288 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



parts of these main trunks and of the connecting network, and the 

 partial or complete atrophy of other portions the definitive circulation 

 of the limb is established. This explains the numerous variations in 

 the blood supply of the limbs, both in the distal parts and in the origin 

 of the main trunks, which may arise from the dorsal aorta or from the 

 radices as far forward as the third aortic arch. 



The main trunk of the fore limb may have different names in differ- 

 ent parts of its course. It is the subclavian artery as it leaves the 

 dorsal aorta, the axillary as it enters the limb, and the brachial in the 

 upper arm. It divides near the elbow into radial and ulnar arteries, 

 which run near the corresponding bones into the podium. 



There are some additional elements of complexity in the develop- 

 ment of the arteries of the hind leg. As in front several somatic vessels 

 are concerned and there is the same formation of a capillary network. 

 Two of the arteries attain special prominence. In front is the epigas- 

 tric artery, which descends from the aorta to the ventral side of the 

 body and runs forward to supply the lower portion of the myotomes, 

 becoming connected at first with the epigastric veins, although later 

 they may anastomose with the hinder ends of the cutaneous arteries 

 (infra). When the hind limb grows out, the epigastric sends a branch, 

 the external iliac or femoral artery, into its anterior side. As the 

 leg increases in size this may surpass the parent epigastric in size, the 

 latter now appearing as a side branch. 



The second pair of somatic arteries are the sciatic (ischiadic) 

 arteries. These descend into the posterior side of the leg, the name 

 changing at the angle of the knee to popliteal artery, and farther 

 down it divides into peroneal and anterior and posterior tibial 

 arteries, the peroneal supplying the calf of the leg, the others continuing 

 into the foot. 



The arrangement of vessels thus outlined is characteristic of the 

 lower tetrapoda where the femoral artery is small. It is also character- 

 istic of the embryos of the mammals, but in the latter, before birth, the 

 femoral artery grows down, joins the popliteal, and thus becomes the 

 chief supply of the limb. These trunks and the hypogastric do not 

 always remain distinct, but may fuse in different ways at the base. 

 Epigastric and hypogastric arteries are distinct in many reptiles and in 

 birds, but elsewhere they fuse to form the common iliac artery, so 

 called since the proximal portion of the femoral is often called the 

 external, the hypogastric the internal iliac artery. The sciatic, too, 



