THE DEVELOPMENT OF THE VASCULAR SYSTEM. 217 



During foetal life they carry all the blood that passes to the placenta. At an 

 early period a branch from each iliac artery anastomoses with the corre- 

 sponding umbilical, and the portion of the umbilical artery between the 

 aorta and the anastomosis then disappears. This makes the umbilical 

 artery a branch of the iliac; and the blood then passes from the aorta into 

 the proximal part of the liiac which becomes the common iliac artery of the 

 adult. At birth, when the umbilical cord is cut, the umbilical arteries no 

 longer carry blood to the placenta, and their intraembryonic portions, 

 often called the hypogastric arteries, persist only in part; their proximal 

 ends persist as the superior vesical arteries, while the portions which accom- 

 panied the urachus degenerate to form the lateral umbilical ligaments. 



So far as a complete history of the growth of the arteries of the extremities 

 is concerned, knowledge is lacking. The facts of comparative anatomy and 

 the anomalies which occur in the human body have led to certain conclusions 

 which have been largely confirmed by embryological observations; but much 

 more work on the development of the arteries is yet necessary to complete 

 their history. The extremities represent outgrowths from several segments 

 oft the body, the nerve supply is derived from several segments, and the 

 limb buds are likewise primarily supplied by plexuses of vessels arising from 

 several branches of the aorta. In the upper extremity the subclavian, which 

 represents the seventh cervical branch of the aortic root, is the single vessel 

 which eventually develops out of the original plexus. In the lower extremity 

 the common iliac, which represents the fifth lumbar branch of the aorta, 

 is the single vessel which develops out of the plexus supplying the lower 

 limb bud. 



In the upper extremity the subclavian grows as a single vessel to the wrist 

 and then divides into branches corresponding to the fingers. In the forearm 

 it lies between the radius and ulna. In a short time a branch is given off 

 just distal to the elbow and accompanies the median nerve. As this branch 

 increases, the original vessel in the forearm diminishes to form the volar 

 interosseous artery; and at the same time the branch unites again with the 

 lower end of the interosseous, takes up the digital branches and becomes 

 the chief vessel of the forearm at this stage, forming the median artery. 

 Later, however, it diminishes in size as another vessel develops, the ulnar 

 artery, which arises a short distance proximal to the origin of the median and, 

 passing along the ulnar side of the forearm, unites with the median to form 

 the superficial volar arch. From the artery of the arm, which is called the 

 brachial artery, a branch develops about the middle and extends distally 

 along the radial side of the forearm. A little later another branch grows out 

 from the brachial just proximally to the origin of the ulnar and extends across 

 to, and anastomoses with, the first branch. Then the portion of the first 



