The twisting is in a great measure owing to the manner in which the bones of the wing are twisted upon themselves, and the spiral nature of their articular surfaces; the long axes of the joints always intersecting each other at nearly right angles. 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 of the wing rotating in the direction of their length 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 membrane or feathers which they bear, the precise angles necessary for flight. It, in fact, insures that the wing, and the curtain, sail, or fringe of the wing shall be screwed into and down upon the air in extension, and unscrewed or withdrawn from it during flexion. The wing of the bat and 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 frenum or sail (figs. 95 and 97). [Illustration: FIG. 96. FIG. 96.--Right wing of the Red-legged Partridge (_Perdix rubra_), dorsal aspect. Shows extreme example of short rounded wing; contrast with the wing of the albatross (fig. 62, p. 137), which furnishes an extreme example of the long ribbon-shaped wing; _d e f_, anterior margin; _b a c_, posterior ditto, consisting of primary (_b_), secondary (_a_), and tertiary (_c_) feathers, with their respective coverts and subcoverts; the whole overlapping and mutually supporting each other. This wing, like the kestrel’s (fig. 61, p. 136), was drawn from a specimen held against the light, the object being to display the mutual relation of the feathers to each other, and how the feathers overlap.--_Original._] [Illustration: FIG. 97. FIG. 97.--Right wing of Red-legged Partridge (_Perdix rubra_), seen from behind and from beneath, as in the beetle (fig. 93) and bat (fig. 95). The same lettering and explanation does for all three.--_Original._] THE WINGS OF BATS.