14 THE SKELETON OF THE CAT. 



The Atlas (Fig. lo, i; Fig. 12). — The first cervical verte- 

 bra or atla^ has somewhat the form of a seal ring. The 

 centrum is absent; it has united with the second vertebra to 

 form the odontoid process or dens. Its place is taken in the 

 atlas by a narrow flat arch of bone, narrower at the ends than 

 in the middle, the ventral arch (Fig. 12, a) of the atlas. This 

 connects the lateral, thicker portions of the ring ventrally and 

 bears on its caudal margin a blunt tubercle (tuberculum 

 anterius, Fig. 12, b). Laterally the ring is thickened, forming 

 thus the lateral masses {c) which are continued into the broad 

 thin transverse processes (Fig. 10, g\ Fig. 12, d). Each 

 lateral mass bears at its cranial end on its medial surface a 

 concave, pear-shaped facet, cranial (or superior) articular facet, 

 (Fig. 12, e) for articulation with the condyles of the skull. 

 These facets look craniomediad. Dorsad of each is a foramen, 

 the atlantal foramen (Fig. 10, /'; Fig. 12, g), which pierces 

 the dorsal arch at its junction with the lateral mass. Caudal 

 to the facet, on the medial face of each lateral mass, within the 

 vertebral canal, is a tubercle. To the two tubercles are 

 attached the transverse ligament (Fig. 14, b) which holds in 

 place the odontoid process (dens) of the axis. 



That part of the lateral mass which bears the articular facet 

 projects craniad of the dorsal arch and is separated by a deep 

 triangular notch from the transverse process. Along the 

 bottom of this notch runs a groove (Fig. 12, f), convex 

 craniad, which connects the cranial end of the foramen trans- 

 versarium and the atlantal foramen. The vertebral artery 

 passes along it. The foramen transversarium is circular. It 

 is bounded laterally by the lateral masses, and dorsally by the 

 dorsal arch. 



The dorsal arch (Fig. 10, h) is two to three times as broad 

 as the ventral, has a thick convex cranial border with a median 

 notch, and a thin concave caudal border. 



The caudal articular facets (Fig. 12, h) are borne by the 

 caudal ends of the lateral masses. They are slightly concave, 

 triangular, and look caudomediad, so that their dorsal borders 

 form with the caudal border of the dorsal arch nearly a semi- 

 circle. The transverse processes are flat and directed laterad. 



