. Cunningham's Text-book of anatomy. Anatomy. Anterior cardinal veins Sinus venosus UmBilical arteries > Vena umbilicalis impar Fig. 83.—Schema of Vascular System of an Embryo with twenty- three Somites. (Arteries after Felix, modified.) heart. The more cranially situated parts of the primitive ventral aortas remain separate and take part in the formation of ventral roots of the aortic arches. Before the single heart is formed other blood-vessels have appeared, which return blood from the chorion and 'the yolk-sac to the heart. These vessels are the primitive veins. Two veins pass from the
. Cunningham's Text-book of anatomy. Anatomy. Anterior cardinal veins Sinus venosus UmBilical arteries > Vena umbilicalis impar Fig. 83.—Schema of Vascular System of an Embryo with twenty- three Somites. (Arteries after Felix, modified.) heart. The more cranially situated parts of the primitive ventral aortas remain separate and take part in the formation of ventral roots of the aortic arches. Before the single heart is formed other blood-vessels have appeared, which return blood from the chorion and 'the yolk-sac to the heart. These vessels are the primitive veins. Two veins pass from the chorion into the body-stalk, where they fuse together to Posterior cardinal veins f()rm the vena umbfflcaKs .Ste"=su^r impar. This divides, at mesenteric) ^q cauclal end of the embryo, into the two lateral umbilical veins, which run to the heart ''one along each lateral margin of the embryo. In an embryo 1*3 mm. long (Eternod), in which the paraxial mesoderm had not yet commenced to segment into meso- dermal somites, each lateral umbilical vein received, as it entered the embryo, a large efferent vein from the yolk-sac. This condition, if regular, is very transitory. After a very short time the connexion of the vitelline veins with the caudal ends of the lateral umbilical veins is lost, and the blood is returned from the yolk-sac directly to the heart by two vitelline veins, one on each side, which run along the sides of the vitello-intestinal duct and receive the lateral umbilical veins close to the heart (Fig. 81). In the meantime a number of branches have been developed from both the dorsal and the ventral walls of the primitive dorsal aortas; the former are the somatic pre-segmental and seg- mental arteries, and the latter are the primitive vitelline arteries. In a human embryo which has de- veloped six distinct mesodermal somites the vitelline arteries form a plexus on the sides of the hind-gut area of the wall of the entodermal vesicle, from which the umbilical art
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1914
