SKELETON. 49 
the deposition of cartilage in the cups (fig. 43, II) producing inter- 
vertebral constrictions of the cord. As this progresses absorption of the 
cartilage begins between the ends of the vertebra (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 (x) the cervical, in the neck, with great reduction or even absence 
of ribs; (2) the thoracic, following the cervical, with distinct ribs; (3) 
Fic. 44.—Section through atlas Fic. 45.—Proatlas, atlas 
(at) and axis (ax) of fowl, cut sur- and axisofalligator. a, atlas; 
faces lined. e, epistropheus; /, facet e, epistropheus (axis); p, pro- 
for articulation with skull; /, trans- atlas; 7, rib of third vertebra; 
verse ligament. ra, re, ribs of altas and epis- 
tropheus. 
lumbar, without ribs; (4) sacral, including one or more vertebre 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 vertebra 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 hemal arches are transformed into ribs. 
One or two of the anterior vertebra 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. Inthe amniotes the second vertebra, the axis or epistropheus is 
also specialized. On the anterior face of its centrum is a pivot (the 
4 
