M10 



ZOOLOGY 



SECT. 



The first two. however, differ greatly from the others. The first 

 is the atlas (C, D). It has no distinct centrum, but is in the form 

 of a ring ; ventrally on its anterior face it bears a smooth articular 

 facet for the occipital condyle of the skull. It consists of three 

 distinct ossifications, one ventral, the others dorso-lateral : the 



latter do not quite 

 meet dorsally, being 

 separated by a space 

 bridged over by mem- 

 brane. The second or 



/-I cen.1 



< JC E 



neit-r > 



,lig 



cent 



lot 



vent 



FIG. 961. Vertebrae of Lizard. A, anterior, B, posterior, 

 view of a thoracic vertebra ; C, lateral, D, anterior, view 

 of atlas vertebra ; E, lateral view of axis. cent, centrum ; 

 hyp. hypapophysis of axis ; hit. lateral piece of atlas ; 

 lift, ligamentous band dividing the ring of the atlas 

 into two ; neur. neural arch of atlas ; o<l. odontoid pro- 

 cess ; pr. zi/. pre-zygapophysis ; pt. zy. post-/ygapophysis ; 

 rb. rib ; sp. spine ; rent, ventral piece of atlas. 



axis (E) has a short 

 conical process the 

 odontoid process (od) 

 projecting forwards 

 from its centrum. In 

 the natural position of 

 the parts the odontoid 

 process, whicli is a part 

 of the centrum of the 

 atlas, and is not actu- 

 ally fused with, though 

 firmly fixed to, the axis, 

 lies in the lower or 



ventral part of the opening of the atlas, separated by a liga- 

 mentous band from the upper portion, which corresponds to the 

 neural arch, and lodges the anterior end of the spinal cord. On 

 the ventral surface of the axis and of each of the following 

 five or six vertebrae, is a distinct bony nodule, sometimes termed 

 the intercentrum (see p. 299) or hypapophysis (hyp). The sacral 

 vertebrae have short centra and strong expanded processes the 

 transverse.processes which abut against the ilia; these are separately 

 ossified, and are to be looked upon as sacral ribs. The anterior 

 caudal vertebras are like the sacral, but have the centra longer, 

 the transverse processes more slender, and the neural spines 

 longer. The posterior caudal vertebrae become gradually smaller 

 a? we pass backwards, and the various processes reduced in 

 prominence, until, at the posterior end of the tail, the whole 

 vertebra is represented merely by a rod-like centrum. Attached 

 to the ventral faces of the centra of a number of the anterior caudal 

 vertebrae are Y- sna P e d bones the chevron bones the upper limbs 

 of the Y articulating with the vertebra, while the lower limb 

 extends downwards and backwards. In nearly all the caudal 

 vertebrae the centrum is crossed by a narrow transverse unossified 

 zone through which the vertebra readily breaks. The ribs are 

 slender curved rods, the vertebral ends of which articulate only 

 with the capitular facets of the corresponding vertebrae, there being 

 no direct articulation with the transverse processes. The ribs of 



