VERTEBRA AND RIBS 



47 



some cormorants and gulls. Only the atlas is ever procoelous. All 

 other centra have saddle-shaped (heteroccele) ends, the anterior end 

 being concave from right to left, convex dorso-ventrally, the posterior 

 end having these outlines reversed, the saddle shape being most 

 n^arked in the neck (fig. 52). As a result sagittal sections of a cen- 

 trum appear as if opisthocoelous; horizontal sections as if procoelous. 

 The neural arches bear zygapophyses, the postzygapophysis 

 overriding the prezygapophysis of the next vertebra. Each anterior 

 vertebra has dia- and parapophyses. The cervical vertebrae (from 

 eight or nine up to twenty-five in swans, fourteen or fifteen being 

 usual) have elongate centra and usually all are free and very mobile, 

 the exceptions being the fused atlas and epistropheus of hornbills 

 or the union of the last cervical with the first thoracic (tinamous, 

 etc.). Atlas, epistropheus and dens are as in all Amniotes, and 



Fig. 52. — Front and side of cervical vertebra of fowl showing cervical rib. c 

 centrum; cs, spinal canal; d, diapophysis; p, parapophysis; r, rib; va, vertebrarterial 

 canal, arrow passing through it in side view. 



these, like all cervicals, bear short ribs with well-developed verte- 

 brarterial canal, the ribs long remaining distinct in Ratites and 

 permanently so in Archseopteryx. The rib of the last cervical, which 

 does not reach the sternum, usually has an uncinate process. 



The thoracic vertebrae, six to ten in number (twelve in Archceop- 

 teryx) are usually more or less fused (not in ArchcBopteryx), strength- 

 ening the thorax. The last thoracic (sometimes several of them) 

 may be free, these having saddle-shaped ends, and in diving birds 

 may have bifurcate hypapophyses. All of the thoracic vertebrae 

 have bicipital ribs (single headed in ArchcBopteryx) articulated to di- 

 and parapophyses. These ribs are fully ossified and are divided by 

 a joint into vertebral and sternal parts. At least the anterior ribs 

 (except in some Lamellirostres and ArchcBopteryx) have uncinate 

 processes on the vertebral part, arising as discrete cartilages, which, 

 except in moas and many water birds, fuse later with the ribs, giving 



