CIRCULATORY ORGANS. 287 
origin, by a longitudinal vessel, the vertebral artery, which, in the 
higher vertebrates, runs through the vertebraterial canal (p. 54) of 
. the vertebre. ; 
In the region of the aortic roots, after the formation of the vertebral 
artery, all of the segmental.arteries except the last of the series lose 
their connexion with the radix and 
henceforth are supplied by way of the 
posterior segmental and the vertebral 
(fig. 291). Anteriorly the vertebral 
arteries pass to the ventral side of the 
spinal cord (or medulla oblongata) 
dividing there into two branches, one 
of which, joining its fellow of the 
opposite side, runs back beneath the 
spinal cord as a spinal artery, while 
the anterior branches unite in the same fic, 292. Dike of anata of 
way to forma basilar artery, running blood supply of vertebrate appendage. 
v, abdominal vein; da, dorsal aorta; 
forward beneath the medulla (fig. 291, si, subintestinal vein; so, somatic (seg- 
B). At-the point just behind the ™en%)) vascular arch. 
hypophysis the basilar divides, one-half passing on either side of 
that structure and receiving the internal carotid of that side. The 
trunks thus formed unite in front in the region of the optic chiasma. 
There is thus formed an arterial ring, the circle of Willis, round 
the hypophysis. 
Fic. 293.—Three stages in the development of the arteries of the forelimb of the white 
mouse, after Géppert. 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.) 
As the limbs grow out, segmental arteries, corresponding in number 
to the somites concerned in the appendages, grow out into the member. 
Distally these arteries become connected with each other and with the 
veins of the limb by a network of small vessels. By enlargement of 
