PROGRKSSION IN OR THROUGH THE AIR. 



179 



vertical, horizontal, and intermediate movements,^ The long 

 axis of the joint is directed vertically; the joint itself some- 

 what backwards. It is otherwise with the elbow-joint, which 

 is turned forwards, and has its long axis directed horizontally, 

 from the fact that the humerus is twisted upon itself to the 

 extent of nearly a quarter of a turn. The elbow-joint is 

 decidedly spiral in its nature, its long axis intersecting that of 

 the shoulder-joint at nearly right angles. The humerus 

 articulates at the elbow with two bones, the radius and the 

 ulna, the former of which is pushed from the humerus, while 

 the other is drawn towards it during extension, the reverse 

 occurring during flexion. Both bones, moreover, while those 

 movements are taking place, revolve to a greater or less extent 

 upon their own axes. The bones of the forearm articulate at 

 the wrist with the carpal bones, which being spirally arranged, 

 and placed obliquely between them and the metacarpal bones, 

 transmit the motions to the latter in a curved direction. The 

 long axis of the wrist-joint is, as nearly as may be, at right 

 angles to that of the elbow-joint, and more or less parallel 

 with that of the shoulder. The metacarpal or hand-bones, 

 and the phalanges or finger-bones are more or less fused 

 together, the better to support the great primary feathers, on 

 the efficiency of which flight mainly depends. They are 

 articulated to each other by double hinge-joints, the long axes 

 of which are nearly at right angles to each other. 



As a result of this disposition of the articular surfaces, the 

 wing is shot out or extended and retracted or flexed in a 

 variable plane, the bones composing the wing, particularly 

 those of the forearm, rotating on their axes during either 

 movement. 



This secondary action, or the revolving of the component 

 bones upon their own axes, is of the greatest importance in 

 the movements of the wing, as it communicates to the hand 



1 The OS humeri, or bone of the arm, is articuhated by a small rounded 

 surface to a corresponding cavity formed between the coracoid bone and the 

 scapula, in such a manner as to allow great freedom of motion." — Macgillivray's 

 Brit. Birds, vol. i. p. 33. 



" The arm is articulated to the trunk by a ball-and-socket joint, permitting 

 all the freedom of motion necessary for flight."— Cyc. of Anat. and Phys., 

 vol. iii. p. 424. 



