394 ME. ST. GEORGE MIVAET ON THE 



The neural spine is excavated by a fossa (for a ligament) on its postaxial surface (fig. 

 10, f); but preaxially it dips gradually to the preaxial border of the neural arch, mth- 

 out either excavation or process of any kind. 



The neural arch encloses a space which is smaller than that enclosed by the neural 

 arch of the atlas ; and the transverse diameter of this space is especially diminished, 

 the arch thus appearing less low relatively. 



When viewed in its dorsal aspect (fig. 11), the neural arch is seen to expand 

 greatly postaxiad, its transverse diameter between the postzygapophyses being double 

 that between the prezypapophyses. Its preaxial margin is concave, with a median 

 preaxial projection; its postaxial margin is generally more or less slightly concave. 



No foramina are developed other than those already noted. 



The immature condition of the axis vertebra (as seen in the specimen 1397 a of the 

 College-of-Surgeons Museum) shows that the neural laminae become perfectly anchy- 

 losed together and to the centrum at a time when the neural laminae of the atlas remain 

 altogether distinct, as also the odontoid process (or rather " bone ") itself. 



The odontoid bone forms a very large part of the so-called centrum of the axis, as 

 well as the whole of the odontoid process itself (fig. 14, ac). All the articular surfaces 

 of the axis for the quasi-body of the atlas are formed by this odontoid bone, except 

 the little surfaces at the sides of the ventral surface of the root of the apparent odon- 

 toid process of the adult. These are seen to be formed by the preaxial ventral ends 

 of the neural laminae themselves (fig. 13, ac-'). 



At this early stage, the lateral perforations of the vertebra (one just postaxial to the 

 diapophysis, and one at the side of the centrum and placed more ventrally) are relatively 

 larger and much more conspicuous. 



The Third Vertebka. 



The third vertebra exeeds the axis in size very much less indeed than the axis exceeds 

 the atlas. 



Its most striking diflerence in appearance from the axis depends on the absence of 

 the odontoid process and the larger development of the prezygapophyses, which give 

 the bone a quadrate appearance when viewed dorsally, instead of that preaxial tapering 

 which marks the axis (fig. 15). 



The vertebra consists of the same parts as the axis, except the odontoid process ; and 

 these are similarly fused into one solid and complex bone. 



As to the centrum, its preaxial surface, which joins the postaxial surface of the 

 centrum of the axis, presents a transversely extended articular surface, very concave 

 from side to side and convex dorso-ventrally ; both the dorsal and ventral margins of 

 this surface are strongly concave, while its lateral margins are slightly convex. The 

 whole articular surface looks more ventrad than preaxiad (figs. 16, 17, ac). 



The postaxial surface does not difier from its preaxial surface in shape nearly so 



