. Elementary text-book of zoology. Zoology. DEVELOPMENT. DC Tf" In the foetus, respiration is effected through the placenta, and the lungs are functionless. In correspondence with this the circulation of the foetus differs from that of the animal after birth (tig. 680). From the heart the blood is driven into the descending aorta, which sends off behind two large vessels to the placenta (nntl> iliad or (dlanloic arteries). The blood, returning from the placenta in the allantoic vein, passes in great part through a connecting vessel (ductus venosus Arantii) into the inferior vena cava,
. Elementary text-book of zoology. Zoology. DEVELOPMENT. DC Tf" In the foetus, respiration is effected through the placenta, and the lungs are functionless. In correspondence with this the circulation of the foetus differs from that of the animal after birth (tig. 680). From the heart the blood is driven into the descending aorta, which sends off behind two large vessels to the placenta (nntl> iliad or (dlanloic arteries). The blood, returning from the placenta in the allantoic vein, passes in great part through a connecting vessel (ductus venosus Arantii) into the inferior vena cava, and thence in part passes into the right auricle, but the greater part passes, in conse- quence of a special arrangement of valves, directly into the left auricle through an opening in the interauricu- lar septum, called the foramen ovale. The blood which reaches the right ventricle passes through a vessel (ductus arteriosus Botalli}, connect- ing the pulmonary artery with the aorta, directly into the systemic cir- culation, except a small portion which goes to the lungs. From this condition of the circulation, it results that all the arterial vessels, except the allantoic vein, contain mixed blood. As remains of the first stage of the circulation before the develop- ment of the placenta, the omphalomeseraic vessels—an artery and a vein—which belong to the umbilical vesicle, still persist. The duration of gestation depends on the size of the borlv and the. Am 'jff FIG. 680.—Diagram of the arrangement of the principal vessels in a human foetus (after Huxley). S, Ventricle; V, Auricle ; Ao, Aortic trunk; Cc, common Carotid ; O, external Carotid ; G', internal Carotid; S, subclavian artery; 1, '2, 3, 4, 5, the arterial arches—the persistent left aortic arch is not visible ; Aod, descending aorta; O, Ornphalomeseritic (vitelline) Artery ; O', Omphalomeseriiic (vitelliue) Vein ; U, Umbilical (allantoic) arteries with their placental ramifications (U") ; Z7', Umbilical (
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
