THE FOETAL CIRCULATORY SYSTEM 115 



since from its beginning at the posterior end of the abdominal 

 cavity to its termination on the right atrium it Hes wholly to the 

 right of the median plane. The azygos vein of the thorax (Plate 

 VII), a vessel uniting the majority of the paired intercostal veins, 

 and interesting as a remnant of the primitive circulation, is also 

 asymmetrical, since the trunk lies to the right of the bodies of the 

 vertebrae, and is connected at its base with the right superior cava! 

 vein. These dispositions have been derived from originally sym- 

 metrical ones through complicated embryological changes. 



The Foetal Circulation and Its Transformation 

 to the Adult Condition 



Besides the features of the circulatory s^^stem discussed above, 

 which are explained by their embryonic origin, others reflect later 

 stages of prenatal development, when the aeration of the blood and 

 the absorption of nourishment are accomplished in the peculiar 

 organ characteristic of the higher, or placental mammals, the 

 placenta. This organ is formed in part by the wall of the maternal 

 uterus and in part by an outgrowth (the allantois) from the embry- 

 onic alimentary canal. It provides for a close interlacement of the 

 maternal and embryonic bloodvessels so that, without any mixing 

 of the two blood-streams, an exchange of dissolved material can 

 occur between them by diffusion. The course of the foetal blood 

 at this time is represented diagrammatically in figure 65. 



After aeration in the placenta, the blood returns to the body 

 of the foetus through the umbilical vein, unshaded in the diagram, 

 and is carried by it to the liver, which it traverses through a wide 

 channel, the ductus venosus. Here it is mixed with unoxygenated 

 blood from the portal vein and is then emptied into the inferior 

 vena cava, there mingling with a second stream of unoxygenated 

 blood coming from the posterior parts of the body. This mixed 

 blood is indicated in the diagram by stippling. Entering the right 

 atrium of the heart, such blood mixes very little with that coming 

 through the superior caval veins but mostly flows directly through 

 the foramen ovale (Fig. 66), a wide passage leading through the 

 median septum into the left atrium, whence the blood is pumped 

 to the left ventricle and out through the aorta. 



