The Framework of the Bird 81 
of this bone which is most characteristic of birds is the 
central ridge or keel which projects straight out from 
the surface of the sternum. This is of the utmost im- 
portance in giving firm anchorage for the great flight- 
muscles of the breast. 
Fic. 58.—Ribs and sternum of Ostrich; notice absence of keel correlated with 
loss of power of flight. The suture through the centre of the sternum reveals 
its paired origin. 
The keel is of many shapes, but when well developed 
is generally very high at the upper end of the breast- 
bone and becomes lower as it slopes gradually backward. 
In birds which have lost the use of their wings the keel 
has disappeared completely, the sternum being flat, as 
in ourselves. This is its condition in the ostrich and 
cassowary, and it is the character which has given a 
