II SKELETONS OF BIRD AND REPTILE 23 
latter is a ball-and-socket. At the hinder end of each 
vertebra there is a protuberance, rounded on its upper 
side but nearly flat below, which fits into a hollow in 
the vertebra behind. The fact that the protuberance is 
Fic. 7.—(A) Two vertebra of Snake. (2) Two neck vertebra of Eagle. 
I. Anterior end. II. Posterior. 
B, ball; cr, cervical rib; sp, ‘‘saddle”’; sk, socket ; sp, spine ; va, tunnel through 
which vertebral artery passes. 
not a perfect ball but has its underside flattened limits 
the freedom of movement, and, in addition to this, each 
vertebra bears a spine (SP, fig. 1) upon the top, the spine 
of one being very near to that of the next, and thus 
a further limit is put to movement up and down. The 
