.380 THE VERTEBRATE SKELETON. 



THE CERVICAL VERTEBRAE. 



These are seven in number, as in almost all mammals. 

 They are characterised by the fact that they have small ribs 

 fused with them, forming transverse processes perforated by 

 canals through which the vertebral arteries run. 



The first, or atlas vertebra (fig. 69, A), differs much from 

 all the others ; it is drawn out into a pair of wide wing-like 

 transverse processes (fig. 69, A, 1), and forms a ring surround- 

 ing a large cavity. This cavity is during life divided into 

 two parts by a transverse ligament ; the upper cavity is the 

 true neural canal, while the lower lodges the odontoid pro- 

 cess of the second vertebra, which is the detached centrum 

 of the atlas. The neural arch is broad and regular ; it has 

 no spinous process, and is perforated in front by a pair of 

 foramina for the passage of the first spinal nerves. The mid- 

 ventral portion of the atlas is rather thick, and bears a minute 

 backwardly-projecting hypapophysis. The bases of the broad 

 transverse processes .are perforated by the vertebrarterial 

 canals (fig. 69, A, 2). The atlas bears at each end a pair of 

 large articulating surfaces; those at the anterior end articulate 

 with the condyles of the skull, and are very deeply concave ; 

 those at the posterior end for articulation with the axis, are 

 nearly as large, but are flattened. The atlas ossifies from 

 three centres, one forming the mid-ventral portion, the others 

 the two halves of the remainder. 



The second, or axis vertebra (fig. 69, B), also differs much 

 from the other cervicals. The long and broad centrum has 

 a very flat dorsal surface, and is produced in front into 

 the conical odontoid process (fig. 69, B, 5), and bears a 

 pair of very large convex outwardly-directed surfaces for arti- 

 culation with the atlas. At its posterior end it is drawn 

 out into a pair of small backwardly-directed spines, the 

 transverse processes ; these are perforated at their bases by 

 the vertebrarterial canals. The neural arch is deeply notched 

 in front and behind for the passage of the spinal nerves, and 



