792 DEVELOPMENT OF THE VASCULAR SYSTEM. 



terior venous trunks disappear more or less completely, to be replaced 

 in importance by some of those which enter upon the sides. The area 

 vasculosa is therefore an appendage to the circulatory apparatus of the 

 embryo, spread out over the surface of the vitellus, and absorbing from 

 it the requisite materials for nutrition. 



In man and the mammalians, the first formation of the area vasculosa 

 is not essentially different from that presented in fishes and birds. But 

 owing to the small size and rapid exhaustion of the vitellus as a source 

 of nourishment, this form of the circulation never acquires a high degree 

 of development, and soon becomes retrograde. It presents, however, 

 certain modifications, which are of importance as indicating the mode 

 of origin of various parts of the permanent vascular system. 



These modifications relate mainly to the arrangement of the arteries 

 and veins distributing the blood to the external vascular plexus, and 

 returning it thence to the body of the embryo. As the embryo and the 

 entire egg increase in size, there are two arteries and two veins which 

 become larger than the others, and which subsequently do the whole 

 work of conveying the blood to and from the area vasculosa. The two 

 arteries emerge from the lateral edges of the embryo, on the right and 

 left sides ; while the two veins enter at about the same point and nearly 

 parallel with them. These four vessels are termed the omphalo-mesen- 

 teric arteries and veins. 



The arrangement of the circulatory apparatus in the interior of the 

 body at this time is as follows : The heart is situated at the median 

 line, immediately beneath the head, and in front of the oesophagus. It 

 receives at its lower extremity the united trunks of the two omphalo- 

 mesenteric veins, and at its upper extremity gives off two vessels which 

 almost immediately divide into two sets of lateral arches, bending back- 

 ward along the sides of the neck, and again uniting into two trunks 

 near the anterior surface of the vertebral column. These trunks then 

 run from above downward, in a nearly similar direction, on each side 

 the median line. They are called the vertebral arteries, on account of 

 their situation, which is parallel with that of the vertebral column. 

 They give off, throughout their course, small lateral branches, which 

 supply the body of the foetus, and also two larger branches the 

 omphalo-mesenteric arteries which pass out, as above described, into 

 the area vasculosa. The two vertebral arteries remain separate in the 

 upper part of the body, but fuse with each other a little beneath 

 the level of the heart ; so that, below this point, there remains but one 

 large artery, the aorta, running from above downward along the median 

 line, giving off the omphalo-mesenteric arteries to the area vasculosa, 

 and supplying smaller branches to the body, the walls of the intestine, 

 and the other organs of the embryo. 



This is the condition which marks the first or vitelline circulation. 

 A change now begins to be established, by which the vitellus is super- 

 seded, as an organ of nutrition, by the placenta ; and the second or pla- 

 cental circulation takes its place. 



