i 4 THE STRUCTURE AND LIFE OF BIRDS chap. 



weak. Its chief component, the Sternum (ST, fig. 

 5a) or breastbone, is a level expanse with ribs 

 attached to its margin. Down the middle may be 

 seen running a thicker and stronger bone, which throws 

 out a branch on either side, so that the whole makes 

 something of a T shape. This bone is called the Inter- 

 clavicle (ICL), and it will be seen that the Clavicles 

 (CL) or collar-bones converge upon it. This inter- 

 clavicle was formerly thought to correspond to the 

 keel, the high, projecting ridge upon a bird's breast. 

 But this is not the case, for the interclavicle is a 

 membrane bone — i.e., a hardened ossified membrane 

 — while the keel of a bird's breastbone originates from 

 cartilage or gristle. There is also a bone called the 

 Coracoid (CO) running from the shoulder-joint to the 

 fore part of the sternum, and this bone in the lizard 

 divides into two, the fore part being called the Pre- 

 coracoid (PCO). Both parts are thin and weak. At 

 the shoulder-joint the Coracoid and Clavicle are met 

 by another bone, the Scapula (SC) or shoulder-blade, 

 which with the Coracoid forms the socket. The 

 Scapula is a thin expanse of bone approaching close 

 to the back-bone, to which it is attached by muscles. 

 The upper part, the Supra-scapula, consists of gristle 

 or cartilage not completely ossified. In the bird all 

 this has been metamorphosed, though the materials are 

 in the main the same. There is the sternum with a 

 high ridge added which ossifies — i.e., becomes bone — 

 from a different centre. In the young bird the keel is 

 at first mere gristle, which at a certain point begins to 

 ossify, and the process continues till the bone of the 

 keel meets and joins the bone of the sternum proper. 



