STRUCTURE OF THE WING. 



31 



§ 56. The Mechanism of these bones is admirable. The shoulder- 

 joint is loose, much like ours, and allows the humerus to swing all about, 

 though chiefly up and down. The elbow-joiut is tight, permitting only 

 bending and unbending in a horizontal line. The finger bones have scarcely 

 any motion. But it is in the wrist that the singular mechanism exists. In 

 the first place, the two forearm bones are fixed with relation to each other 

 so that they cannot roll over each other, like ours. Stretch your arm out 

 on the table ; without moving the elbow, you can turn the hand over so that 

 either its palm or its back lies flat on the table. It is a motion {rotation) 

 of the bones of the forearm, resulting in what is called pronation and su- 

 pination. This is absent from the bird's arm, necessarily ; for if the hand 

 could thus roll over, the air striking the pinion-feathers, when the bird is 



flying, Avould throw them up, and render 



flight 



diflicult or impossible-. 



Next, the hinging of the hand upon the wrist is such, that the hand does not 

 move up and down, like ours, in a plane perpendicular to the plaue of the 

 elbow-bend, but back and forwards, iu a plaue horizontal to the elbow ; 

 it is as if we could bring our little finger and its side of the baud around to 

 touch the corresponding border of the forearm. Thus, evidently, extension 

 of the hand upon the wrist-joint increases and completes the unfolding 

 of the wing that commenced by straightening out the forearm at the elbow. 

 There is another essential feature in a bird's wing. In the figure, 6, abo 

 represents a deep angle formed by the bones, but none such is seen upon 

 the outside of the wing. This is because this triangular space is filled up 

 by a fold of skiu stretched over a cord that passes straight from near a to c. 

 But A and c approach or recede as the wing is folded or unfolded, and a 

 simple cord long enough to reach the full distance a — c would be slack in 

 the folded wing ; so the cord is made elastic, like an India rubber band ; it 

 stretches when the wing is unfolded, and contracts when the wing is shut ; 

 it is thus always hauled taut. The cord makes the always straightish and 

 smooth anterior border of the wing. The carpus c, or the always promi- 

 nent point of the anterior border, is a highly important landmark in de- 

 scriptions, and should be thoi-oughly understood ; it is also called the "bend 

 (See under Directions for Measurement; see also explana- 



of the wiug." 

 tion of fig. 6. 



) 



Fig. 6, taken from a young chicken (right wing, upper surface), shows the composition 

 and mechanism of a bird's wing, a, shoulder; b, elbow; c, wrist or carpus; d, tip of prin- 



