222 THE SIXTH DAY. [CHAP. 



Through the other aortic root, viz. that from the left 

 ventricle, the blood flows into the third and fourth pairs of 

 arches. That part of the blood which flows into the third 

 pair of arches, passes almost entirely to the head and upper 

 extremities by the external and internal carotids ; that which 

 flows into the right arch of the fourth pair, is chiefly brought 

 to the dorsal aorta, but some of it passes to the right wing ; 

 that, on the contrary, which goes into the left fourth arch, is 

 for the most part sent to the wings, a small part only reaching 

 the dorsal aorta. There is still a mixture of the blood from 

 the two chambers of the heart, so that the blood in the 

 dorsal aorta is composed partly of the blood from the left, 

 and partly from the right chambers. The blood of the upper 

 (anterior) end of the body comes entirely from the left ventricle. 



The blood of the dorsal aorta passes to the yolk-sac and 

 allantois, and to all the hinder parts of the body. It 

 is brought back from the yolk-sac, from the allantois, and 

 to a certain extent from the intestines, by the portal vein, 

 part of the blood from which passes to the inferior vena cava 

 by the direct course (ductus venosus), and part indirectly by 

 the more circuitous course of the capillaries of the liver and 

 hepatic veins. 



The blood from the generative and urinary organs, and 

 from the hinder extremities, is brought back to the heart by 

 the vena cava inferior ; that from the upper extremities and 

 head by the jugular, vertebral and wing veins into the two 

 venae cavae of the right and left side, and so to the heart. 

 Of these three venae cavae, the left superior and the inferior 

 open into the heart independently. The right superior enters 

 with the inferior. All of these enter the right auricle, but 

 the common opening of the inferior and right superior venae 

 cavae is so directed, that the blood carried by those vessels 

 flows chiefly through the foramen ovale into the left auricle. 

 The blood from the left superior vena cava enters the right 

 auricle only. Now the blood of the inferior vena cava 

 lias been partly aerated by the allantois ; and, since it is this 

 blood which passing through the left auricle and ventricle is 

 distributed to the third aortic arch, unmixed by any blood 

 from the right ventricle (the mixture with the blood from the 

 fifth arch taking place in the fourth arch only), it happens 

 that the blood which flows to the anterior extremities and 



