"Ideal plan of the double-ringed body of a vertebrate. N, neural canal; H, haemal canal; the body separating them is the centrum of any vertebra, bearing e, and epapophysis, and y, a hypapophysis; n, n, neurapophyses; d, d, diapophyses; ns, bifid neural spine; pl, pl, pleurapophyses; h, h, haemapophyses; hs, bifid haemal spine. Drawn by Dr. R. W. Shufeldt, USA, After Owen. The Axial Skeleton of a bird or any vertebrated animal, that is, one having a back-bone, exhibits in cross-section two rings or hoops, one above and the other below a central point, like the upper and lower loops of a figure 8. The upper ring is the neural arch, so called because such cylinder encloses a section of the cerebro-spinal axis, or principal nervous system of a vertebrate (brain and spinal cord, whence arise all the nerves of the body, excepting those of the sympathetic nervous system). The lower ring is the haema arch, which similarly contains a section of the principal blood vessals and viscera.