LOCOMOTION OF ANIMALS IN AIR. 191 



are associated circumstances which have been supposed to result in an impor- 

 tant diminution of the specific gravity of the animal ; but the difference in 

 weight between air at the ordinary temperature, and at 108, that of the 

 hottest bird, is insignificant, in proportion to the weight of the entire mass of 

 the bird. The chief relation between the energetic respiration of birds, and 

 their adaptation to flight, consists in the provision, through the former, for 

 the rapid decomposition and oxidation of the large muscles engaged in that 

 movement ; and also in its endowing the muscular fibre with an unwonted 

 degree of contractility. It is supposed that the air-cavities which occupy 

 spaces between the abdominal viscera, may assist respiration during flight, 

 when the sternum and ribs require to be comparatively fixed, and cannot be 

 used in the respiratory movements, which must then be performed chiefly by 

 the action of the abdominal muscles. 



The dorsal and lumbar regions of the spine, in birds, are strong, and com- 

 paratively immovable, so that the trunk forms a firm basis for the support of 

 the vibration of the wings ; the consolidation of the trunk being, as a rule, 

 proportioned to the powers of flight. The head is usually tapering, so as to 

 offer slight resistance to the air ; the neck is long, and can be extended or 

 drawn back, so as, amongst other purposes, to shift the position of the centre 

 of gravity in flight. Moreover, the length of the neck, and the conformation 

 of the jaws, convert the head into a prehensile organ ; and as the feet are 

 organized for standing, walking, perching, climbing, or swimming, or for 

 prehensile purposes, the wings are left free to be specially organized for aerial 

 locomotion. Turning to the special contrivances in these parts, we find, first, 

 a double bony arch between the shoulders, the one, posterior, formed by the 

 coracoid bones, resting on the sternum, and articulating with the scapula ; 

 and the other, anterior, formed by the merry-thought, or furcular bone, con- 

 sisting of the two clavicles united together in front. This compound arch 

 gives stability to the shoulder-joint, by resisting inward thrust. Secondly, 

 must be noticed, the vast surfaces of the sternum, which reaches backwards, 

 sometimes as far as to the pubes, with its deep projecting keel for the attach- 

 ment of the large, and the two smaller, pectoral muscles, which, often weigh- 

 ing as much as all the other muscles of the body, serve powerfully to depress, 

 and more gently to elevate, and slightly rotate the wings. Other remarkable 

 points of structure are the length of the humerus, radius, and ulma, the sim- 

 plicity and solidity of the tarsus, and the degradation of the phalanges to a 

 few flat supporting bones for the attachment of feathers. As regards the 

 joints, their movements are specially limited, those of the elbow and tarsus 

 performing simple fringe-like movements ; the latter joint being limited to ad- 

 duction and abduction, so as to have special firmness when extended ; and the 

 shoulder-joint moving merely in the directions of extension and flexion, and 

 in the upward and downward direction. The wings present many points of 

 special contrivance ; as, for example, the strong attachment of the stiff quill- 

 feathers to the periosteum of the bones of the fore-arm and hand ; the curved 

 form, from quill to tip, of these feathers ; their peculiar structure, the partial 

 hollowness of their stems, their stiff, horny exterior, and the light pithy char- 

 acter of their contents ; the close parallel arrangement and vertical depths of 

 the barbs ; the shortness and stiffness of the anterior barbs, as compared with 

 the greater length of the posterior barbs ; the secondary barbs, or barbules ; the 

 interlocking of the barbules of each feather ; the overlapping of the several 

 quill-feathers, the position of which is regulated by multitudes of small mus- 

 cular slips lying in the skin, there being sometimes four or five slips to each 

 quill-feather ; and, lastly, the stiffness of the anterior margin of the wing, as 

 compared with its hinder edge, and the marked concavity of the under sur- 

 face of the wing, as contrasted with the convexity of its upper surface. 



The rarity of the medium in which flight takes place, the slight mechanical 

 resistance it offers, and its feeble sustaining power, dependent on the extreme 

 difference between its specific gravity and that of the bird, necessitate special 

 contrivances, and an enormous relative amount of effort on the part of a flying 

 animal, to sustain or support its weight in the air ; but, on the other hand, the 

 resistance to motion through such a medium is so slight, that comparatively 

 little forward impulse is sufficient to propel it through the air. In accordance 



