28 



THE BONES. 



ATLAS ; INFERIOR SDTIFACE. 



1, Articular cavities for condyles of 

 the occipital bone ; 2, articulai facet; 

 3, vertebral or antero-internal fora- 

 men ; 4, posterior, or cervical fora- 

 men ; 5, transverse process or wing ; 



quite at their base, and on each side of the spinal foramen, they show two large 

 vertical facets which represent the posterior articular processes ; these facets are 



uneven, are confounded with the articular cavity 

 of the upper face of the body, and correspond to 

 the two analogous facets of the axis. Each trans- 

 verse process is pierced at its base by two foramina, 

 which traverse it from below upwards. The 

 posterior represents the vertebral foramen of the 

 other vertebrae ; while the anterior is continued 

 to the external surface of the process by a wide, 

 deep, but very short channel, running from with- 

 out to within, and joins a third foramen, which 

 enters the spinal canal. These last two openings, 

 with the demi-canal which unites them, replace 

 the anterior notch ; the posterior is altogether 

 absent. Lastly, an inflected venous canal, the 

 position of which varies, and it is also sometimes 

 6, tubercle representing the inferior absent, crosscs the laminae of the atlas, and opens, 



spinous process ; 7 superior arch, ^^ ^^q g^[^Q j^to ^J^g Spinal Canal, and OU the 

 forming the roof of the spinal fora- , , ,i ,i , mi n 



men. Other, beneath the transverse process, ihe atlas 



contains much compact tissue, and is generally 

 developed from six centres of ossification : two for the body, which at an early 

 period becomes a solid piece, and two for the annular part ; the other two are 

 complementary centres, each of which forms one of the two posterior undulated 

 facets and lip of the corresponding transverse process. 



Second. — This is named the axis, or dentata (Fig. 16). It is the longest of 



all the cervical vertebrae ; those which 

 succeed it gradually diminish in length 

 and in thickness. The body of the axis 

 has no increase anteriorly, but a conical 

 process termed the odontoid, which is 

 flattened above and below, concave and 

 rough from one side to the other on its 

 superior face ; convex in the same direc- 

 tion and perfectly smooth on its inferior 

 face. The latter represents an articular 

 half-hinge, around which glides the con- 

 cave articular surface on the superior 

 face of the body of the atlas. The 

 anterior articular processes are carried 

 to the base and to each side of the 

 odontoidian pivot, in the shape of two 

 undulated facets, which are confounded with th .' gliding surface of the latter, 

 the destination of which has been already noted. The spinous process, very 

 large and elongated antero-posteriorly, is divided behind into two roughened lips. 

 The transverse processes are slightly developed, and terminate posteriorly in a 

 single tubercle, directed backwards. The anterior notches are very deep, and 

 are most frequently converted into foramina. This vertebra, although voluminous, 

 is light, in consequence of its containing much spongy substance. In the young 

 animal, the odontoid process and the articular surfaces on each side, constitute 



Fig. 16. 



THE AXIS, OR DENTATA ; LATERAL VIEW. 



1, Superior spinous process ; 2, odontoid pro- 

 cess ; 3, intervertebral foramen, or hole of 

 conjugation; 4, body; 5, inferior spinous 

 process ; 6, 7, inferior and superior articu- 

 lating processes. 



