EXTERNAL PARTS OF BIRDS. — THE WINGS. 109 



quite peculiar. lu the first place the two bones of the forearm are so fixed iu relation to each 

 other, that the radius cauuot roll over the ulua, like ours. If you stretch your arm upon the 

 table, you can, without moving the elbow, tufti the haud over so that either the palm or the 

 knuckles are downward. This is a rotary motion of the bones of the forearm, called pronation 

 and supination ; the prone when the palm touches the table, supine when the knuckles are 

 do\viiward. This rotatit)n is absent from the bird's arm ; if it could occur, the action of the air 

 upon the pinion-feathers would throw them all " at sea " during the strokes of the wing, render- 

 ing rtight difficult or iini)()ssible. The hingeiug of the hand upon the wrist is such, also, that the 

 hand does not move up and down, as ours can, in a plane p(;rpendicular 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 finger and its border of the haud so far aroimd as to touch the 

 corresponding border of the forearm. It is a motion of add action, not of iiexion, 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 completing 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 lie upon the back 

 of the upper arm, and the fi'ont of the forearm and haud, 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 wiui^s 

 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 tlie up and 

 down wing-strokes. Tendons are generally strong inelastic cords ; but there is an interesting 

 arrangement of an elastic cord in a bird's wing. In fig. 27, ^ B C is a deep angle formed by 

 tlie 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-C 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 explanation of fig. 28.) 



The point C is a highly important landmark in practical ornithology ; 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 convexity 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 be evident from the 



consideration that they are tlie bird's chief orgam of locomotion ; for without them the win^ 



wo 



uld 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 the pteryla alaris. They are 

 of two main sorts : the flight-feathers proper, or long stiff quiUs, collectively called remiges 

 (Lat. remex, pi. remiges, rowers) ; and the smaller, weaker feathers overlying them, and hence 

 called coverts, or tectrices (Lat. tectrix, pi. tectrices, coverers). To these may be added as a 

 third distinct group tlie bastard quills, which constitute the 



Alula, or Ala Spuria (Lat. alula, little wing, diminutive of ala, wing ; spuria, spurious, 

 i.astard). The "'little wing" is simply the small parcel of feathers whicli grow upon the 

 •' thumb " (see fi-. 27, d2; 29, d and h; 30, al). Highly significant as these may be in a mor- 

 phological point of view, as representing what this part of" the wing may have been in early times. 



