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HANDBOOK OF PHYSIOLOGY. 



foetal life under the name of ductus arteriosus : the branches of the pul- 

 monary artery, to the right and left lung, are very small, and most of 

 the blood which is forced into the pulmonary artery passes through the 

 wide ductus arteriosus into the descending aorta. All these points will 

 become clear on reference to the accompanying diagram (Fig. 476). 



As the umbilical vesicle dwindles in size, the portion of the omphalo- 

 mesenteric arteries outside the body gradually disappears, the part inside 

 the body remaining as the mesenteric arteries. 



Meanwhile with the growth of the allantois two new arteries (umbil- 

 ical) appear, and rapidly increase in size till they are the largest branches 

 of the aorta: they are given off from the internal iliac arteries, and for a 

 long time are considerably larger than the external iliacs which supply 

 the comparatively small hind-limbs. 



Veins. The chief veins in the early embryo may be divided into two 

 groups, visceral and parietal: the former includes the omphalo-mesen- 



FIG. 479. Diagrams illustrating the development of veins about the liver. B, d c, ducts of 

 Cuvier, right and left; ca, right and left cardinal veins; o, left oniphalo-mesenteric vein; o', right 

 omphalo-mesenteric vein, almost shrivelled up; u, u', umbilical veins, of which w', the right one 

 has almost disappeared. Between the venae cardinales is seen the outline of the rudimentary liver 

 with its venae hepaticae advehentes, and revehentes. D, ductus venosus; l\ hepatic veins; ci, vena 

 cava inferior; P, portal vein; P', P', venae advehentes; m, mesenteric veins. (Kolliker.) 



teric and umbilical, the latter the jugular and cardinal veins. The 

 former may be first considered. 



The earliest veins to appear in the foetus are the omphalo-mesenteric 

 or vitelline, which return the blood from the yolk-sac to the developing 

 auricle. As soon as the placenta with its umbilical veins is developed, 

 these unite with the omphalo-mesenteric, and thus the blood which 

 reaches the auricle comes partly from the yolk-sac and partly from the 

 placenta. The right omphalo-mesenteric and the right umbilical vein 

 soon disappear, and the united left omphalo-mesenteric and umbilical 

 veins pass through the developing liver on the way to the auricle. Two 

 sets of vessels make their appearance in connection with the liver (venae 

 hepaticae advehentes, and revehentes), both opening into the united 

 omphalo-mesenteric and umbilical veins, in such a way that a portion of 

 the venous blood traversing the latter is diverted into the developing 



