422 SKELETON OF THE FOWL. 



surface, articulating with the anterior end of 

 the sternum, 

 iii. The clavicles are a pair of slender curved bones, 

 whose upper ends are slightly expanded, and 

 connected, as described above, with the cora- 

 coids ; while their lower ends are fused together 

 to form a median laterally compressed plate. 



The two clavicles together form the fur- 

 cula or ' merrythought.' 



B. The Fore-limb or Wing. 



The wing of the bird corresponds to the fore-limb of the 

 rabbit, but differs from it in several respects, the more 

 important of which are : (1) the presence of only three digits ; 

 and (2) the fusion of the distal row of carpal bones with 

 the three persistent metacarpals to form a compound bone, 

 the carpo-metacarpus. 



When the wing is extended for flight, its surfaces and 

 borders correspond to those of the primitive vertebrate limb, 

 the preaxial border being directed forwards, the postaxial 

 backwards, and the dorsal and ventral surfaces upwards and 

 downwards respectively. When the wing is folded, in the 

 position of rest, the three main divisions arm, forearm, and 

 hand are bent on each other like the letter Z, the elbow 

 pointing backwards and the wrist forwards. Owing to a 

 slight rotation at the shoulder- joint, and a slight obliquity of 

 the elbow- joint, the dorsal surface of the arm now faces 

 inwards and upwards ; that of the forearm outwards and 

 slightly upwards ; while in the hand, which is bent on the 

 forearm in a position of extreme abduction, the dorsal sur- 

 face faces almost directly outwards. 



1. The arm. 



i. The humerus is an elongated slightly curved 

 bone expanded at both ends. 



The proximal end, or head, presents a 

 large convex vertically elongated surface for 

 articulation with the glenoid cavity, bordered 



