x.] THE CIRCULATING SYSTEM. 415 



The anterior tibial, having passed to the front of the inter- 

 osseous ligament, extends down the front of the leg obliquely 

 to the ankle, and then dips between the first and second 

 metatarsals and anastomoses with the posterior tibial in a 

 plantar arch, like the palmar arch before noticed. Many 

 smaller branches are, of course, given off at intervals. 



6. In their DEVELOPMENT the arteries exhibit some in- 

 teresting phenomena. 



The root of the aorta is, at a very early embryonic period, 

 dilated into a bulb, and from it spring five vessels on each 

 side (successively, however, as never more than three on each 

 side exist at the same time), which arch round and meet 

 together beneath the spinal axis to form the dorsal aorta. 



FIG. 358. DIAGRAM REPRESENTING THE PRIMITIVE AORTIC ARCHES OF 

 MAMMALS AND SAUROPSIDANS. 



(After H. Rathke.} 



a, common trunk, or root, of the aorta ; b, b, the two branches into which it 

 divides, and which give off the successive arches i, 2, 3, 4, and 5, which end 

 in c, c, two vessels which again unite to form d, the descending, or dorsal, 

 aorta. 



The fourth arch on the left side persists, grows, and be- 

 comes the arch of the great aorta. 



The third arch becomes the common carotid and part of 

 the internal carotid arteries. 



The t\vo more pre-axially situate arches may not impro- 

 bably become ultimately the tympanic and stylo-mastoid 

 arteries. 



The fifth arch gives rise to the pulmonary arteries ; a com- 

 munication between it and the great aorta, persisting for a 

 considerable time, is called the ductus arteriosus (Fig. 367, o). 



The descending dorsal aorta is at first double, but its two 

 parts soon coalesce to form a single vessel. It extends 



