VASCULAR SYSTEM. 507 
(6) 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 venze cave 
(into which the superior 
cardinals open as external 
jugulars). The inferior car- 
dinals at first return blood 
from the Wolffian bodies 
and the posterior region ; 
later they atrophy, and are 
replaced by an unpaired 
inferior vena cava which 
eo agli peg Fic. 269.—Diagram of circulation. 
from the liver (hepatics), —After Leunis, 
and from the hind-limbs ~.2., Right auricle receiving superior vena 
except when there is a @y% {0 aoe ences a, ae 
renal portal system. The monary artery to lungs (Z.); .v., right 
azygos vein of Mammals pulmonary vein; 4da., left auricle; 
is a persistent remnant 4., left ventricle; @o., aortic arch; 
fi i : d.ao., dorsal aorta giving off arteries to 
of the inferior cardinals. liver (dé.), to gut (g.), to body (B.); 
(c) In Amphibia 4 vein known —4o.v., portal veins; 4.v., hepatic vein. 
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 
aoa 
dao. 
