4 62 



MANUAL OF ZOOLOGY 



SECT. 



a. 



n.a. 



en, 



with the sternum. In the third to the twelfth there are 

 vestigial ribs (Fig. 261, rb\ each having its head fused with 

 the centrum, and its tubercle with the transverse process. 

 The whole rib thus has the appearance of a short, backwardly- 

 directed transverse process perforated at its base. 



The centra of the cervical vertebrae differ from those of 

 all other Vertebrata in having saddle-shaped surfaces, the 

 anterior face (Fig. 261, A) being concave from side to side 

 and convex from above down- 

 wards. This peculiar form of 

 vertebra is distinguished as hetero- 

 cotlous. 



The first two vertebrae, the 

 atlas and axis, are specially 

 modified. The atlas is a ring- 

 like bone with an articulation on 

 its anterior surface for the single 

 occipital condyle of the skull. 

 The axis has projecting forwards 

 from its centrum a peg-like pro- 

 cess, the odontoid process, which 

 lies in the lower part of the ring 

 of the atlas. 



Between the last cervical 

 vertebra and the pelvic region 



come four thoracic vertebrae (Fig. 260), the first three 

 united into a single mass, the fourth free. They all bear 

 ribs, each consisting of a vertebral (vr. r.) and a sternal 

 (.$/. r.) portion, and articulating with the vertebra by a double 

 head. Springing from the posterior edge of the vertebral 

 rib is an uncinate (unc.}> resembling that of Hatteria and the 

 Crocodile, but formed of bone and ankylosed with the 

 rib. 



crv 



FIG. 261. Columba livia. Cer- 

 vical vertebra ; A, anterior ; 

 B, posterior face. a. zyg. an- 

 terior zygapophysis ; en. cen- 

 trum ; n. a. neural arch ; p. zyg. 

 posterior zygapophysis; rb. rib; 

 vrb.f. vertebrartenal foramen. 



