THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



251 



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 of the 

 body, and the nerve supply is derived from several segments, but the blood is 

 furnished by a single segmental vessel in each extremity. In the upper ex- 

 tremity the subclavian, which represents the seventh cervical branch of the 

 aortic root, is the primary vessel from which all the other vessels are derived. In 



Brachial artery- 



Superficial radia artery 



Median artery 



Interosseous artery 



Ulnar artery - 



Brachial artery 



B 



----- Median artery 



Interosseous artery 



- Ulnar artery 

 * * Radial artery 



FIG. 227. Diagrams showing (^4) an early and (B) a late stage in the development 

 of the arteries of the upper extremity. McMurrich. 



the lower extremity the common iliac, which represents the fourth (or fifth?) 

 lumbar branch of the aorta is the primary vessel. 



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 in- 

 creases, the original vessel in the forearm diminishes to form the volar inter- 

 osseous 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 di- 

 minishes in size as another vessel develops, the ulnar artery, which arises a short 



