306 



ZOOLOGY 



SECT 



lot 



vent 



hyp. 



Fio. 972. Vertebrae of Lizard. A, anterior, S, posterior 

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

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

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

 Hg. ligamentbus band dividing the ring of the atlas 

 into two ; neur. neural arch of atlas : od. odontoid pro- 

 cess ; pr. zy. pre-zygapophysis ; pt. zy. post-zygapophy- 

 sis ; rb. rib ; sp. spine ; cent, ventral piece of atlas. 



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 membrane. The 

 second or 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 pro- 

 cesswhich is a part of 

 the centrum of the atlas, 

 and is not actually fused 

 with, though firmly fixed 

 to, the axis lies in the 

 lower or ventral part of 



the opening of the atlas, separated by a ligamentous 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 

 or Jiypapophysis (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 vertebrae 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 as 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-shaped 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 the five anterior thoracic vertebrae are 

 connected by means of cartilaginous sternal ribs with the sternum. 



