30 ELEMENTARY ANATOMY. [LESS. 



It differs from all the other vertebrae in having a large blunt 

 process of bone (like a peg) continued upwards from its cen- 

 trum. This is the odontoid* process, and it has on its front 

 aspect a smooth articular surface. 



The front surface of the rest of its body often bears a 

 slightly marked median vertical prominence. 



The axis also differs from all the other cervical vertebrae 

 in having its neural spine stouter and rather more projecting, 

 though still bifid at its apex. Moreover there are no pre- 

 zygapophyses, but instead, there is on each side a large 

 articular surface in front of the root of the capitular process. 



ii. The first cervical vertebra is also quite peculiar, and 

 bears the special name "ATLAS," because it supports the 

 head. 



FIG. 42. THE ATLAS VERTEBRA. 



s, rudiment of neural spine ; d, tubercular process ; p, capitular process ; a, ar- 

 ticular surface for skull ; hy, plate of bone holding the place of a centrum, and 

 articulating with the odontoid process of the axis vertebra. 



It differs from every other vertebra in having no true centrum, 

 its two lateral halves being connected together in front by a 

 plate of bone, which articulates by its hinder surface with the 

 front surface of the odontoid process of the axis, while its 

 own front surface developes a slight median prominence. 



While in the axis the neural arch is slightly deeper and the 

 neural spine larger than in the other cervical vertebrae, in 

 the atlas the neural arch is much more slender, while the 

 neural spine is absent, or represented by a small tubercle 

 only. The neural arch is either perforated or deeply notched 

 above, just behind the root of the transverse process, which 

 is longer and larger than in the other cervical vertebrae. 

 Zygapophyses are entirely wanting, but we find a large 

 articular surface developed both below and above the r6ot of 

 the transverse process of each side. 



The two lower of these articular surfaces join those before 



1 From 65ou, a tooth, and i3o, form. 



