MECHANISM FOR FLIGHT. The wings of verte brates are modifications of the fore limbs; these in bats and pterodactyls support, or supported, a thin membrane, while in birds the bones sup port stiff feathers. In bats the thumb is free, the bones corresponding to the fingers of man greatly elongated. The membrane is not only spread between these, but continued from the little finger to the ankle-joint, while there is, in addition, a greater or less amount of membrane sustained between the legs and tail, which helps support the body during flight. In pterodaetyls the thumb was absent, the succeeding three fin gers very small, the fifth or little finger enor mously elongated. This supported the wing-mem brane in front, while the hinder margin reached to the ankle-joint, or possibly in sonic species only to the hip. In birds the bones of the hand are reduced in number, and some of them are fused, to furnish a point of attachment for the feathers. In bats the strain of the wing is sustained by the greatly developed eollar-bone, or clavicle, which runs from the shoulder-blade to the front end of the breast-bone, and furnishes a firm brace for the shoulder-joint. The clavicle is absent in pterodactyls, and the shoulder-joint is formed by the shoulder-blade and a bone termed the coracoid, the two uniting to form a V, with the socket for the wing at the apex. One arm of the V rests against the backbone, the other against the front part of the breast-bone, thus forming an extremely firm brace for the wing. In birds (see BIRD) the shoulder-joint is also formed by the shoulder-blade and eo•acoid, but the shoulder-blade runs backward, parallel with the body, the coraeoid being attached to the front of the breast-bone. This forms the most impor tant support of the wing, for while the collar bones are present in the shape of the wishbone, they are by no means so important as is often supposed. and, as in many small birds, may be of no use at all. The eollar-bones are strong,
and serve to brace apart the shoulders in birds like hawks and eagles. whose vigorous movements demand an extra strong support for the wing; but in humming-birds and swifts the wishbone is feeble, although these birds have extraordinary powers of flight.
In insects the wings may be two or four in number, most commonly the latter, and, unlike the wings of vertebrates, they are not modified limbs, but specially developed parts believed by some to have been derived from the breathing organs of primitive aquatic forms. They are formed of thin, rather stiff membrane, braced by little ridges, termed 'nerves,' which are strongest on the front edge of the wing. This strengthen ing of the front margin is as absolutely essen tial as that the posterior portion should be flexible; any injury to this part of the wings destroys the power of flight, and flight is also impossible when the wing is made rigid. But a considerable part of the hinder portion of the wing may be removed without seriously impair ing the ability to fly.
Fuoicr-Muset.Es. As in insects the hard parts are outside of the muscles, the wings are attached to the exterior of the body. and are not sup ported by an internal skeleton. The wings of vertebrates are moved by powerful muscles at tached to the breast-bone. and the depth of this portion of the skeleton is a rough measure of the muscular power used in flying, In such flight less birds as penguins, the breast-bone is deeply keeled beeause the wings are used for swimming and moved by the muscles employed by other birds in flight. Among dragon-flies the muscles are fastened directly to the base of the wings, much as in vertebrates; but in insects generally the wings are not only moved by these direct muscles, but by powerful cross-muscles, which alter the curvature of the body and by so doing raise or depress the wings.