ARTERIAL AND VENOUS SYSTEMS OF EMBRYO. 



939 



but remains incomplete throughout the whole of foetal life, the opening 

 being termed the foramen ovale. Contemporaneously with the formation 

 of these septa, a transformation occurs in the arrangement of the Arterial 

 trunks proceeding from the heart, which ends in their assumption of the form 

 they present until the end of Foetal life ; and this undergoes but a slight 

 alteration, when the plan of the circulation is changed at the moment of the 



FIG. 314. 



FIG. 345. 



FIG. 344. Diagram of the Circulation in the Human Embryo and its Appendages, as seen in profile 

 from the right side, at the commencement of the formation of the Place'nta. 



FIG. 345. The same, as seen from the front: a, venous sinus, receiving all the systemic veins; 6, 

 right auricle ; &', left auricle ; c, right ventricle ; c', left ventricle ; </, bulbus aorticus ; subdividing into 

 e, e', e", branchial arches ; /,/', arterial trunks formed by their confluence ; g, g', vena azygos superior ; 

 h, h', confluence of the superior and inferior azygos ; j, vena cava inferior ; k, h', vena azygos inferior ; m, 

 descending aorta ; n, n, umbilical arteries proceeding from it : o', o, umbilical veins ; q, omphalo-mesaraic 

 vein ; r, omphalo-mesaraic artery, distributed on the walls of the vitelliue vesicle, t ; v, ductus venosus ; 

 y, vitelliue duct ; z, chorion. 



first inspiration. The number of aortic arches on each side, which was five 

 at first, soon becomes reduced in the Mammalia to three, by the obliteration 

 of the two highest pairs. The Bulbus Aorticus is subdivided by the adhe- 

 sion of its walls at opposite points into two tubes, of which one becomes the 

 origin of the aorta, and the other that of the pulmonary artery, and of the 

 three remaining pairs of vascular (branchial) arches, the third, being con- 

 nected with the aortic trunk, contributes, with portions of the two highest 

 pairs, to the formation of the external and internal carotid arteries; whilst 

 of the second pair, the arch on the right side forms the innominate and the 

 beginning of the right subclavian, and the other becomes the arch of the 

 aorta, and contributes to form the left subclavian. The lowest pair is en- 

 tirely obliterated on the right side. On the left, it gives off the pulmonary 

 artery, and remains throughout foetal life in communication with the aorta, 



