340 



HISTORY OF THE HUMAN BODY 



of the legs correlated with the erect position, which has de- 

 veloped the iliac arteries out of all proportion, giving the er- 

 roneous but inevitable impression that these latter arteries 

 form the real continuation of the aorta, which becomes bi- 

 furcated, and that the arteria sacralis media is an unimportant 

 median branch arising from the point of bifurcation and sup- 

 plying the coccygeal region. 



In the adult selachians, which in their venous system rep- 

 resent practically the starting point of the history so far as 

 vertebrates are concerned, the two sets of cardinal veins, an- 



FIG. 95. Metamerism in the mesenteric arteries of Amphibia. [After 

 KLAATSCH.] 



(a) Siren. (b) Necturus. (c) Cryptobranchus. (d) Cryptobranchus (a second 

 specimen). (e) Anura. 



tenor and posterior, are in control of the venous blood, except 

 that from the alimentary canal, and return it from all parts 

 of the body to the sinus venosus. However, during the em- 

 bryological development of these animals one catches glimpses 

 of a still earlier systemic vein, the sub-intestinal, which, here 

 embryonic and transitory, must have preceded the cardinal sys- 

 tem historically, and have been totally replaced by the latter 

 before the advent of true vertebrates as we now know them. 

 It appears soon after the establishment of the two yolk veins, 

 always for practical reasons the first to appear in vertebrate 

 embryos, and extends from the left yolk vein, from which it 



