Well-developed ribs are attached to the posterior cervical vertebrae as well as to the thoracic vertebrae. The ribs generally have uncinate processes and separate capitula and tubercula, but uncinate processes are absent in Chauna Palamedea and apparently in Archaeopteryx.
The sternum (fig. 63) is greatly developed in all birds. In the embryo[107] it is seen to be derived from the union of right and left plates of cartilage, formed by the fusion of the ventral ends of the ribs. In the Ratitae and a few Carinatae, such as Stringops, it is flat, but in the great majority of birds it is keeled, though the development of the keel varies greatly. It is large in the flightless Penguins, which use their wings for swimming. Traces of an interclavicle may occur in the embryo.
Fig. 63. Shoulder-girdle and sternum of
A. Black Vulture (Vultur cinereus) × 1/3.
B. Peacock (Pavo cristatus) × 3/8.
C. Pelican (Pelicanus conspicillatus) × 1/3. (All Camb. Mus.)
| 1. carina of the sternum. | 6. surfaces for articulation with |
| 2. coracoid. | the sternal ribs. |
| 3. scapula. | 7. xiphoid processes. |
| 4. clavicle. | 8. fontanelle. |
| 5. costal process. |
Pectoral Girdle.