CER VIC A L VER TEBRJE. 



37 



which is the rudiment of a spinous process, and gives origin to the Rectus capitis 

 posticus minor. The diminutive size of this process prevents any interference in 

 the movements between the atlas and the cranium. The posterior part of the 

 arch presents above and behind a rounded edge for the attachment of the posterior 

 occipito-atlantal ligament, while in front, immediately behind each superior 

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

 bony spiculum which arches backward from the posterior extremity of the superior 

 articular process. These grooves represent the superior intervertebral notches, 

 and are peculiar from being situated behind the articular processes, instead of 

 in front of 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 backward and inward. 

 They also transmit the suboccipital (first spinal) nerve. 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 vertebrae. They are much less marked than the superior. The lower 

 border also gives attachment to the posterior atlanto-axial ligament, which con- 

 nects it with the axis. 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 articu- 

 lating processes above, and two below. The two superior are of large size, oval, 



Tubercle, 



Diagram of section of odontoid 

 process. 



Diagram of section of 

 transverse ligament. 



Foramen for 

 vertebral artery. 



Groove for vertebral artery 

 and 1st cervical nerve. 



Rudimentary spinous process.. 



FIG. 2. First cervical vertebra, or atlas. 



concave, and approach each other in front, but diverge behind ; they are directed 

 upward, inward, and a little backward, each forming a kind of cup for the corre- 

 sponding tondyle of the occipital bone, and are admirably adapted to the nodding 

 movements of the head. Not infrequently 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 downward and inward, 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 the transverse 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 poste- 

 rior 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 compression of this 

 structure. The transverse processes are of large size, project directly outward 

 and downward from the lateral masses, and serve for the attachment of special 

 muscles which assist in rotating the head. They are long, not bifid, and perfor- 

 ated at their base by a canal for the vertebral artery, which is directed from below, 

 upward and backward. 



The Axis (Fig. 3) is so named from forming the pivot upon which the first 

 vertebra, carrying the head, rotates. The most distinctive character of this bone 



