CERYICAL VERTEBRAE. 43 



backwards from the posterior extremity of the superior articular process. These 

 grooves represent the superior inter vertebral notches, and are peculiar from being 

 situated behind the articular processes, instead of before them, as in the other 

 vertebras. They serve for the transmission of the vertebral artery, which, ascending 

 through the foramen in the transverse process, winds round the lateral mass in a 

 direction backwards and inwards. They also transmit the sub-occipital nerves. 

 On the under surface of the posterior arch, in the same situation, are two other 

 grooves, placed behind the lateral masses, and representing the inferior inter ver- 

 tebral notches of other vertebrae ; they are much less marked than the superior. 

 The lateral masses are the most bulky and solid parts of the atlas, in order to 

 support the weight of the head ; they present two articulating processes above, 

 and two below. The two superior are of large size, oval, concave, and approach 

 towards one another in front, but diverge behind ; they are directed upwards, 

 inwards, and a little backwards, forming a kind of cup for the condyles of the 

 occipital bone, and are admirably adapted to the nodding movements of the head. 

 Not unfrequently they are partially subdivided by a more or less deep indentation 

 which encroaches upon each lateral margin ; the inferior articular processes are 

 circular in form, flattened, or slightly concave, and directed downwards, inwards, 

 and a little backwards, articulating with the axis, and permitting the rotatory 

 movements. Just below the inner margin of each superior articular surface, is a 

 small tubercle, for the attachment of a ligament which, stretching across the ring 

 of the atlas, divides it into two unequal parts ; the anterior or smaller segment 

 receiving the odontoid process of the axis, the posterior allowing the transmission 

 of the spinal cord and its membranes. This part of the spinal canal is of con- 

 siderable size, to afford space for the spinal cord ; and hence lateral displacement 

 of the atlas may occur without compression of the .spinal cord. (This ligament 

 and the odontoid process are marked in figure 2 in dotted outline.) The trans- 

 verse processes are of large size, for the attachment of special muscles which 

 assist in rotating the head long, not bifid, perforated at their base by a canal 

 for the vertebral artery, which is directed from below, upwards and backwards. 

 The Axis (fig. 3) is so named from forming the pivot upon which the head 



Fig. 3. 2d Cervical Vertebra or Axis. 

 Odo.ntoid 



Surf.fr CAec'Jt Ii^ _____ ___ 



-Artie. Surf, for Atlas 



VH(!lkXT3n\ 



Artie. Swrj.fer 



Spin. 



Infer. A. rtic. I 



rotates. The most distinctive character of this bone is the strong prominent 

 process, tooth-like in form (hence the name odontoid), which rises perpendi- 

 cularly from the upper part of the body. The body is of a triangular form ; 

 deeper in front than behind, and prolonged downwards anteriorly so as to overlap 

 the upper and fore part of the adjacent vertebra. It presents in front a median 

 longitudinal ridge, separating two lateral depressions for the attachment of the 

 Longi colli muscles. The odontoid process presents two articulating surfaces: 



