16 ELEMENTARY SKETCH OF THE OSTEOLOGY OF BIRDS. 



centrum of each vertebra there is a small cup-shaped cavity 

 which articulates with the head or capitulum* of the rib. 

 It is very characteristic of birds that the facets for the articu- 

 lation of the ribs occupy the same position throughout the entire 

 series of dorsal vertebra, whereas in reptiles the articulations 

 vary very much in their relative position at the two extremities 

 of the series. 



The anchylosed mass of vertebrse which forms the sacrum of 

 a bird (pi. I. sm.),t probably, as we have already seen, repre- 

 sents the lumbar, sacral, and some of the anterior caudal vertebras 

 of other Vertebrata, and it is probable, from considerations into 

 which we need not enter here, that only some five of the 

 middle vertebrae of this mass correspond to the true sacral ver- 

 tebrse of a mammal or a reptile. The free caudal vertebrse (c, vt) 

 are generally of simple structure and few in number ; they are 

 usually terminated by a three-cornered bone (ex.) usually 

 known as the coccyx| or ploughshare bone (os en charrue) 

 which appears to consist of several incomplete and agglomerat- 

 ed vertebrse. This bone has no cavity for the reception of the 

 spinal marrow. In an extinct bird [Archaopteryx) the caudal 

 Tcrtebrse formed a long tapering series, as in reptiles and most 

 mammals. In another group of extinct birds {Icthyornis)^ {Apator- 

 nis) II the vertebrae, instead of having modified ball and socket 

 joints, were concave on both articular surfaces, (amphicoelous) 

 and inclosed a bi-convex disc of cartilage between every two of 

 them. 



Ribs. — The ribs of birds (pi. I., r.) are flattened bones, which, 

 as we have already seen, carry widely separated capitula and 

 tubercula for articulation with the centra and transverse pro- 

 cesses of the vertebrae. They are generally few in number, and 

 the most anterior have uncinate processes (p. u.) which project 

 upwards and backwards from their posterior margins. Similar 

 processes occur in the ribs of crocodiles, but never in those 

 of mammals. The true or vertebral ribs are directed almost 

 immediately downwards, and are always completely ossified. 

 At their distal extremities they articulate with other frequently 

 less well ossified ribs (sternal ribs) which are directed upwards 

 and forwards, and which articulate at their free extremities 



* Capitulum, diminution of caput, the head, applied to the terminal vertebral 

 articulation of a rib. 



f The sacral vertebrae are not really seen in the figure, as they are concealed by 

 the pelvis, the letters merely indicate their position. 



J Coccyx, from kokkv^, a cuckoo. The last anchylosed vertebra ; so called 

 from the supposed resemblance of these vertebrae in man to a cuckoo's beak. 



§ Icthyornis, see previous note. 



II Apatornis (from uTraTEw to deceive, and opvig a bird). A fossil American 

 bird with fish-like vertebrae, from which deceptive character the name is taken. 



