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 voluntary 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, r s, t u, 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 processes are also seen at m, n of fig. 100.—Original.
The manner in which the roots of the primary, secondary, and tertiary feathers are geared to each other in order to rotate in one direction in flexion, and in another and opposite direction in extension, is shown at figs. 98, 99, 100, and 101, p. 181. In flexion the feathers open up and permit the air to pass between them. In extension they flap together and render the wing as air-tight as that of either the insect or bat. The primary, secondary, and tertiary feathers have consequently a valvular action.
The Wing of the Bird not always opened up to the same extent in the Up Stroke.—The elaborate arrangements and adaptations for increasing the area of the wing, and making it impervious to air during the down stroke, and for decreasing the area and opening up the wing during the up stroke, although necessary to the flight of the heavy-bodied, short-winged birds, as the grouse, partridge, and pheasant, are by no means indispensable to the flight of the long-winged oceanic birds, unless when in the act of rising from a level surface; neither do the short-winged heavy birds require to fold and open up the wing during the up stroke to the same extent in all cases, less folding and opening up being required when the birds fly against a breeze, and when they have got fairly under weigh. All the oceanic birds, even the albatross, require to fold and flap their wings vigorously when they rise from the surface of the water. When, however, they have acquired a certain degree of momentum, and are travelling at a tolerable horizontal speed, they can in a great measure dispense with the opening up of the wing during the up stroke—nay, more, they can in many instances dispense even with flapping. This is particularly the case with the albatross, which (if a tolerably stiff breeze be blowing) can sail about for an hour at a time without once flapping its wings. In this case the wing is wielded in one piece like the insect wing, the bird simply screwing and unscrewing the pinion on and off the wind, and exercising a restraining influence—the breeze doing the principal part of the work. In the bat the wing is jointed as in the bird, and folded during the up stroke. As, however, the bat’s wing, as has been already stated, is covered by a continuous and more or less elastic membrane, it follows that it cannot be opened up to admit of the air passing through it during the up stroke. Flight in the bat is therefore secured by alternately diminishing and increasing the area of the wing during the up and down strokes—the wing rotating upon its root and along its anterior margin, and presenting a variety of kite-like surfaces, during its ascent and descent, precisely as in the bird (fig. [80], p. 149, and fig. [83], p. 158).