STRUCTURE OF THE WING. 31 
§ 56. Tae Mecuanism 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-joint 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, would throw them up, and render flight difficult 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 plane of the 
elbow-bend, but back and forwards, in a plane horizontal to the elbow; 
it is as if we could bring our little finger and its side of the hand 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, aBc 
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 skin stretched over a cord that passes straight from near 4 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 thoroughly understood ; it is also called the “bend 
of the wing.” (See under Directions for Measurement; see also explana- 
tion of fig. 6.) 
Fic. 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- 
