AYES. 



281 



muscles (pectoral muscles) which move the wings. As a general 

 rule, the size of this crest or keel gives a tolerably just estimate 

 of the flying powers of the bird to which it belonged. The keel is, 

 of course, most largely developed in those birds which possess the 

 power of flight in its greatest perfection ; and in certain birds which 

 do not fly, such sis the Ostrich, there is no sternal keel at all. The 

 pectoral arch or shoulder-girdle of birds, consists of the shoulder- 

 blades {scapulce), the clavicles or collar-bones, and of two bones. 



Fig. 202. — A, Breast-bone, shoulder-girdle, and fore-limb of Penguin (after Owen): h 

 Breast-bone (sternum), with its prominent ridge or keel ; s s Shoulder - blades 

 {scapulce}, k h Coracoid bones; c Furculum or merry-thought, composed of the 

 united collar-bones (davicUs); h Bone of the upper arm or huTneru^, r Radius, 

 and u TJlna, forming together the fore - arm ; g Bones of the wrist or carpus ; t 

 Thumb ; m Metacarpus ; p Phalanges of the fingers. B, Ribs of the Golden Eagle : 

 a a Ribs giving off processes (& &); c c Sternal Ribs. 



which are distinct in Birds, and are called the " coracoid bones.' 

 The shoulder-blades {s s) are usually long and narrow bones. The 

 coracoid bones (k k) correspond with the part of the shoulder-blade 

 which is known in most of the Mammals as the "coracoid pro- 

 cess " ; and in Birds they are not only separate bones, but they are 

 the strongest bones of the pectoral arch. They are more or less 

 nearly vertical, and they form fixed points for the downward stroke 

 of the wing. The collar-bones or clavicles (c) in the great majority 

 of Birds are united together in front, so as to form a somewhat V- 



