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1 96 EMBRYOLOGY. 



former becomes the round ligament of the liver, while the ductus 

 venosus remains as a small fibrous cord. 



What is the course of the foetal blood through the heart ? 



It enters the right auricle, and is thence sent in part to the 

 right ventricle, and in part to the left auricle, 

 FIG. 68. through the foramen ovale, an opening left 



in the development of the auricular septum. 

 The blood which enters the left ventricle 

 from the left auricle is forced out through 

 the aorta (Fig. 68). The right ventricle, 

 however, sends but a small part of its blood 

 to the lungs, but delivers it through the 

 ductus arteriosus to the aorta. The blood is 

 in this way sent into the systemic circulation, 



Foetal Heart still farther a part going to the placenta through the inter- 

 nal iliac, hypogastric, and umbilical arteries. 



'' 4 ' What effect has the Eustachian valve upon 



the blood-stream in the foetal heart ? 

 It throws the blood from the inferior vena cava through the 

 foramen ovale into the left auricle. In this way the stream of 

 blood coming from the superior vena cava crosses that from the 

 inferior cava on entering the heart, inasmuch as the blood from 

 the superior vena cava enters the right ventricle. 



What effect does this division of the blood-stream have upon the 



distribution of the blood in the foetus ? 



The circulation of the blood is made more perfect, for the 

 branches of the aorta given off to the head and upper extremities 

 distribute blood from the inferior vena cava ; while the ductus 

 arteriosus, carrying the blood from the superior cava and right 

 ventricle, enters the aorta in such a way that most of its blood is 

 sent to the lower extremities and abdominal organs and umbilical 

 arteries. In this way the deoxidized blood is sent back to the 

 placenta for the renewal of its oxygen. 



How does this result in the development of the lower extrem- 



ities ? 



They develop less rapidly than the upper. There are probably 

 two reasons for this: 1, the blood is less well aerated and less 

 nutritious ; 2, the internal iliac arteries, giving off the umbilical 

 arteries, probably divert a considerable portion of the blood-supply 

 of the external iliacs which go to the lower extremities. 



