3o6 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



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

 (fig. 326), 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. This will explain the shiftings of the subclavian artery 

 shown in fig. 319. 



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

 of its course. It is_the^ubclavianjMtery as it leaves the dorsalaorta, the 

 axillary as it enters the limb, and the brachia l in the upper arm It~3ivides 

 near the elbow into radial and ulnar arteries, which run near the corresponding 

 bones into the podium. 



/^r ^ 



Fig. 326. — Three stages in the development of the arteries of the fore limb of the 

 white mouse, after Goppert. A, 8 days; B, 9 days; C, 10 days; a, aorta; b, brachial 

 plexus. (The vessels are extremely variable, not agreeing even on the two sides of a 

 single individual.) 



There are some additional elements of complexity in the develop- 

 ment of the arteries of the hind leg. As in front, several somatic ves- 

 sels are concerned and there is the same formation of a capillary 

 network. Two of the arteries attain special prominence. In front 

 is the epigastric 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 cuta- 

 neous arteries (infra). When the hind hmb 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 epi- 

 gastric 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, 



