256 



VERTEBKATE ANIMALS. 



FIG. 128. A, Breastbone, shoulder-girdle, and fore-limb of Penguin (after Owen); & Breast- 

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

 Coracoid bones ; o Furculum or Merry -thought, composed of the united collar-bones 

 (clavicles) ; h Bone of the upper arm or humerus ; r Kadius ; u Ulna, forming together 

 the forearm; q Bones of the wrist or carpus; t Thumb; m Metacarpus; p Phalanges 

 of the fingers. B, Kibs of the Golden Eagle ; a a Bibs giving off processes (b V) ; c c 

 Sternal ribs. 



process ; " 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 clavi- 

 cles (c) in the great majority of birds are united together in 

 front, so as to form a somewhat V-shaped bone, which is tech- 

 nically called the "furculum," but is familiarly called the 

 " merry-thought." The function of this clavicular arch is to 

 keep the wings asunder during their downward stroke, and 

 the strength of the furculum varies, therefore, with the powers 

 of flight enjoyed by each bird. The bones which form the 

 limb proper, or " wing," are considerably modified to suit the 

 special function of flight, but essentially the same parts are 

 present as in the fore-limb of the Mammals. The upper arm 

 is constituted by a single bone, the humerus (h\ which is gen- 

 erally short and stout, The forearm is composed of two 

 bones, the radius (r) and the ulna (u), of which the ulna is 

 the bigger. These are followed by the small bones, which 



