Wing, flexion of, necessary to flight of birds, 183 Wing flexed and partly elevated by action of elastic ligaments, 191 Wing, power of, to what owing, 194 Wing, effective stroke of, why delivered downwards and forwards, 195 Wing acts as an elevator, propeller, and sustainer both during extension and flexion, 197 Wings, points wherein the screws formed by, differ from those in ordinary use, 151 Wings at all times thoroughly under control, 154 Wings of insects, consideration of forces which propel, 186 Wings of bats and birds, consideration of forces which propel, 189 [Illustration] PRINTED BY T. AND A. CONSTABLE, PRINTERS TO HER MAJESTY, AT THE EDINBURGH UNIVERSITY PRESS. End of the Project Gutenberg EBook of Animal Locomotion, by J. Bell Pettigrew