14 THE STRUCTURE AND LIFE OF BIRDS cwap. 
weak. Its chief component, the Sternum (ST, fig. 
Sa) 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—zz., 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—z.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. 
