CHAP. X.] THE VASCULAR SYSTEM. 117 



the upper extremities. Returned from the upper extremities 

 by the superior vena cava the blood enters the right auricle 

 and, passing over the Eustachian valve, descends into the right 

 ventricle, and from the right ventricle into the pulmonary 

 artery. As the lungs in the foetus are solid, they require very 

 little blood, and the greater part of the blood passes through 

 the ductus arteriosus into the descending aorta, where, mixing 

 with the blood delivered to the aorta by the left ventricle, it 

 descends to supply the lower extremities of the foetus, the chief 

 portion of this blood, however, being carried back to the pla- 

 centa by the two umbilical arteries. 



From this description of the foetal circulation, it will be 

 seen : 



1. That the placenta serves the double purpose of a respi- 

 ratory and nutritive organ, receiving the venous blood from 

 the foetus, and returning it again charged with oxygen and 

 additional nutritive material. 



2. That the greater part of the blood traverses the liver 

 before entering the inferior vena cava ; hence the large size of 

 this organ at birth. 



3. That the blood from the placenta passes almost directly 

 into the arch of the aorta, and is distributed by its branches to 

 the head and upper extremities ; hence the large size and 

 perfect development of those parts at birth. 



4. That the blood in the descending aorta is chiefly derived 

 from that which has already circulated in the upper extremities, 

 and, mixed with only a small quantity from the left ventricle, 

 is distributed to the lower extremities ; hence the small size and 

 imperfect development of these parts at birth. 



Development of blood-vessels and corpuscles. The blood-vessels and 

 red corpuscles are formed very early in the embryo. They are developed in 

 that portion of the primitive tissue called the mesoblast. The cells which 

 are to form the vessels become extended into processes of varying length, 

 which grow out from the cells in two or more directions. The cells become 

 united with one another, either directly or by the junction of their processes, 

 so that an irregular network is thus formed. Meanwhile the nuclei in the 

 cells multiply, and each nucleus surrounds itself with a small amount of 

 cell protoplasm . The corpuscles thus formed acquire a reddish colour, and 

 the protoplasmic network in which they lie becomes hollowed out into a 

 system of branched canals containing fluid, in which the nucleated coloured 

 corpuscles float. The protoplasmic walls of the vessels gradually change 



