376 SKELETON OF THE FOWL 



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 im- 

 portant of which are, (1) the complete absence of the two outer 

 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 pre- 

 axial 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 shght 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 surface 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 prominent 

 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 

 wiug. 



