VASCULAR SYSTEM. 



459 



d.ao. 



fo.v. 



though there are no gills beyond Amphibians. From a ventral arterial 

 stem arches arise, which are connected so as to form the roots of the 

 dorsal aorta. This aorta gives off vessels to the body, while in embry- 

 onic life it sends important vitelline arteries to the yolk, and (in 

 Reptiles, Birds, and Mammals) 

 equally important allantoic ar- 

 teries to the allantois. 



Returning to the arterial system 

 of a fish, we must consider the 

 arches more carefully, and com- 

 pare them with those of Saurop- 

 sida and Mammals, where they 

 are no longer connected with 

 functional gill-clefts, and also 

 with those of Amphibians, where 

 the complications due to lungs, 

 etc. , begin (see Table, p. 458). 



The important features in the 

 development of the venous sys- 

 tem 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 re- 

 turned 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 stomach 

 and intestine is carried 

 to the liver, and not 

 directly to the heart. 



(b) At an early stage in develop- 



ment the blood is brought 

 back from the anteiior 

 region by the superior 

 cardinal veins, from the 

 posterior region by the 

 inferior cardinals. The 

 two cardinals on each side 

 unite to form the short 



transverse ductus Cuvieri, the two ducts entering the sinus 

 venosus of the heart. In Fishes the superior cardinals per- 

 sist, the inferior cardinals bring back blood from the kidneys, 

 and also to some extent, by means of their union with the 

 caudal vein, from the posterior region of the body. In some 

 cases this union with the caudal is only indirect, through 



Fig. 195. — 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. , pul- 

 monary artery to lungs (L.) ; p.v. , right 

 pulmonary vein; /.a., left auricle; l.v., 

 left ventricle; ao., aortic arch; d.ao., 

 dorsal aorta giving off arteries to liver (#.), 

 to gut (jr.), to body (B.) ; po.v., portal 

 veins ; h. v., hepatic vein. 



