518 Comparative Morphology of Chordates 



in a thick layer of horn, as in Chelonia, forming the beak, or bill, 

 which is usually sharp-pointed and provided with cutting edges so that 

 it in part serves the function of teeth. 



The ribs and sternum (Fig. 401) enclose the thoracic region of the 

 trunk. The thoracic ribs, commonly five pairs, but only three pairs in 

 Rhea ("South American ostrich") and nine in the swan, articulate with 

 their corresponding vertebrae and connect ventrally with the sternum. 

 Each rib is divided into a dorsal vertebral part and a ventral sternal 

 part, movably jointed together. In most birds each vertebral part, 

 except the hindmost, bears a strong uncinate process which projects 

 backward to overlap the next rib, to which it is tied by fibrous tissue. 

 Both vertebral and sternal parts of the rib consist of fully ossified and 

 very hard bone, in contrast to the fact that in most animals other than 

 birds the sternal segments of ribs are cartilaginous. 



The sternum is greatly expanded (Fig. 401). It is a broad, thin, 

 undivided plate of bone curving laterally upward to fit the contour of 

 the breast region of the bird. It provides extensive bony enclosure for 

 the anterior ventrolateral part of the trunk in much the same way that 

 the expanded pelvic girdle serves for the posterior dorsolateral region. 

 Along the median ventral line of the sternum, a thin blade of bone 

 projects outward in the median plane. The entire sternum, with this 

 blade extending lengthwise of the externally convex surface of it, 

 strongly resembles the under side of a round-bottomed boat with a 

 keel (Latin carina). The carina is prominently developed in all strong- 

 fliers. In the ostrich and similar "running birds" and in various birds 

 which are poor fliers or incapable of flight (e.g., certain ground-living 

 rails of New Zealand, and the recently extinct dodo and solitaire of 

 islands near Madagascar), the carina is more or less reduced or entirely 

 lacking. There are aquatic birds which fly poorly (some members of the 

 auk family) or not at all (penguins and the recently extinct "great 

 auk"), but make very effective use of their short wings in swimming. 

 In these birds the carina is strongly developed. Our common domestic 

 fowl, however, in the course of its brief (in terms of geologic time) 

 period of existence under conditions of domestication, has become a 

 very feeble and reluctant flier, but it still retains a well-developed 

 carina. It is a noteworthy fact that the sternum of reptilian pterosaurs 

 and mammalian bats is provided with a carina. 



Muscles 



The massive muscles which operate the wings occupy the angular 

 recess, on either side, between the carina and the main body of the 

 sternum. Most of this space is filled by the enormous pectoralis 



