AXIAL SKELETON OF THE OSTEICH. 



391 



AXIS VEETEBEA (natural size). 

 Fig. 9. 



Fig. 10. 



Fig. 11. 



Fig. 12. 



Fig. 13. 



Fig. 14. 



Aspects. 

 Fig. 8, lateral ; 9, preaxial ; 10, postaxial ; 11, dorsal ; 12, ventral : 13, preaxial ; 14, lateral of immature bone. 

 Letters as before, except az, prezygapophysis ; «s, neural spine ; o, odontoid process ; y?, rib-like or pleurapophysial lamella ; 

 fs, postzygapophysis ; r, running backwards and outwards to parapophysis ; /, fossa (for ligament) on postaxial aspect of neural 

 spine. 



The whole vertebra in the adult consists of one bone. 



The centrmn may be considered as regards its four surfaces : — 



The preaxial surface, which joins the postaxial surface of the quasi-body of the atlas, 

 presents a crescentic, transversely extended articular surface, more or less strongly 

 concave dorso-ventraUy, but nearly straight transversely. 



Its ventral margin is slightly convex, generally flattened, and never produced me- 

 dianly. 



Its dorsal margin is not easUy defined, as the preaxial articular surface of the centrum 

 is prolonged dorsaUy on to each side of the ventral aspect of the root of the odontoid 

 process. Its dorsal margin, however, if we count such prolongations as belonging to the 

 preaxial surface of the centrum, must be described as strongly concave (figs. 8, 9, a c). 



The postaxial surface differs greatly in shape from the preaxial surface. Its ventral 

 part is prolonged ventrad iuto a hypapophysial process ; but apart from this it exhibits 

 an articular siuface the outline of which approximates slightly to that of an hour-glass 

 (fig. 10, }) c). This articular surface is strongly concave dorso-ventrally and strongly 

 convex transversely. Its dorsal and ventral margins are both very decidedly concave, 



