VASCULAR SYSTEM. 



507 



(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 each side unite to form the short transverse ductus Cuvieri, 

 the two ducts entering the 

 sinus venosus of the heart. 

 In Fishes the superior car- 

 dinals persist, 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 pos- 

 terior region of the body. 

 In some cases this union 

 with the caudal is only in- 

 direct, through the medium 

 of the kidney (Elasmo- 

 branchs) ; in this way the 

 renal portal system is con- 

 stituted. In higher Verte- 

 brates, before development 

 is completed, the superior 

 cardinals are replaced by 

 the superior venae cavae 

 (into which the superior 

 cardinals open as external 

 jugulars). The inferior car- 

 dinals at first return blood 

 from the Wolfnan bodies 

 and the posterior region ; 

 later they atrophy, and are 

 replaced by an unpaired 

 . inferior vena cava which 

 brings back blood from the 

 kidney (efferent renals), 

 from the liver (hepatics), 

 and from the hind-limbs 

 except when there is a 

 renal portal system. The 

 azygos vein of Mammals 



d.ao. 



po.v. 



FIG. 269. Diagram of circulation. 

 After Leunis. 



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 (.)> A 7 '-> right 

 pulmonary vein; /.#., left auricle; 

 l.v.y left ventricle; ao., aortic arch; 

 d.ao.) dorsal aorta giving off arteries to 

 liver (//.), to gut (g:), to body (B.)', 

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



is a persistent remnant 

 of the inferior cardinals. 

 (c) In Amphibia a vein known 

 as the epigastric (anterior 

 abdominal) carries blood from the hind-limbs into the hepatic 

 portal system. This vein also receives blood from the allantoic 

 bladder, a fact which is of great theoretical importance. In 

 all higher Vertebrates in embryonic life, the blood from the 

 allantois passes through the liver, and to a greater or less 

 extent into its capillaries, on its way to the heart. In 



