DEVELOPMENT OF THE ARTERIAL SYSTEM. 671 



though at first a mere branch of communication between the caro- 

 tid and the vertebral, has now increased in size to such an extent 

 that it has become the principal trunk, from which the vertebral 

 itself is given off as a small branch. Immediately below this point' 

 of intersection, also, the vertebral artery diminishes very much in 

 relative size, loses its connection with the abdominal aorta, and 

 supplies only the first two intercostal spaces, under the name of the 

 superior intercostal artery (4, 4). The second cervical arch becomes 

 altered in a very different manner on the two opposite sides. On 

 the left side, it becomes enormously enlarged, so as to give off, as 

 secondary branches, all the other arterial trunks which have been 

 described, and is converted in this manner into the arch of the 

 aorta (o). On the right side, however, the corresponding arch (e) 

 becomes smaller and smaller, and at last altogether disappears ; so 

 that, finally, we have only a single aortic arch, projecting to the 

 left of the median line, and continuous with the thoracic and abdo- 

 minal aorta. 



The first cervical arch remains during foetal life upon the left 

 side, as the " ductus arteriosus," presently to be described. In the 

 adult condition, however, it has disappeared equally upon the right 

 and left sides. In this way the permanent condition of the arterial 

 circulation is gradually established in the upper part of the body. 



Corresponding changes take place, however, during the same 

 time, in the lower part of the body. Here the abdominal aorta 

 runs undivided, upon the median line, quite to the end of the 

 spinal column; giving off" on each side successive lateral branches, 

 which supply the intestine and the parietes of the body. When 

 the allantois begins to be developed, two of these lateral branches 

 accompany it, and become, consequently, the umbilical arteries. 

 These two vessels increase so rapidly in size, that they soon appear 

 as divisions of the aortic trunk ; while the original continuation of 

 this trunk, running to the end of the spinal column, appears only 

 as a small branch given off at the point of bifurcation. When the 

 lower limbs begin to be developed, they are supplied by two small 

 branches, given off from the umbilical arteries near their origin. 



Up to this time the pelvis and posterior extremities are but 

 slightly developed. Subsequently, however, they grow more 

 rapidly, in proportion to the rest of the body, and the arteries 

 which supply them increase in a corresponding manner. That 

 portion of the umbilical arteries, lying between the bifurcation of 

 the aorta and the origin of the branches going to the lower ex- 



