978 SPECIAL PHYSIOLOGY. 



course. The inferior vena cava is developed as a median vein accom- 

 panying the aorta, altogether independently of the posterior lateral 

 cardinal veins. It is chiefly formed by the upper part of the omphalo- 

 mesenteric trunk, which is joined by the umbilical vein, forming the 

 ductus venosus ; it also receives the common trunk of the future iliac 

 veins. It terminates, at first, in the left half of the common auricle, 

 but is soon shut off from that by the lower fold of the inter-auricular 

 septum. The mode in which the pulmonary veins are formed, and 

 brought into relation with the left auricle, is not known. 



Embryonal and Foetal Circulations. 



The circulation established between the early embryo and the va- 

 cular area upon the yolk, has been called the first embryonal circulation ; 

 it is intended for nutrient and respiratory purposes. In the ovum of all 

 the Vertebrata it soon ceases to be sufficient as a respiratory circula- 

 tion. Thus, in the Fish and Amphibia, the branchial arches give ori- 

 gin to external or internal gills, which carry on the embryonic respi- 

 ration ; but in the Reptile, Bird, and Mammal, the allantoid sac is 

 developed, the bloodvessels of which convey blood outwards from the 

 body of the embryo, in order that it may be aerated, and then return 

 it to the embryo again, thus assisting in the respiratory function. 

 Moreover, in the Mammalian ovum, in which the yolk is so minute, the 

 allantois and its vessels, as we have seen, assist in the formation of 

 the placenta, which is not only a respiratory, but a nutrient organ. 

 This allantoid or placental circulation constitutes what has been called 

 the second embryonal circulation. It is thus carried on : From the 

 iliac arteries of the embryo, the two hypogastric or umbilical arteries 

 proceed outwards along the allantoid sac, and, in the eggs of the Rep- 

 tile and Bird, ramify in the outer layer of the allantois, immediately 

 beneath the permeable shell arid shell-membrane. In the Mammalian 

 embryo, the blood enters the membranous or villous placenta, and is 

 returned by the single umbilical vein, which runs beneath the liver and 

 joins the inferior vena cava just below the heart. In passing beneath 

 the liver, the umbilical vein communicates freely with the portal vein. 

 The blood brought back by the umbilical vein is arterialized ; it partly 

 enters the liver through its portal veins, and is thence returned by the 

 hepatic veins to the inferior vena cava, but it is partly conducted on 

 directly to this last-named vein, the portion of the venous trunk which 

 so conducts the blood being named the ductus venosus. In the vena 

 cava inferior, this mixed hepatic and arterial blood becomes further 

 mingled with the venous blood from the hinder part of the embryo, and 

 the combined stream then enters the right auricle, and here, guided by 

 the Eustachian valve, is chiefly, and, in the earlier stages of develop- 

 ment, entirely, conveyed through the foramen ovale in the auricular 

 septum, directly into the left auricle. On the other hand, the purely 

 venous blood returned from the anterior half of the embryo by the 

 superior vena cava, separated by the same valve, remains in the right 

 auricle. The contents of the auricles being, by their contraction, 



