388 SKELETON OE THE FOWL 



B. The'^Fore-Limb or Wing. 



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

 rabbit, but differs from it in several respects, the naore im- 

 portant of which are, (1) the complete absence of the twa 

 outer digits ; and, (2) the fusion of the distal row of carpal 

 bones with the three persistent metacarpals to form a com- 

 pound 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, fore-arm, 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 looks 

 inwards and upwards ; that of the fore-arm outwards and 

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

 fore-arm in a' position of extreme abduction, the dorsal sur- 

 face looks almost directly outwards. 



1. The arm has only a single bone. 



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 

 by two tuberosities, which are preaxial and 

 postaxial respectively. 



The preaxial tuberosity is small, but is 

 continued along the bone as the promiuent 

 forwardly directed deltoid ridge : this gives 

 insertion to the great pectoral and deltoid 

 muscles ; while a small tubercle at its proximal 

 end, and slightly on the extensor surface, 

 marks the insertion of the tendon of the second 

 pectoral muscle, or elevator of the wing. 



