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HANDBOOK OF PHYSIOLOGY. 



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- 



Fig. 496. 



Fig. 497. 



Fig. 496. Diagram of young embryo and its vessels, showing course of circulation in the 

 umbilical vesicle; and also that of the allantois (near the caudal extremity), which is just com- 

 mencing. (Dalton. ) 



Fig, 497. Diagram of embryo and its vessels at a later stage, showing the second circula- 

 tion. The pharynx, oesophagus, and intestinal canal have become further developed, and the mes- 

 enteric arteries have enlarged, while the umbilical vesicle and its vascular branches are very 

 much reduced in size. The large umbilical arteries are seen passing out in the plaeenta. (Dalton.) 



mesenteric 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 veins 

 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 



