1044 



EMBRYOLOGY. 



Fig 580. 



diagoam of the cir- 

 culation at the 

 commencement of 

 the formation of 

 the placenta 



(seen from the 

 front). 



a, Venous sinus re- 

 ceiving aH the sys- 

 temic veins; 6, right 

 auricle ; 6', left 

 auricle ; c, right 

 ventricle; c', left 

 ventricle ; d, bulbus 

 aorticus,subilivi(iing 

 into, e, e', e", bran- 

 chial branches; /, 

 /', arterial trunks 

 formed by their con- 

 fluence; g, g', vena 

 azygos superior; h. 

 h'y confluence of the 

 superior and inferior 

 azygos; j, vena cava 

 inferior ; k, k', vena 

 azygos inferior ; m, 

 descending aorta ; 

 n, n, umbilical arte- 

 ries proceeding from 

 it ; 0. o', umbilicil 

 veins ; g, omphalo- 

 mesenteric vein ; r, 

 om I .halo-mesenteric 

 artery distributed 

 on the walls of the 

 vitelline vesicle, t\ 

 V, ductus venosus; 

 (/, vitelline duct ; z, 

 chorion. 



branches, which are hollowed in the centre by a cavity in 

 which the cells become free. In proportion as these new 

 vessels are developed, the omphalo-mesenteric vessels dis- 

 appear, until at last there only remain one or two ducts 

 that pass to the umbilical vesicle. 



The umhilical veins are developed immediately after the 

 formation of the omphalo-mesenteric veins ; they enter the 

 common trunk of the latter, and when its ramifications 

 diminish in volume, the umbilical veins increase rapidly ; 

 when the liver is formed around them, they throw into it 

 branches, which are the rudiments of the hepatic network. 

 Between the hepatic and sublobular veins, the umbilical 

 vein communicates with the vena cava by the ductus venosus 

 of Aranzi, which, according to M. Colin, does not exist in 

 the foetus of Solipeds in the last moiety of uterine life. 



The veins of the embryo form four principal trunks at 

 first — two anterior, the anterior cardinal veins ; and two 

 posterior — the posterior cardinal veins. The veins of the 

 same side unite in twos, from which result the Cuvierian 

 ducts ; these open transversely into the omphalo-mesenteric 

 trunk, close to the auricular cavity. 



The anterior cardinal veins issue from the cranium ; 

 they form the jugular veins, and communicate by an anasto- 

 mosis that extends transversely from left to right. Below 

 this anastomosis, the left vein gradually atrophies, as does 

 the Cuvierian duct of the same side ; but the canal on the 

 right side increases, and becomes the anterior vena cava. 



The posterior vena cava appears behind the liver towards 

 the fifth month ; it receives the veins of the kidneys and 

 Wolffian bodies, and, behind, it anastomoses with the cardinal 

 veins. The latter disappear in their middle portion, and are 

 replaced by the vertebrcd veins, the right of which forms the 

 vena azygos. There only remain the two extremities of the 

 cardinal veins ; tlie anterior enters the Cuvierian duct, and 

 the posterior constitutes the hypogastric and crural veins. 

 From this disposition, it will be seen that at first the venous 

 system of the foetus is perfectly symmetrical, but that in the 

 adult animal it becomes asymmetrical. 



In consequence of these successive developments, the 

 placental circulation is instituted, and continues the same 

 until the termination of intra-uterine life. The heart is 

 always the organ that propels the blood, and this passes 

 into the arteries, reaches the umbilical arteries, and is can'ied 

 to the placenta. There it is renewed — becomes r^?7^r?>// through 

 contact with the maternal blood — and is returned by the 

 umbilical veins. In the substance of the liver it is mixed 

 with the venous blood of the intestines and posterior ex- 

 tremities, through the medium of the ductus venosus. and at 

 last arrives at the right auricle, then passes into the right 

 ventricle, from which it is propelled by a conti-action. 



