THE FCETAL CIRCULATION. 929 



venosus degenerates to a solid cord up to the point where the hepatic veins, developed from the 

 hepatic net-work, unite with it. The umbilical vein also degenerates to form the round ligament 

 of the liver, which frequently presents more or less distinct evidences of its original lumen. 



The Veins of the Limbs. — The details of the development of the limb veins are not as yet 

 thoroughly known. The superficial veins are the first to form, the basilic vein of the arm and 

 the long saphenous of the leg being the primary vessels and the axillary and subclavian and the 

 iliac veins their respective continuations. The remaining superficial veins and the deep veins 

 are later formations. 



The Pulmonary Veins. — The pulmonary veins make their e.xit from the lungs as four 

 vessels, two belonging to each lung, but as they approach the heart they unite first in pairs and 

 then to form a single trunk which opens into the right auricle. . Later a considerable portion of 

 the original veins is taken up into the wall of the auricle, the absorption eventually extending 

 beyond the point of the union of the original veins in pairs, so that in the adult all four veins. 

 open independently into the auricle. 



THE FCETAL CIRCULATION. 



The primary or vitelline circulation of the mammalian embryo, formed 

 by the ramifications of the viteUine arteries and veins over the yolk-sac (umbihcal 

 vesicle), must be regarded as an inheritance from ancestors in whom the yolk 

 provided the ntitrition for the developing- animal. While in birds and reptiles 

 the vitelline veins are important channels for the conveyance of the nutritive 

 materials taken up from the yolk, in mammals in this respect they are of little 

 consequence, thus affording an example of structures that, although no longer 

 useful, recur in the development. The \'itelline circulation is soon followed by a 

 second, the allantoic circulation, which in man and the higher mammals provides 

 the vessels connecting the foetus with the placenta — the organ whereby respiration 

 and nutrition are secured to the foetus during the greater part of its sojourn within 

 the uterus. 



The blood is carried from the foetus to the placenta by the hypogastric arteries 

 and ' their prolongations, the two lunbilical arteries. After passing through the 

 vascular tufts of the chorionic villi that constitute the essential structures of the foetal 

 part of the placenta, the foetal blood, renewed in oxygen and laden with nutritive 

 material derived from the maternal circulation, is carried by venous tributaries that 

 unite into the single umbilical vein. The latter vessel accompanies the umbilical 

 arteries within the umbilical cord as far as the umbilicus, from which point it then 

 passes along the free margin of the crescentic peritoneal fold, the falciform ligament, 

 to the under surface of the liver to join the portal vein and pour its stream of freshly 

 oxygenated blood into the current of venous blood returned from the digestive tract 

 to the liv^er. For a short time the rapidly growing liver is capable of transmitting 

 all the blood brought to it by the vitelline (later portal) and the umbilical veins, 

 and this blood is returned to the heart after making the circuit of the vessels of the 

 liver. Soon, however, the latter organ can no longer accommodate the entire 

 volume of blood conveyed to it by the portal and umbilical veins, and the necessary 

 relief is afforded by the development of a short vessel, the ductus venosus, or 

 ductus Arantii, that e.xtends from the portal vein to the inferior vena cava and 

 thus establishes a by-pass for the greater part of the oxygenated blood returned 

 from the placenta. On reaching the inferior cava, this pure blood is mingled with 

 the venous blood being returned from the lower half of the body and the abdominal 

 viscera, the mixed stream so formed being poured into the right auricle. On entering 

 the heart the current is directed by the Eustachian valve towards the foramen 

 ovale in the auricular septum and enters the left auricle. After receiving the 

 meagre additions returned by the pulmonary veins from the uninflated lungs, the 

 blood passes through the left auriculo-ventricular opening into the left ventricle. 

 Contraction of this chamber forces the blood into the systemic aorta and thence to 

 all parts of the body. 



After traversing the vessels of the head, neck, upper extremities, and thorax, 

 the venous blood from these parts is returned to the heart by the superior vena cava, 

 but on entering the right auricle does not mingle to any extent with the current 

 returned by the inferior cava, but passes through the auriculo-ventricular orifice into 

 the right ventricle. With contraction of the ventricles the blood is propelled into 



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