorsal aorta from the fifth pair of arches, which com- 

 municate only with the right ventricle, become 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 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 

 venae cavse. 



The portal veins henceforward receive blood from the intes- 

 tines only, and the ductus venosus is obliterated, so that 

 all the blood of the portal vein passes through the capil- 

 laries of the liver. 



The partition between the auricles is rendered complete 

 by the closure of the foramen ovale ; into the right auricle 

 the veins of the body enter, and into the left the pulmonary 

 veins. 



There is thus a completely double circulation formed, in 

 which all the blood of the left ventricle is arterial, and all the 

 blood of the right ventricle venous, and there is at no part 

 of the circulation a mixture of venous and arterial blood. 



12. As early as the sixth day, movements, as we have 

 said, may be seen in the limbs of the embryo upon opening 

 the egg. We may conclude that after this epoch spontaneous 

 movements occur from time to time in the unopened egg. 

 They cannot however be of any great extent until the four- 

 teenth day, for up to this time the embryo retains the position 



