CHANGES IN THE OVUM. 



99 



being the organ which circulates the blood in the foetus. This fluid, car- 

 ried by the arterial vessels, reaches the umbilical arteries, and from them 

 is distributed to the placenta. There it is respired, depurated, arterialized, 

 through indirect contact with the blood of the parent, and is returned by 

 the umbilical veins. In the texture of the liver it is mixed with the 

 venous blood of the intestines and the hinder part of the body, conveyed 

 by the ductus venosus, and is finally thrown into the right auricle, then 

 into the ventricle, whence it is expelled by a contraction of that cavity. 

 Instead of entering the lungs, which do not act during fcetal life as res- 

 piratory organs, the blood is forced into the ductus arteriosus, and thence 

 into the aorta. So that the organs of the young creature are never sup- 



Fig. 45- 



F<ETAL Circulation : Advanced Period. > 



A, Placentulae; Bi Bi, Umbilical Veins, with their Common Trunk, B ; D, Vena Portse, and its 

 Anastomosis, C ; E, Ductus Venosus ; F, Posterior Vena Cava ; G, Right Ventricle of Heart ; 

 H, Pulmonary Artery ; J J, Aorta ; I, Ductus Arteriosus ; K, Umbilical Arteries, with their 

 Anastomosis at the extremity of the Umbilical Cord. 



plied with pure blood, but with a mixture of arterialized and venous 

 blood ; this mingling taking place through the foramot ovale, in the aorta 

 by the ductus arteriosus, and in the liver by the ductus venosus. The head 

 and neck receive the purest blood, a circumstance which probably 

 explains the predominance in size of the upper to the lower parts of the 

 body of the foetus. 



At birth the conditions of existence being suddenly changed, very 

 marked modifications occur in the circulation. The lungs then become 



