DEVELOPMENT OF THE FECUNDATED EGG. 519 



veins). Having become venous, this blood from the head 

 and upper extremities returns through the superior vena 

 cava to the right auricle, thence to the right ventricle (see p. 

 516) and the pulmonary artery. Since the lung at this 

 period forms a compact mass, and scarcely permeable, the 

 blood of the pulmonary artery passes directly into the duc- 

 tus arteriosus, and thence down the descending portion of 

 the aorta, where it mingles with that small amount of arterial 

 blood, which is not sent from the arch of the aorta to the 

 lower extremities of the foetus. Having arrived at the 

 primary iliac arteries, a large amount of the blood is diverted 

 through the umbilical arteries for the purpose of undergoing 

 ha3inatosis in the placenta, whilst a smaller amount continues 

 on its course through the iliacs, in order to nourish the pelvis 

 and lower extremities of the foetus. 



Respecting the character of the blood, which the different 

 portions of the body of the embryo receive, it may be noticed 

 that its upper part receives arterial, mixed with a small quan- 

 tity of venous blood ; whilst the parts below the umbilicus 

 receive venous, mixed with a small quantity of arterial blood. 

 This difference corresponds with what we have mentioned in 

 respect to the two lobes of the liver, and explains the pre- 

 ponderance of size, and increased development, of the upper 

 over the lower part of the body of the embryo. 

 . This placental or secondary circulation, with its mode of 

 nutrition and respiration to which it is adapted, continues 

 until the birth of the embryo. At this latter stage the 

 placental circulation ceases, and is replaced by the functions 

 of nutrition and respiration which we have already studied in 

 the adult. The disappearance of the secondary circulation 

 follows the same order that we have just studied: first, the 

 placenta, which is thrown off after the expulsion of the 

 foetus (under the name of after-birth) ; then the umbilical 

 vein, which is cut and obliterated by the teeth of animals, 

 or by direct section after ligation in mankind. That portion 

 of this vein, which goes from, the umbilicus to the liver, is 

 also obliterated by a retraction of its sides, as also the ductus 

 venosus of Arantius; these vessels are replaced by the 

 fibrous ligaments which we have studied in descriptive 

 anatomy. The Eustachian valve in the heart undergoes 

 atrophy, the foramen ovale is obliterated, and the two auri- 

 cles thenceforth are separated ; the right auricle transmits to 

 the right ventricle the blood from the inferior vena cava as 

 well as that from the superior vena cava. 



