SEC. Ill EXTERIOR PARTS OF BIRDS i6i 



hand upon the wrist is such, also, that the hand does not move up 

 and down, as ours can, in a plane perpendicular to the surface of 

 the wing, but in the same plane as that surface. The motion is 

 that which would take place in our hand if we could bring the little 

 linger and its border of the hand so far around as to touch the 

 corresponding border of the forearm. It is a motion of adduction, 

 not of flexion, and its opposite, abduction, not extension, by which 

 a wing is folded and spread. Such abduction is the way in which 

 the hand is extended upon the wrist -joint, increasing and com- 

 pleting the unfolding of the wing that begins by the true extension 

 of the forearm upon the elbow and abduction of the upper arm from 

 the body. In a word, a wing is spread by the motion of abduction 

 at the shoulder and wrist, of extension at the elbow ; it is closed 

 by adduction at the shoulder and wrist, and flexion at the elbow. 

 The numerous muscles which unfold or straighten out the wing are 

 called extensors ; those that bend or close it are flexors. Extensors 

 he upon the back of the upper arm, and the front of the forearm 

 and hand, their " leaders " or tendons passing over the convexities of 

 the elbow and of the wrist. The flexors occupy the opposite sides 

 of the limb, with tendons in the concavities of the joints. The 

 most powerful muscles of the wings are the great pectoral or breast 

 muscles, acting upon the upper end of the humerus ; there are 

 several of them, exerted in throwing out the arm from the body, 

 and in giving both the up and down wing-strokes. Tendons are 

 generally strong inelastic cords ; but there is an interesting arrange- 

 ment of an elastic cord in a bird's wing. In Fig. 27, A B G is a, 

 deep angle formed by the naked bones, but none such is visible from 

 the exterior, because the space is filled by a fold of skin passing 

 from C to near A. But C approaches and recedes from A as the 

 wing is folded or unfolded, and a cord long enough to reach A-G 

 would be slack in the folded wing, did not its elasticity enable it to 

 contract and stretch, keeping the anterior border of the wing straight 

 and smooth. (For another automatic mechanism, see Fig. 28.) 



The point (7 is a highly important landmark in practical orni- 

 thology ; it represents, in any folded wing, a very prominent point, 

 the distance from which to the tip of the longest flight-feather is a 

 special measurement known as that of "the wing." It is the con- 

 vexity of the carpus, commonly called the " carpal angle," or " bend 

 of the wing." Having thus glanced at the bony structure and 

 mechanism of the wing, we are ready to examine the 



Feathers of the Wing (Fig. 30). — How important these are will 

 he evident from the consideration that they are the bird's chief 

 organs of locomotion ; for without them the wing would be useless 

 for flight. We also remember that such means of locomotion is the 

 great specialty of birds. Wing-feathers are those which grow upon 



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