DEVELOrMEXT OF THE FCETUS. 107 



postcrior cardinal veins. The veins on each side unite in pairs to form 

 the ductus Cuvieri, which opens transversely into the omplialo-meseraic 

 trunk near the auricular cavity. The anterior cardinal veins issue from 

 the cranium, forming the jugular veins, and communicate by a trans- 

 verse anastomosis between the right and left veins. Below this the 

 left vein gradually atrophies, as does the ductus Cuvieri of the same 

 side ; while the right vessel enlarges, and is ultimately the anterior vena 

 cava. 



The posterior vena cava appears in the liver about the fifth week ; it 

 receives the veins of the kidneys and the Woltlian bodies, and behind 

 it anastomoses with the cardinal veins. The middle portion of the 

 latter disappear and are replaced by the vertebral veins, the right of 

 which forms the vena azygos. There, then, only remain two caixlinal 

 veins for the two extremities — the anterior which enters the ductus 

 Cuvieri, and the posterior which constitutes the hypogastric and crural 

 veins. It therefore happens that the venous system of the foetus, which 

 was at first symmetrical, becomes asymmetrical in the adult animal. 



As a result of these successive developments, the placental circulation 

 is instituted, and continues the same until the end of gestation, the 

 heart being the organ which circulates the blood in the foetus. This 

 fluid, carried by the arterial vessels, reaches tlie umbilical arteries, and 

 from them is distributed to the placenta. There it is respired, depurated, 

 or arterialised, through indirect contact with the blood of the parent, 

 and is returned by the umbilical veins. In the texture of the liver it is 

 mixed with the venous blood of the intestines and the hinder part of 

 the body, conveyed by the ductus vcnosus, and is finally thrown into tlie 

 right auricle, then into the ventricle, whence it is expelled by a contrac- 

 tion of that cavity. Instead of entering the lungs, which do not act 

 during fcctal life as respiratory organs, the blood is forced into the 

 ductus arteriosus, and thence into the aorta. So that the organs of the 

 young creature are never supplied with pure blood, but with a mixture 

 of arterialised and venous blood ; this mingling taking place through 

 the foramen ovale, in the aorta by the ductus arteriosus, and in the 

 liver by the ductus veiiosus. The head and neck receive the purest 

 blood, a circumstance which probably explains the predominance in 

 size of the upper to the lower parts of the body of the fa^tus. 



At birth, the conditions of existence being suddenly changed, very 

 marked modifications occur in the circulation. The lungs then become 

 tlie organs of respiration, and rapidly increase in capacity, while the 

 thorax enlarges in a commensurate degree ; the pulmonary artery also 

 dilates to admit the increased flow of blood, and the ductus arteriosus 

 is obliterated to prevent the mixture of arterial and venous blood. The 

 ductus venosus also aids in the isolation of the two kinds of blood by 

 becoming atrophied, as does also the occlusion of the foramen ovale. 

 This opening, however, not infrequently remains intact in young 

 animals ; though, as a rule, tliis does not greatly affect the circulation, 

 for when the heart contracts the auricles are isolated by the narrowing 

 of the orifice and the elevation of a valve. 



The Bespirator;/ Apparatus. 



There is an absence of unanimity as to the mode of development of 

 the respiratory apparatus, and particularly the lumjs. Some assert that 

 the latter are derived from two little distended cellular masses attached 

 to the anterior part of the intestinal tube, and which afterwards become 



