. Cunningham's Text-book of anatomy. Anatomy. 68 HUMAN EMBRYOLOGY. External carotids Internal carotid 2nd arch Internal carotid - Internal carotid " Internal Bight common ;,. rsal root of 3rd archâ Right sub- clavian artery Innominate artery--- r^"SSi Right pulmonary-^â artery '>* + f fV. f"l f of a foetus seen from the Front, showing the me lurtner nistory oi tne arterial system, origin of the positions of the first and second arches, See Under Vascular System.) the dorsal roots of the third arches on both sides, The Primitive Venous System. The and t
. Cunningham's Text-book of anatomy. Anatomy. 68 HUMAN EMBRYOLOGY. External carotids Internal carotid 2nd arch Internal carotid - Internal carotid " Internal Bight common ;,. rsal root of 3rd archâ Right sub- clavian artery Innominate artery--- r^"SSi Right pulmonary-^â artery '>* + f fV. f"l f of a foetus seen from the Front, showing the me lurtner nistory oi tne arterial system, origin of the positions of the first and second arches, See Under Vascular System.) the dorsal roots of the third arches on both sides, The Primitive Venous System. The and the dorsal roots of the fourth and fifth arches n , -i r> â , j_ i . on the right side are shown in dotted lines. The first definite venous trunks to appear positions of the fifth arches are not shown. are the umbilical veins, returning blood from the chorion, and the vitelline veins, which convey the blood from the yolk-sac. Two umbilical veins enter the body-stalk and unite to form the vena umbilicalis impar (Fig. 81). This divides, at the posterior margin of the umbilicus, into the right and left lateral umbilical veins, which run round the lateral margins of the umbilical orifice, in the lateral margins of the body wall of the embryo. At the cranial margin of the umbilicus they turn medially, enter a transverse bar of mesoderm which forms the caudal boundary of the pericardium and is known as the septum transversum, and pass through it into the caudal end of the heart (Figs. 81, 82). The blood from the yolk-sac passes, for a short time, into the lateral umbilical veins at the posterior margin of the umbilicus. This is a very transitory arrange- ment, and it is soon replaced by the formation of two proper vitelline veins, one on each side, which ascend, along the vitello-intestinal duct, to the cranial margin Dorsal root of right 6th arch. Dorsal root of left 3rd arch Left common carotid Arch of aorta (4th arch) \ Ductus arteriosus â¢N Arch of aorta \ Left pulmon- ary
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