EXTERNAL PARTS OF BIRDS.—THE WINGS. 109 
quite peculiar. In the first place the two bones of the forearm are so fixed in relation to each 
other, that the radius cannot roll over the ulna, like ours. If you stretch your arm upon the 
table, you can, without moving the elbow, turn the hand 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 
downward. This rotation 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 flight difficult or impossible. The hingeing of the 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 finger 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 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 flezors. Extensors lie 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 
arrangement of an elastic cord in a bird’s wing. In fig. 27, 4 BC is adeep 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—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 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 the pteryla alaris. They are 
of two, main sorts: the flight-feathers proper, or long stiff quills, collectively called remiges 
(Lat. remesx, pl. remiges, rowers) ; and the smaller, weaker feathers overlying them, and hence 
called coverts, or tectrices (Lat. tectrix, pl. tectrices, coverers). To these may be added as a 
third distinct group the bastard quills, which constitute the 
- Alula, or Ala Spuria (Lat. alula, little wing, diminutive of ala, wing ; spuria, spurious, 
bastard). The ‘‘little wing” is simply the small parcel of feathers which grow upon the 
‘‘thumb” (see fig. 27,d2; 29,dandk; 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, 
