456 



STRUCTURE OF VERTEBRATA. 



The important features in the development of the venous system are 

 as follows : 



(a) In the embryo the vitelline veins bring back blood from the yolk 

 sac, at first directly to the heart, and later to the liver. Into 

 these veins, blood returned from the intestine is poured in 

 increasing quantity by other veins. In the adult these persist to 

 form the hepatic portal system by means of which blood from the 



FIG. 146. Diagram of Circulation. (After LEUNIS.) 



r.a., Right auricle receiving superior vena cava (s.v.c.) and inferior 

 vena cava (i.v.c.') ; r.v., right ventricle ; p. a., pulmonary artery to 

 lungs (Z,) ; p.v., right pulmonary vein ; /.#., left auricle ; /.z/., left 

 ventricle ; ao., aortic arch ; d.ao., dorsal aorta giving off arteries to 

 liver (//.), to gut (-.), to body (B.) ; #o.v., portal veins ; k.v., hepatic 

 vein. 



stomach and intestines is carried to the liver, and not directly to 

 the heart. 



(b) At an early stage in development the blood is brought back from 

 the anterior region by the superior cardinal veins, from the 

 posterior region by the inferior cardinals. The two cardinals on 



