CLASS AVES 581 



use this organ as a rudder while flying and as a balancer while 

 perching. 



There are two cervical ribs and five thoracic ribs on each side. 

 The second cervical and first four thoracic ribs bear each an 

 uncinate process which arises from the posterior margin and 

 overlaps the succeeding rib, thus making a firmer framework. 

 The thoracic ribs are connected with the sternum or breastbone. 

 The sternum is united in front with the coracoid (Fig. 474, 32) 

 of the pectoral girdle and bears on its ventral surface a large 

 crest or keel {carina, Fig. 474, 33) to which the muscles that move 

 the wings are attached. 



The pectoral girdle consists of a pair of blade-like scapula, 

 the shoulder-blades, which lie above the ribs one on either side 

 of the vertebral column in the thorax. The coracoids (Fig. 474, 

 32) connect the sternum with the anterior end of the scapulae 

 at the shoulders. A concavity in these bones at their junction 

 furnishes the articular surface for the long wing bone (humerus), 

 and is called the glenoid cavity. The two clavicles (Fig. 474, 31) 

 connect proximally with the shoulder and are fused together 

 distally, forming a V-shaped furcula or " wishbone." The 

 clavicles are homologous to the collar-bones of man, and serve 

 to brace the shoulders. 



The fore limb or wing of the pigeon (Fig. 471) is greatly 

 modified. There are but three digits, and only one of these is 

 well developed. The distal row of carpal bones and the three 

 metacarpals are fused together forming a carpo-metacarpus 

 (Fig. 471, M1-M3) ; this adds to the rigidity of the wing. The 

 arm contains, as in other vertebrates, a single bone, the humerus 

 (X), with a convex head which lies in the glenoid cavity. The 

 fore arm possesses two bones, the radius (!') and ulna (Z). 

 The wrist contains two carpal bones (UC and RC); the ether 

 carpal bones are fused with the three metacarpals (M1-M3), 

 forming the carpo-metacarpus, as stated above. Besides the 

 carpo-metacarpus, the hand possesses a preaxial digit with two 

 small bones (Ni), which supports a small tuft of feathers and 



