TSE FCETUS. 



917 



tlie anterior cardinal veins ; and two posterior, tte posterior cardinal veins. 

 The veins of the same side unite in twos, from which result the Guvierian 

 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 anastomosis that extends transversely 



PLAN OP THE AORTA AND ITS ARCHES AT AN EARLY PERIOD. 



1, Truncus arteriosus, witli one pair of aortic arches, and dotted lines indicating the 

 position of the second and third pairs. 2, The same, with four pairs of aortic 

 arches, and indications of the fifth. 3, The same, with the three posterior pairs 

 of aortic arches, from which the permanent vessels of the embryo are developed, 

 with dotted outlines showing the position of the two (now) obliterated anterior 

 arches. 4, Permanent arterial trunks in their primitive form, the obliterated 

 portions still shown in dotted outline, 1-5, Primitive aortic arches. — a, Aorta ; p, 

 Pulmonary artery ; p', p", Branches to the lungs ; aw', Boot of thoracic aorta 

 (ad) on left side ; aw, Obliterated root springing from right side ; s", s', Subclavian 

 artery ; v, Vertebral ; ax, Axillary ; o, Common carotid ; c'. External carotid ; 

 c". Internal carotid. 



from left to right. Bejow this anastomosis, the left vein gradually atrophies, 

 as does the Guvierian 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 anasto- 

 moses with the cardinal veins. The latter disappear in their middle portion, 

 and are replaced by the vertebral veins, the right of which forms the vena 

 azygos. There only remain the two extremities of the cardinal veins ; the 

 anterior enters the Guvierian duct, and the posterior constitutes the hypo- 

 gastric and crural veins. Prom this disposition, it will be seen that at first 

 the venous system of the fcetus 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-uterme life. 

 The heart is always the organ that propels the blood, and this passes into the 

 arteries, reaches the umbilical arteries, and is carried to the placenta There 

 it is renewed becomes arterial 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 extremities through 

 the medium of the ductus venosus, and at last arrives at the right auricle, 

 then the risht ventricle, from which it is propelled by a contraction Instead 

 of eoing to the lungs, which do not act as respiratory organs, the blood, 

 being pressed by the contraction of the right ventricle, passes into the aorta 

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