VERTEBRATES. 



291 



the lower, as in the sharks, it can be recognized through- 

 out life. This notochord is enveloped in a membranous 

 notochordal sheath, and in this sheath are formed rings of 

 cartilage which give rise to the bodies (centra) of the verte- 

 brae. Between these rings no cartilage is formed and hence 

 the whole column is jointed and flexible. In the sharks 

 these rings and other parts of the skeleton remain carti- 

 laginous ; in other vertebrates any or all may be converted 

 into bone. In a typical vertebra, for instance, in the tail 

 of a fish (fig. 101, A), outgrowths from the centrum occur 



A 



FIG. 101. Different vertebrae and connected structures. A, in tail region of 

 teleost; B, in body region of teleost; C, in tail region of salamander; D, in 

 mammal; c, centrum; h, hsemal arch (rib in B); n, neural arch; r, rib; s, 

 sternum; t, tran verse process. 



above and below, forming two arches. The upper of these 

 (neural arch, n) encloses the spinal cord, the lower (hcemal 

 arch, h) extends around the blood-vessels of the tail. 

 Farther forward, in the trunk region of the bony fish, the 

 two halves of the hsemal arch do not meet below, but form 

 slender threads (ribs, B, h) which support the flesh around 

 the viscera. In the forms above the fishes an outgrowth 

 (transverse process, C, D, t) may rise on either side of the 

 vertebral centrum, and the ribs, when they occur, are con- 

 tinuations of these transverse processes, and have nothing 



