98 OBSTETRICAL PHYSIOLOGY. 



tricuspid valves. The heart now contains three cavities, two ventricles 

 and an auricle, but it is soon to have a fourth ; for the auricle becomes 

 partitioned into two portions, and externally this division is marked by 

 another depression, corresponding to the septum formed in the cavity. 

 It is to be remarked, however, that the partition remains incomplete 

 during the whole period of uterine life, being perforated by an opening 

 — Xh^ foramen ovale or foramen of Botal. The aortic bulb now contracts 

 and forms two vessels — the aorta and pulmonary artery. 



The arteries are developed, partly at the expense of the primary circu- 

 lation, and partly in the vascular layer of the embryo. The aortic 

 arches, situated at the inner face of the first two pharyngeal arches, form 

 five new vessels, which are placed within the other arches \ all these, 

 however, do not exist at the same time, some becoming atrophied while 

 the others are forming. For instance, the first two completely disappear ; 

 the third form the carotids ; the fourth, the axillary artery and the arch 

 or cross of the aorta ; the fifth is atrophied on the right, but on the left 

 forms the pulmonary artery, the ductus arteriosus^ and the aorta. The 

 latter is continued along the spine by the fusion of the two primitive 

 aortae, and shows at its posterior termination the small pelvic vessels 

 and the large umbilical arteries. 



The peripheral vessels originate, independently of the central vessels, 

 in the interior of the vascular layer. They appear as solid cellular 

 branches, which soon become hollow in their interior, and free cells 

 become visible. As these new channels are developed, the omphalo- 

 meseraic vessels gradually diminish and disappear, until at last only one 

 or two pass to the umbilical vesicle. 



The umbilical veins are developed immediately after the formation of 

 the omphalo-meseraic veins, and join these ; as the ramifications of the 

 latter diminish in size the former increase rapidly, and when the liver is 

 formed around them they throw it into those branches which are the 

 rudiments of the hepatic plexus. Between the portal and hepatic veins, 

 the umbilical vein communicates with the vena cava by the ductus vetiosus 

 of Aranzi, which does not exist in the fcetus of solipeds in the last 

 month of gestation. The embryonic veins form four chief trunks : two 

 anterior, the anterior cardinal veins ; and two posterior, the posterior car- 

 dinal veins. The veins on each side unite in pairs to form the ductus 

 Cuvieri, which opens transversely into the omphalo-meseraic trunk near 

 the auricular cavity. The anterior cardinal veins issue from the cranium, 

 forming the jugular veins, and communicate by a transverse 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 carva. 



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

 receives the veins of the kidneys and the Wolfiian 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 cardinal veins for the 

 two extremities : the anterior, which enters the ductus Cuvieri, and the 

 posterior, which constitutes the hypogastric and crural veins. It there- 

 fore 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 



