[85] “The os humeri, or bone of the arm, is articulated by a small rounded surface to a corresponding cavity formed between the coracoid bone and the scapula, in such a manner as to allow great freedom of motion.”--Macgillivray’s Brit. Birds, vol. i. p. 33.
“The arm is articulated to the trunk by a ball-and-socket joint, permitting all the freedom of motion necessary for flight.”--Cyc. of Anat. and Phys., vol. iii. p. 424.
As a result of this disposition of the articular surfaces, the wing is shot out or extended and retracted or flexed in a variable plane, the bones composing the wing, particularly those of the forearm, rotating on their axes during either movement.
This secondary action, or the revolving of the component bones upon their own axes, is of the greatest importance in the movements of the wing, as it communicates to the hand and forearm, and consequently to the primary and secondary feathers which they bear, the precise angles necessary for flight; it in fact insures that the wing, and the curtain or fringe of the wing which the primary and secondary feathers form, shall be screwed into and down upon the air in extension, and unscrewed or withdrawn from it during flexion. The wing of the bird may therefore be compared to a huge gimlet or auger; the axis of the gimlet representing the bones of the wing, the flanges or spiral thread of the gimlet the primary and secondary feathers (fig. 63, p. 138, and fig. 97, p. 176).
_Traces of Design in the Wing of the Bird--the arrangement of the Primary, Secondary, and Tertiary Feathers, etc._--There are few things in nature more admirably constructed than the wing of the bird, and perhaps none where design can be more readily traced. Its great strength and extreme lightness, the manner in which it closes up or folds during flexion, and opens out or expands during extension, as well as the manner in which the feathers are strung together and overlap each other in divers directions to produce at one time a solid resisting surface, and at another an interrupted and comparatively non-resisting one, present a degree of fitness to which the mind must necessarily revert with pleasure. If the feathers of the wing only are contemplated, they may be conveniently divided into three sets of three each (on both sides of the wing)--an upper or dorsal set (fig. 61, _d_, _e_, _f_, p. 136), a lower or ventral set (_c_, _a_, _b_), and one which is intermediate. This division is intended to refer the feathers to the bones of the arm, forearm, and hand, but is more or less arbitrary in its nature. The lower set or tier consists of the primary (_b_), secondary (_a_), and tertiary (_c_) feathers, strung together by fibrous structures in such a way that they move in an outward or inward direction, or turn upon their axes, at precisely the same instant of time,--the middle and upper sets of feathers, which overlap the primary, secondary, and tertiary ones, constituting what are called the “coverts” and “sub-coverts.” The primary or rowing feathers are the longest and strongest (_b_), the secondaries (_a_) next, and the tertiaries third (_c_). The tertiaries, however, are occasionally longer than the secondaries. The tertiary, secondary, and primary feathers increase in strength from within outwards, _i.e._ from the body towards the extremity of the wing, and so of the several sets of wing-coverts. This arrangement is necessary, because the strain on the feathers during flight increases in proportion to their distance from the trunk.