CERVICAL VERTEBRAE. 119 



it is concave, and marked by a smooth oval or circular facet, for articulation 

 with the odontoid process of the axis. The posterior arch forms about two-fifths 

 of the circumference of the bone ; it terminates behind in a tubercle, which is 

 the rudiment of a spin-cms process, and gives origin to the Rectus Capitis Pos- 

 ticus Minor. The diminutive size of this process prevents any interference in 

 the movements between it and the cranium. The posterior part of the arch 

 presents, above, a rounded edge ; whilst, in front, immediately behind each su- 

 perior articular process, is a groove, sometimes converted into a foramen by a 

 delicate bony spicula which arches backwards from the posterior extremity of 

 the superior articular process. These grooves represent the superior interver- 

 tebral notches, and are peculiar from being situated behind the articular pro- 

 cesses, instead of before them, as in the other vertebrae. 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 sur- 

 face of the posterior arch, in the same situation, are two other grooves, placed 

 behind the lateral masses, and representing the inferior intervertebral notches 

 of other vertebras. 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 to- 

 wards 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 down- 

 wards, inwards, and a little backwards, articulating with the axis, and permit- 

 ting 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 pos- 

 terior allowing the transmission of the spinal cord and its membranes. This 

 part of the spinal canal is of considerable size, to afford space for the spinal 

 cord ; and hence lateral displacement of the atlas may occur without compres- 

 sion of the spinal cord. The transverse processes are of large size, for the at- 

 tachment 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. 70), (so named from forming the pivot upon which the head 

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

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

 larly 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 

 Longus colli muscles of each side. The odontoid process presents two articu-" 

 lating surfaces: one in front of an oval form, for articulation with the atlas; 

 another behind, for the transverse ligament; the latter frequently encroaching 

 on the sides of the process; the apex is pointed. Below the apex, the process 

 is somewhat enlarged, and presents on either side a rough impression for the 

 attachment of the odontoid or check ligaments, which connect it to the occipital 

 bone; the base of the process, where it is attached to the body, is constricted, 

 so as to prevent displacement from the transverse ligament, which binds it in 

 this situation to the anterior arch of the atlas. Sometimes, however, this pro- 

 cess does become displaced, especially in children, in whom the ligaments are 

 more relaxed: instant death is the result of this accident. The pedicles are 



