[84] The furcula are usually united to the anterior part of the sternum by ligament; but in birds of powerful flight, where the wings are habitually extended for gliding and sailing, as in the frigate-bird, the union is osseous in its nature. “In the frigate-bird the furcula are likewise anchylosed with the coracoid bones.”--Comp. Anat. and Phys. of Vertebrates, by Prof. Owen, vol. ii. p. 66. [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the primary and secondary feathers form, shall be screwed into and down upon the air in extension, and unscrewed or withdrawn from it during flexion.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).