vertical, horizontal, and intermediate movements.1 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.

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

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.

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).

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 (b), 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 (b), 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

increase in strength from within outwards, i.e. from the body. secondaries. The tertiary, secondary, and primary feathers

FIG. 99.

FIG. 100.

FIG. 101.

FIGS. 98, 99, 100, and 101 show the muscles and elastic ligaments, and the arrangement of the primary and secondary
feathers on the ventral aspects of the wing of the crested crane. The wing is in the extended condition.
y (fig. 98), Great pectoral muscle which depresses the wing.
a b, Voluntary muscular fibres terminating in elastic band k. This band splits up into two portions (k, m). A
somewhat similar band is seen at j. These three bands are united to, and act in conjunction with, the great fibro-
elastic web c, to flex the forearm on the arm. The fibro-elastic web is more or less under the influence of the vol-
untary muscles (a, b).
o, p, q, Musculo-fibro elastic ligament, which envelopes the roots of the primary and secondary feathers, and
forms a symmetrical network of great strength and beauty, its component parts being arranged in such a manner
as to envelope the root of each individual feather. The network in question supports the feathers, and limits their
peculiar valvular action. It is enlarged at figs. 99 and 101, and consists of three longitudinal bands, rs, tu, v, w.
Between these bands two oblique bands, g and h, run; the oblique bands occurring between every two feathers.
The marginal longitudinal band (v, w) splits up into two processes, one of which curves round the root of each
feather (x) in a direction from right to left (c, b, a), the other in a direction from left to right (d, e, f). These pro-
cesses are also seen at m, n of fig. 100.-Original.

the wing of the bat and bird. The principle is, however, in both cases the same, the loops ultimately terminating in a waved track. The impulse is communicated to the insect wing at the heavy parts of the loops a b c d e f g h i j k l m n of fig. 71; the waved tracks being indicated at p q r s t of the same figure. The recoil obtained from the air is represented at corresponding letters of fig. 72, the body of the

insect being carried along the curve indicated by the dotted line. The impulse is communicated to the wing of the bat and bird at the heavy part of the loops a b c d e f g h i j k l m n o of fig. 73, the waved track being indicated at p s t u v w of this figure. When the horizontal speed attained is high, the wing is successively and rapidly brought into contact with innumerable columns of undisturbed air. It, consequently, is a matter of indifference whether the wing is carried at a high speed against undisturbed air, or whether it operates upon air

travelling at a high speed (as, e.g. the artificial currents produced by the rapidly reciprocating action of the wing). The result is the same in both cases, inasmuch as a certain quantity of air is worked up under the wing, and the necessary degree of support and progression extracted from it. It is, therefore, quite correct to state, that as the horizontal speed of the body increases, the reciprocating action of the wing decreases; and vice versa. In fact the reciprocating and nonreciprocating action of the wing in such cases is purely a matter of speed. If the travel of the wing is greater than the horizontal travel of the body, then the figure-of-8 and the reciprocating power of the wing will be more or less perfectly developed, according to circumstances. If, however, the

horizontal travel of the body is greater than that of the wing, then it follows that no figure-of-8 will be described by the wing; that the wing will not reciprocate to any marked

bd

j

FIG. 74.

FIG. 75.

Figs. 74 and 75 show the more or less perpendicular direction of the stroke of the wing in the flight of the bird (gull)-how the wing is gradually extended as it is elevated (e f g of fig. 74)-how it descends as a long lever until it assumes the position indicated by h of fig. 75-how it is flexed towards the termination of the down stroke, as shown at hij of fig. 75, to convert it into a short lever (a b), and prepare it for making the up stroke. The difference in the length of the wing during flexion and extension is indicated by the short and long levers a b and c d of fig. 75. The sudden conversion of the wing from a long into a short lever at the end of the down stroke is of great importance, as it robs the wing of its momentum, and prepares it for reversing its movements. Compare with figs. 82 and 83, p. 158.-Original.

extent; and that the organ will describe a waved track, the curves of which will become less and less abrupt, i.e. longer and longer in proportion to the speed attained. The more

K