358 



ZOOLOGY 



SECT. 



ribs (Fig. 969, 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 pro- 

 cess perforated at its base ; the perforation transmits the vertebral 

 artery, and is called the vertebrarterial foramen (vrb. /.) 



The centra of the cervical vertebra differ from those of all other 

 Vertebrata in having saddle-shaped surfaces, the anterior face 

 (Fig. 969, A) being concave from side to side and convex from 

 above downwards, the posterior face (B) convex from side to side 

 and concave from above downwards. Thus the centrum in sagittal 

 section appears opisthoccelous, in horizontal section proccelous. 

 This peculiar form of vertebra is distinguished as heterocaelous. 



f.trs 



* 



th.v.-f 



SCb 



th.v.S 



* scr ct.tr 



a 



St 



car 



FIG. 968. Columba livia. The bones of the trunk, acr. cor. acrocoracoid ; u.tr. anti-trochanter ; 

 actb. acetabulum ; car. carina sterni ; cd. r. caudal vertebras ; cor. coracoid ; a: r. cervical 

 ribs ; /. t /.?. probe passed into foramen triosseum ; far. furcula ; gl. ci\ glenoid cavity ; il. 

 ilium; is. ischium ; is. for. ischiatic foramen; obt. n. obturator notch; pu. pubis ; jiwi.st. 

 pygostyle ; scj). scapula; s. scr. syn-sacrum ; st. sternum; st. r. sternal ribs; th. c. 1, first, 

 and th. v. 5, last thoracic vertebra ; unc. uncinates ; vr. /. vertebral ribs. 



The centra articulate with one another by synovial capsules each 

 traversed by a vertical plate of cartilage, the meniscus, with a 

 central perforation through which a suspensory ligament passes 

 from one centrum to the other. 



The first two vertebrae, the atlas and axis, resemble those of the 

 Lizard, but have the various elements of which they are composed 

 completely fused. The small size of the ring-like atlas is notice- 

 able. 



Between the last cervical vertebrae and the pelvic region come 

 four thoracic vertebra? (Fig. 968), the first three united into a single 

 mass, the fourth free. The anterior thoracic as well as the posterior 

 cervical vertebrae have the centrum produced below into a com- 

 pressed plate, the hypapophysis, for the origin of the flexor muscles 



