696 THE BLOOD-VASCULAR SYSTEM 



Arterial blood, transmitted from the placenta through the umbilical vein, 

 passes almost entirely by way of the ductus venosus to the vena cava inferior. 

 From here it passes through the right atrium; then, obliquely between the atrial 

 septa into the left atrium, from which it passes through the left ventricle and 

 into the ascending aorta. Escaping largely through the branches of the aortic 

 arch, it is distributed to the head and upper extremities, and returned to the vena 

 cava superior. Having reached the right atrium it passes, to the right of the 

 stream from the vena cava inferior (p. 513), through the atrio-ventricular ostium 

 into the right ventricle. The blood issuing from the right ventricle into the 

 pulmonary artery goes almost entirely (the lungs being functionless) into the 

 ductus arteriosus and so into the descending aorta. Having performed two 

 circuits, the blood returns to the placenta through the umbilical branches of the 

 hypogastric arteries. 



The two streams, arterial and semi-venous, cross one another in the right 

 atrium. The degree of intermixture, if any, which occurs in this cavity has been 

 the subject of much discussion; for literature and experimental evidence on this 

 point see Pohlmann, A. (Johns Hopkins Hosp. Bui., Vol. 18, 1907.) 



When the lungs assume their function at birth the pressure in the left atrium 

 is suddenly raised by an inrush of blood. The overlapping atrial septa (primum 

 and secundum) are brought into lateral apposition and thus the blood entering 

 the right atrium finds but one exit — the atrio-ventricular ostium. Since the 

 vessels of the expanded lungs now transmit a greatly increased volume of blood, 

 the stream passing through the ductus venosus is diminished proportionately. 

 The blood traversing the aortic arch, released from the check exerted by the lateral 

 stream pouring from the ductus arteriosus, passes more readily into the descending 

 aorta; thus the adult equihbrium is established. 



References for blood-vascular system. — A. Heart: {Development) Born, 

 Archiv f. mikr. Anat., Bd. 33, 1889; His, Anatomie menschl. Embryonen, 

 1880-85, Anatomie des menschl. Herzens, 1886; Tandler, in Keibel and Mall's 

 Human Embryology. (Morphology) MacCallum, Johns Hopkins Hospital 

 Reports, vol. 9, 1900; Mall, Amer. Jour. Anat., vol. 11, 1911, vol. 13, 1912; 

 (Atrio-ventricular bundle) Keith and Flack, Jour. Anat. and Physiol., vol. 41, 1907. 

 B. Arteries. (Development) Evans, in Keibel and Mall's Human Embryology; 

 (Pulmonary) Bremer, Anat. Rec, vol. 3, 1908; (Internal mammary) Mall, Johns 

 Hopkins Hospital Bui., 1898; (Cephalic) Tandler, Morph. Jahrb., Bd. 30, 1902; 

 (Celiac) Tandler, Anat. Hefte, Bd. 25, 1904; (Extremities) Miiller, Anat. Hefte, 

 Bd. 22, 1903; de Vriese, Arch, de Biol., T. 18, 1902; (Variations) Goppert, 

 Morph. Jahrb., Bd. 40, 1909; C. Veins. (Development) Davis, Amer. Jour. 

 Anat., vol. 10, 1910; (Brain) Mall, Amer. Jour. Anat., vol. 4, 1904; (Liver) Mall, 

 Amer. Jour. Anat., vol. 5, 1905; (Cervical) Lewis, F. T., Amer. Jour. Anat., vol. 

 9, 1909; (Upper extremity) Berry and Newton, Anat. Anz., Bd. 33, 1908; (Vena 

 cava inferior) Nouberger, Anat. Anz., Bd. 43, 1913; v. Alten (ibid): (Sinusoids) 

 Minot, Proc. Boston Soc. Nat. Hist., vol. 29, 1900. 



