6 SKELETON CHAP. 
present a considerable number of processes or projections, which 
protect certain blood- vessels, and serve for the attachment of 
the muscles which turn the flexible neck.. The dorsal vertebrae 
follow, and some not unfrequently coalesce with each other, but 
this is always so with the sacrals, and in nearly all existing 
Birds with the terminal portion of the caudals, which are fused 
together to, form a“ pygostyle ” or upright triangular plate to carry 
the tail-feathers.| Archaeopteryx, so far as is known, stands 
alone in having all the caudal vertebrae free. 
A typical vertebra consists of a centrum, and an arch, with 
articular surfaces for two ribs, and is called heterocoelous when the 
facets, or connecting surfaces, are 
saddle-shaped, a condition charac- 
teristic of, and restricted to, Birds. 
It is amphicoelous, or biconcave, 
when each end is hollowed, as in 
the dorsal region of Ichthyornis 
and probably in Archaeopteryx ; 
procoelous, when concave in front 
Fia. 1.—Third cervical vertebra of Wood- (as is common in Reptiles) ; opis- 
pecker (Picus viridis). (Viewed thocoelous when concave behind 
anteriorly.) 7%, vertebrarterial fora- ( as in many Mamm als). 
men; Ob, upper arch; Pa, articular 
process ; Psi, haemal spine ; Pt, Pt, 2. The Ribs are doubly attached 
the two bars of the transverse process, he aeaiecasya Iyer livennal Gan 
shewn on one side ancylosed with the to the vertebrae bY & Neac (capi- 
cervical rib (#2); Sa, articular surface ¢ulum) and a knob (tuberculum) : 
of centrum. (From Wiedersheim.) - 
and have a neck, a dorsal, and a 
ventral portion, each dorsal section (save on the last rib) 
possessing an “ uncinate process” or thin, bony posterior projec- 
tion, except in Archaeopteryx and the Palamedeidae. Should the 
ventral piece articulate with the sternum the rib is “ true,” 
otherwise it is called “false”; moreover the cervical and 
frequently the post-thoracic ribs are fused with the cervical 
vertebrae and the ilia respectively. 
3. The Breast-bone (Sfernuwm) presents two different styles— 
according to whether it exhibits on its ventral surface a median 
ridge or keel (carina), or not. In the former ease, which is that 
of by far the greater number of existing Birds (hence termed 
Carinatae), the keel is of variable size, being correlated with the 
power of flight. It is exceedingly deep in the Swifts, Humming 
1 The Ratitae, Crypturi and Hesperornis have no pygostyle. 
