650 BIRDS. 
There ts a well-developed sternum, generally with a keel, 
with a separate centre of ossification, to which the pectoral 
muscles are in part attached. The strong coracoids reach 
and articulate with the sternum. In flying birds the clavicles 
Fic. 355.—Fore-limb and hind-limb compared. 
H., Humerus; &., radius; U., ulna; ~, radiale; ~., ulnare; C., distal 
carpals united to carpo-metacarpus; CC., the whole carpal region; 
MC./., metacarpal of the thumb; /., phalanx of the thumb; A7C.//., 
second metacarpus; //., second digit; J¢C.///., third metacarpus ; 
177, third digit, , femur; 7.7\, tibio-tarsus; 77, fibula; Pz., 
proximal tarsals united to lower end of tibia;-d¢., distal tarsals united 
to upper end of tarso-metatarsus (7.77.); 7., entire tarsal region; 
MT.1., first metatarsal, free ; /.-/V., toes. 
are usually well developed, and connected by an tnterclavicle, 
which may be connected with the apex of the sternum. The 
fore-limb has not more than three digits (I., I1., and III.), the 
three metacarpals are fused (except in Archeopteryx), and 
