_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. [Illustration: FIG. 98. FIG. 99. FIG. 100. FIG. 101. FIGS. 98, 99, 100, and 101 show the muscles and elastic ligaments, and the arrangement of the primary and secondary feathers on the ventral aspects of the wing of the crested crane. The wing is in the extended condition. _y_ (fig. 98), Great pectoral muscle which depresses the wing. _a b_, Voluntary muscular fibres terminating in elastic band _k_. This band splits up into two portions (_k_, _m_). A somewhat similar band is seen at _j_. These three bands are united to, and act in conjunction with, the great fibro-elastic web _c_, to flex the forearm on the arm. The fibro-elastic web is more or less under the influence of the voluntary muscles (_a_, _b_).