CIRCULATORY ORGANS. 



287 



origin, by a longitudinal vessel, the vertebral artery, which, in the 

 higher vertebrates, runs through the vertebra terial canal (p. 54) of 

 the vertebrae. 



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 

 way to form a basilar artery, running 

 forward beneath the medulla (fig. 291, 

 B). At the point just behind the 

 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 hi the region of the optic chiasma. 

 There is thus formed an arterial ring, the circle of Willis, round 

 the hypophysis. 



FIG. 292. Diagram of origin of 

 blood supply of vertebrate appendage. 

 V, abdominal vein; da, dorsal aorta; 

 si, subintestinal vein; so, somatic (seg- 

 mental) vascular arch. 



FIG. 293. Three stages in the development of the arteries of the forelimb 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.) 



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 



