VASCULAR SYSTEM 



563 



Returning to the arterial system of a fish, we must consider the 

 arches more carefully, and compare them with those of Sauropsida 

 and Mammals, where they are no longer connected with functional 

 gill-clefts, and also with those of 

 Amphibians, where the complica- 

 tions due to lungs, etc., begin (see 

 the Table opposite). 



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. 



i.v.c. 



Into these veins, blood returned 

 from the intestine is poured in 

 increasing quantity by other 

 veins. In the adult these per- 

 sist 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 anterior 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 persist, the F^g. 320 

 inferior cardinals bring back 

 blood from the kidneys, and 



ao. 



d.ao. 



po.v. 



-Diagram of circulation. 

 -After Leunis. 



also to some extent, by means ''•^•' Right auricle receiving superior vena 



cava (s.v.c.) and inferior vena cava 



(x.v.c.) ; r.v., right ventricle ; p.a., pul- 

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

 pulmonary vein ; l.a., left auricle ; 

 I.V., left ventricle ; ao., aortic arch ; 

 d.ao., dorsal aorta giving off arteries to 

 liver {U.), to gut [g.), to body (jB.) ; 

 po.v., portal veins ; h.v., hepatic vein. 



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 the 



medium of the kidney (Elas- 



mobranchs) ; in this way the 



renal portal system is constituted. In higher Vertebrates, 



before development is completed, the superior cardinals are 



replaced by the superior vense cava3 (into which the superior 



cardinals open as external jugulars). The inferior cardinals 



at first return blood from the Wolffian bodies and the posterior 



region ; later they atrophy, and are replaced by an un- 



