186 . THE HUMAN EMBRYO. 



the left ventricle, and is thence driven along the systemic trunk 

 (Pig. 246, ex), and through the carotid and subclavian arteries: 

 to the head and fore-limbs. 



It is probable that very little, if any, blood from the left 

 ventricle gets into the dorsal aorta, for this is already filledr 

 through the ductus arteriosus, from the right ventricle ; and as- 

 the, two ventricles have at this stage walls of about equal thick- 

 ness, and presumably of equal strength, there will be as strong a 

 tendency for the blood of the right ventricle to pass forwards 

 along the arch of the aorta, as for the blood from the left ven- 

 tricle to pass backwards along the dorsal aorta. 



Theoretically, the aorta might be ligatured just in front of 

 the point at which the ductus arteriosus joins it, without in any 

 way disturbing the foetal circulation ; and instances of mal- 

 formation have occurred, in which the aorta was completely 

 obliterated at this place, and yet development in other respects- 

 proceeded normally. Such a malformation, though causing no 

 disturbance of the circulation so long as the foetus is receiving 

 nourishment through the placenta, is fatal at the time of birth, 

 as the arterial supply of the whole body behind the arms is then 

 cut off. 



6. The Changes in the Circulation at the Time of Birth. 



At birth, the placental circulation is arrested, and the 

 allantoic and vitelline vessels are interrupted ; and, as the lungs 

 become inflated, the pulmonary circulation is fully established. 



In connection with this shifting of the seat of respiration, 

 from the placenta to the lungs, important changes are effected 

 in the circulation, the principal of which are : 



(i) Shrinking and obliteration of the ductus arteriosus, and 

 of the hypogastric, or allantoic, arteries. 



(ii) Obliteration of the ductus venosus, and of the part of 

 the allantoic vein within the body of the child. 



(iii) Closure of the foramen ovale. 



By these changes it is brought about that the blood in the 

 posterior vena cava, which is now entirely venous, is no longer 

 able to get into the left auricle, owing to closure of the foramen 

 ovale, but passes, with that of the anterior vena cava, from the 

 right auricle to the right ventricle. , From the right ventricle, 

 owing to the obliteration of the ductus arteriosus, it can no 



