60 GENERAL EMBRYOLOGY. 
the aorta, the main vessels of the head and neck, the pulmonary artery and its 
primary branches, the common and internal iliac arteries, and the hypogastric arteries. 
The blood distributed by the various arteries is returned to the heart by vessels 
called veins, which are developed in the substance of the mesoderm in the same 
manner as the arteries. From the yolk-sac the blood returns by the vitelline 
veins ; from the alimentary canal and its appendages, through the portal and hepatic 
veins ; from the head and neck, by the ees veins and the superior vena cava ; 
and from the body and lower limbs, first by the cardinal veins, and afterwards by 
the inferior vena cava and the azygos veins. 
The heart is formed by the fusion of portions of the anterior ventral sections of 
the primitive aorte behind the origins of the cephalic aortic arches, and, therefore, 
it is primitively a bilateral organ. Subsequently it possesses for a time a single 
chamber, but this is afterwards divided. During the greater part of foetal life the 
heart, as in the adult, possesses four chambers—two auricles or upper chambers 
and two ventricles or lower chambers, right and left. The two auricles com- 
municate with the corresponding ventricles through auriculo-ventricular apertures, 
7th pair of Vertebral 
Caudal arches Dorsal aorte segmental arteries arteries aR 
1st pair of 
nen rare i segmental arteries 
: 7™ 
Umbilical vein 
Splanchnic arterie. ~ 
a i i isu cephalic aortic arch 
’ ji A Seta fl 2nd cephalic aortic arch 
es 3rd cephalic aortic arch 
4th cephalic aortic arch 
if 5th cephalic aortic arch 
Aortie bulb 
Ventricle 
Auricle 
Hypogastric artery 
Yolk-sac 
Sinus venosus 
Vitelline vein 
Fic. 46.—DIAGRAM OF THE BLOOD-VESSELS OF A MAMMALIAN EMBRYO AFTER THE FORMATION OF THE HEART. 
and with each other through a foramen, the foramen ovale, in the septum between 
them. 
In the adult the blood enters the right auricle by the superior and inferior 
vene cave and the coronary sinus; from the right auricle it passes into the right 
ventricle, by which it is propelled through the pulmonary arteries and lungs; re- 
turning to the heart by the pulmonary veins it passes into the left auricle, and then 
into the left ventricle, by the contraction of which it is forced into the systemic 
aorta. From the aorta, by v various branches, it traverses the organs and tissues of 
the body, and is returned again to the right ‘auricle. 
The course of the foetal circulation differs from that of the adult: the blood 
passes out of the body into the placenta, to be oxygenated and purified, the lungs 
of the foetus remaining functionless until the time of birth. Very little of the 
blood which is ejected from the right ventricle at every contraction of that 
chamber reaches the lungs; the greater part is transferred from the pulmonary 
artery to the aorta by an anastomosing channel, the ductus arteriosus, which 
disappears after the pulmonary circulation i is esta blished. 
During the later months of foetal life, blood enters the right auricle by the 
superior and inferior ven cave and thr ough the coronary sinus; by the latter the 
small amount only which is returning from the walls of the heart. The blood 
poured into the right auricle by the superior vena cava is returned from the head, 
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