IX.] SUMMAHT. 301 



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 cavse of the right and 

 left side, and so to the heart. Of these three vense 

 cavse, the right superior and the inferior join the 

 auricle by a common entrance, but the left superior 

 has an entrance of its own. AU of these open into 

 the cavity of the right auricle, but the opening of 

 the inferior vena cava is so directed (vide Chap. viii. 

 p. 263) that the blood carried by this vessel flows 

 chiefly through the foramen ovale into the left auricle. 

 The blood from the two superior vense cavse enters the 

 right auricle only. Now the blood of the inferior 

 vena cava has 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 ven- 

 tricle (the mixture with the blood from the fifth arch 

 reaching only as far as the fourth arch), it happens 

 that the blood which flows to the anterior extremities 



