SKELETON. 



49 



the deposition of cartilage in the cups (fig. 43, IP) producing inter- 

 vertebral constrictions of the cord. As this progresses absorption of the 

 cartilage begins between the ends of the vertebrae (ic) and continues 

 in such a way that the result is a ball of cartilage attached to the hinder 

 vertebra and a corresponding cup in the one in front; in other words, 

 an opisthoccelous condition. 



Several regions may be differentiated in the vertebral column, these 

 being the most numerous in the higher groups of vertebrates. These 

 are (i) the cervical, in the neck, with great reduction or even absence 

 of ribs; (2) the thoracic, following the cervical, with distinct ribs; (3) 



FIG. 44. Section through atlas 

 (at) and axis (ax) of fowl, cut sur- 

 faces lined, e, epistropheus; /, facet 

 for articulation with skull; /, trans- 

 verse ligament. 



FIG. 45. Proatlas, atlas 

 and axis of alligator, a, atlas ; 

 e, epistropheus (axis) ; p, pro- 

 atlas; r, rib of third vertebra; 

 ra, re, ribs of altas and epis- 

 tropheus. 



lumbar, without ribs; (4) sacral, including one or more vertebras with 

 which the pelvis is connected; (5) caudal, the tail, behind the sacrum. 

 Sometimes the ribs extend back to the sacrum so that thoracic and 

 lumbar cannot be distinguished, all being then grouped as dorsal. 

 Then in the fishes and some higher vertebrates (snakes, whales, etc.) 

 sacral vertebrae are not differentiated, and in the fishes there is no line 

 between cervicals and dorsals, so that only trunk or abdominal, and 

 caudal regions can be distinguished, the line being drawn (fishes) at the 

 point where haemal arches are transformed into ribs. 



One or two of the anterior vertebrae are modified in the higher 

 (tetrapodous) vertebrates and have received names. The first, which 

 immediately adjoins the skull, is the atlas. It bears on its anterior face 

 an articular surface which receives the one or two condyles of the cra- 

 nium. In the amniotes the second vertebra, the axis or epistropheus is 

 also specialized. On the anterior face of its centrum is a pivot (the 



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