184 THE STRUCTURE OF MAN 



deeper and a superficial, but in the foot ojaly a deep plantar 

 one. It is evident on reflection that a superficial arch cannot 

 exist in the foot on account of its functions as an organ of 

 support, and that the larger pedal arteries, to be free from 

 interference with the circulation, may have had to withdraw 

 into the recesses of the foot. Indications, however, are not infre- 

 quently encountered that the foot formerly possessed a super- 

 ficial arterial arch, and that the arteries for the toes arose from 

 it, in a manner identical with that in which the arteries for 

 the fingers arise from the superficial palmar arch of the hand. 



Finally, as to the intestinal arteries, although our knowledge 

 of the development of these is still very limited, all things point 

 to the fact that originally they were numerous and segmental, 

 and that their final reduction in Man and Mammals to three 

 trunks, the cceliac, omphalo-mesaraic (which later becomes the 

 superior mesenteric), and the inferior mesenteric, is to be con- 

 sidered as secondary. 



THE VEXOTTS SYSTEM 



The developing venous system of Man, like the arterial, 

 shows unmistakable traces of a very primitive condition inherited 

 from the lower Vertebrates. In this connection the anterior 

 and posterior cardinal veins, the ductus Cuvieri, and the sinus 

 venosus cordis, are especially conspicuous. 



The system of the vena cava inferior is a late acquisition, 

 dating [in its fully differentiated form] from the higher Fishes 

 (Dipnoi) and Amphibians. Its phylogenetically recent origin 

 is, even in Man, denoted by the variation and arrested develop- 

 ment which it occasionally exhibits. Several cases of [that which 

 Hochstetter's researches prove to be] the persistence of an early 

 stage in its development have been recorded. I refer to those ] 

 in which the caval vein, from about the level of the superior \l 

 mesenteric, is continued downwards towards the pelvis, owing ' 

 to the retention of the posterior cardinals. 



In these cases we may speak of persistence of the posterior 

 cardinals in the form of a double vena cava inferior. 



In other cases of what we may now regard as arrested develop- 

 ment, the distal portion of the inferior vena cava is formed out 

 of the left instead of the right cardinal vein, there is then a 

 vena cava inferior passing to the left [of the aorta]. 



In very rare cases, where development is arrested at a very 



