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As a consequence, the long axis of the shoulder-joint is nearly at right angles to that of the elbow-joint. Similar remarks may be made regarding the radius (the principal bone of the forearm) (d), and the second and third metacarpal bones with their phalanges (e f), all of which are greatly elongated, and give strength and rigidity to the anterior or thick margin of the wing. The articular surfaces of the bones alluded to, as well as of the other bones of the hand, are spirally disposed with reference to each other, the long axes of the joints intersecting at nearly right angles. The object of this arrangement is particularly evident when the wing of the living bat, or of one recently dead, is extended and flexed
as in Hing bat, or of is particularly right anger, the longd, are
In the flexed state the wing is greatly reduced in size, its under surface being nearly parallel with the plane of progression. When the wing is fully extended its under surface makes a certain angle with the horizon, the wing being then in a position to give the down stroke, which is delivered downwards and forwards, as in the insect. When extension takes place the elbow-joint is depressed and carried forwards, the wrist elevated and carried backwards, the metacarpophalangeal joints lowered and inclined forwards, and the distal phalangeal joints slightly raised and carried backwards. The movement of the bat's wing in extension is consequently a spiral one, the spiral running alternately from below upwards and forwards, and from above downwards and backwards (compare with fig. 79, p. 147). As the bones of the arm, forearm, and hand rotate on their axes during the extensile act, it follows that the posterior or thin margin of the wing is rotated in a downward direction (the anterior or thick one being rotated in an opposite direction) until the wing makes an angle of something like 30° with the horizon, which, as I have already endeavoured to show, is the greatest angle made by the wing in flight. The action of the bat's wing at the shoulder is particularly free, partly because the shoulder-joint is universal in its nature, and partly because the scapula participates in the movements of this region. The freedom of action referred to enables the bat not only to rotate and twist its wing as a whole, with a view to dimin
ishing and increasing the angle which its under surface makes with the horizon, but to elevate and depress the wing, and move it in a forward and backward direction. The rotatory or twisting movement of the wing is an essential feature in flight, as it enables the bat (and this holds true also of the insect and bird) to balance itself with the utmost exactitude, and to change its position and centre of gravity with marvellous dexterity. The movements of the shoulder-joint are restrained within certain limits by a system of check-ligaments, and by the coracoid and acromian processes of the scapula. The wing is recovered or flexed by the action of elastic ligaments which extend between the shoulder, elbow, and wrist. Certain elastic and fibrous structures situated between the fingers and in the substance of the wing generally take part in flexion. The bat flies with great ease and for lengthened periods. Its flight is remarkable for its softness, in which respect it surpasses the owl and the other nocturnal birds. The action of the wing of the bat, and the movements of its component bones, are essentially the same as in the bird.
THE WINGS OF BIRDS. The Bones of the Wing of the Bird—their Articular Surfaces, Movements, etc.— The humerus, or arm-bone of the wing, is supported by three of the trunk-bones, viz. the scapula or shoulder-blade, the clavicle or collar-bone, also called the furculum, and the coracoid bone,—these three converging to form a point d'appui, or centre of support for the head of the humerus, which is received in facettes or depressions situated on the scapula and coracoid. In order that the wing may have an almost unlimited range of motion, and be wielded after the manner of a flail, it is articulated to the trunk by a somewhat lax universal joint, which permits vertical, horizontal, and intermediate movements. The long axis of the joint is directed vertically; the joint itself somewhat backwards. It is otherwise with the elbow-joint, which is turned forwards, and has its long axis directed horizontally, from the fact that the humerus is twisted upon itself to the extent of nearly a quarter of a turn. The elbow-joint is decidedly spiral in its nature, its long axis intersecting that of the shoulder-joint at nearly right angles. The humerus articulates at the elbow with two bones, the radius and the ulna, the former of which is pushed from the humerus, while the other is drawn towards it during extension, the reverse occurring during flexion. Both bones, moreover, while those movements are taking place, revolve to a greater or less extent upon their own axes. The bones of the forearm articulate at the wrist with the carpal bones, which being spirally arranged, and placed obliquely between them and the metacarpal bones, transmit the motions to the latter in a curved direction. The long axis of the wrist-joint is, as nearly as may be, at right angles to that of the elbow-joint, and more or less parallel with that of the shoulder. The metacarpal or hand-bones, and the phalanges or finger-bones are more or less fused together, the better to support the great primary feathers, on the efficiency of which flight mainly depends. They are articulated to each other by double hinge-joints, the long axes of which are nearly at right angles to each other.
1 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.
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).
1 " 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.
Traces of Design in the Wing of the Bird—the arrangement of the Primary, Secondary, and Tertiary Feathers, etc.—There are few things in nature more admirably constructed than the wing of the bird, and perhaps none where design can be more readily traced. Its great strength and extreme lightness, the manner in which it closes up or folds during flexion, and opens out or expands during extension, as well as the manner in which the feathers are strung together and overlap each other in divers directions to produce at one time a solid resisting surface, and at another an interrupted and comparatively non-resisting one, present a degree of fitness to which the mind must necessarily revert with pleasure. If the feathers of the wing only are contemplated, they may be conveniently divided into three sets of three each (on both sides of the wing)—an upper or dorsal set (fig. 61, d, e, f, p. 136), a lower or ventral set (c, a, b), and one which is intermediate. This division is intended to refer the feathers to the bones of the arm, forearm, and hand, but is more or less arbitrary in its nature. The lower set or tier consists of the primary (6), secondary (a), and tertiary (c) feathers, strung together by fibrous structures in such a way that they move in an outward or inward direction, or turn upon their axes, at precisely the same instant of time,—the middle and upper sets of feathers, which overlap the primary, secondary, and tertiary ones, constituting what are called the “coverts” and “subcoverts." The primary or rowing feathers are the longest and strongest (6), the secondaries (a) next, and the tertiaries third (C). The tertiaries, however, are occasionally longer than the
of wing-coverts. This arrangement is necessary, because the towards the extremity of the wing, and so of the several sets
secondaries. increase in strength from within outwards, i.e. from the body
The tertiary, secondary, and primary feathers
y (fig. 98), Great pectoral muscle which depresses the wing.
ab, Voluntary muscular fibres terminating in elastic band k. This band splits up into two portions (k, m). A
0, p, q, Musculo-fibro-elastic ligament, which envelopes the roots of the primary and secondary feathers, and
ength and beauty, its component parts being arranged in such a manner