SEC. Ill 



EXTERIOR PARTS OF BIRDS 



157 



other and to the axes of the body ; the movements of the joints 

 are peculiar in some respects ; and the whole extremity of the 

 wing, from the wrist outward, is peculiarly constructed, by loss of 

 some of the digits that five-fingered animals possess, and by the 

 compression of those that are left. The wing proper begins at the 

 shoulder-joint, where it hinges freely upon the shoulder, in a 

 shallow or glenoid socket formed conjointly by the shoulder-blade 

 or scapula, and by the coracoid bone ; these two, with the clavicles, 

 collar-bones, or merrythought, furculum, forming the shoulder-girdle, 

 or pectoral arch (Figs. 56, 59). 



The wing ordinarily consists, in adult life, of ten or eleven 

 actually separate bones ; in the embryo (see Fig. 29) there are 

 indications of several more at the wrist-joint, which speedily lose 



Fig. 28. — Mechanism of elbow-joint. (See explanation of Fig. 27.) 



their individual identity by fusing together and with bones of the 

 hand. Aside from these, there is often an accessory ossicle at the 

 shoulder-joint (Fig. 56, ohs), sometimes one at the wrist-joint, 

 occasionally an extra bone at the end of the principal finger. The 

 normal or usual number is shown in Fig. 27, taken from a duck 

 {Clangula islandica), in which there are eleven. 



The upper-arm bone h, reaching from the shoulder A to the 

 elbow B, is the humerus. In the closed wing, the humerus lies 

 nearly in the position of the same bone in man when the elbow is 

 against the side of the body ; in extension of the wing the elbow 

 is borne away from the body, as when we raise the arm, but carry 

 it neither forward nor backward. A peculiarity of the bird's 

 humerus is, that it is rotated on its axis through about the quadrant 

 of a circle, so that what is the front of the human bone is the outer 



