THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 587 



divided into two branches. One stream goes directly through the 

 ductus Arantii into the inferior vena cava, and is here mingled with 

 the venous blood which returns to the heart from the posterior limbs 

 and the kidneys. The other stream passes through the liver, where 

 there is added to it the venous blood of the portal vein coming from 

 the intestine ; by this circuitous course it also reaches, through the 

 venae hepaticae revehentes, the inferior vena cava. From the latter 

 the mixed blood flows into the right atrium, but, in consequence of 

 the position of the Eustachian valve and because the foramen ovale 

 is still open, the greater part of it passes through the latter into the 

 left atrium. The other smaller part is again mingled with venous 

 blood, which has been collected by the superior vena cava from the 

 head, the upper limbs, and (through the azygos) from the walls of 

 the trunk, and is propelled into the right ventricle and from there 

 into the pulmonalis. The latter sends a part of its highly venous 

 blood to the lungs, the other part through the ductus Botalli to the 

 aorta, where it is added to the arterial current coming from the left 

 ventricle. 



The blood of the left ventricle comes principally from the infeiior 

 cava, only a small part of it from the lungs, which pour their blood, 

 which at this time is venous, into the left atrium. It is driven into 

 the aortic arch and part of it is given off through lateral branches to 

 the head and upper limbs (carotis communis, subclavia) ; the rest is 

 carried on downwards in the aorta descendens, where the venous 

 current of blood from the right atrium by the way of the ductus 

 Botalli is united with it. The mixed blood is distributed to the 

 intestinal canal and the lower limbs, but the most of it reaches the 

 placenta through the umbilical veins, where it is again made arterial. 



In the distribution of the blood in the anterior and the posterior regions 

 of the body a noteworthy difference is easily recognised. The former receives 

 through the carotis and subclavia a more arterial blood, since to the stream in 

 the aorta descendens is added the venous blood of the right ventricle through 

 the ductus Botalli. Especially in the middle of pregnancy is this difference 

 important. There has been an endeavor to refer to this fact the more rapid 

 growth of the upper part of the body in comparison with the lower. 



As this sketch has shown, there is everywhere a mingling of the 

 different kinds of blood. This, it is true, is not uniform in the different 

 months of embryonic life, because, indeed, the separate organs do not 

 alter in size uniformly, and especially because the lungs during the 

 later stages are in a condition to receive more blood, and finally 

 because the foramen ovale and the ductus Botalli become narrower 



