THE ARTERIES 305 



Behind the hypogastric arteries the caudal aorta is called, the caudal 

 artery (figs. 322, 327). The somatic arteries are more numerous 

 and are metamerically arranged. They are distributed to the body 

 wall and to the parts arising from it by development, and may be 

 subdivided into two groups. The first of these in the early stages 

 are given off in pairs from the radices and the dorsal aorta, an artery 

 on either side extending laterally between each two successive 

 myotomes (fig. 310). These are the upper halves of the transverse 

 somatic vessels alluded to on p. 289. Many of these remain in a 

 slightly modified condition and are called intercostal arteries (includ- 

 ing lumbar and sacral arteries, etc., according to position). These 

 usually become connected on either side (fig. 323), near their 

 origin, by a longitudinal vessel, the 

 vertebral artery, which, in the higher 

 vertebrates, runs through the verte- 

 brarterial canal (p. 6i)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 hence- 

 forth are suppUed by way of the pos- 

 terior segmental and the vertebral 



(fig. 324). Anteriorly the vertebral ,, Tig. 325.— Diagram of origin of 



. 1 • J r 1 blood supply of vertebrate appendage. 



arteries pass to the ventral side of the v, abdominal vein; da, dorsal aorta; 



spinal cord (or medulla oblongata) ^Je't^'frjl/S.'''' ''°""''"' '"'" 

 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. 324, 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 in the region of the optic chiasma. There is thus 

 formed an arterial ring, the circle of Willis (fig. 324, B), around the 

 hypophysis. 



As the limbs grow out, segmental arteries, corresponding in num- 

 ber to the somites concerned in the appendages, extend into the mem- 

 ber. Distally these arteries become connected with each other and 

 with the veins of the limb by a network of small vessels. By enlarge- 



