184 THE STRUCTURE OF MAN 
deeper and a superficial, but in the foot only 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 coeliac, omphalo-mesaraic (which later becomes the 
superior mesenteric), and the inferior mesenteric, is to be con- 
sidered as secondary. 
THE VENOUS 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 
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 
