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
metacarpo-phalangeal 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 diminishing 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_,[84] 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.[85] 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.
[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).
_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 “sub-coverts.” 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 secondaries. The tertiary, secondary, and
primary feathers increase in strength from within outwards, _i.e._ from
the body towards the extremity of the wing, and so of the several sets
of wing-coverts. This arrangement is necessary, because the strain on
the feathers during flight increases in proportion to their distance
from the trunk.
[Illustration: FIG. 98. 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).
[Illustration:
FIG. 102.--Shows the upward inclination of the body and the flexed
condition of the wings (_a b_, _e f_; _a´ b´_, _e´ f´_) in the
flight of the kingfisher. The body and wings when taken together
form a kite. Compare with fig. 59, p. 126, where the wings are fully
extended.]
_Flexion of the Wing necessary to the Flight of Birds._--Considerable
diversity of opinion exists as to whether birds do or do not flex their
wings in flight. The discrepancy is owing to the great difficulty
experienced in analysing animal movements, particularly when, as in
the case of the wings, they are consecutive and rapid. My own opinion
is, that the wings are flexed in flight, but that all wings are not
flexed to the same extent, and that what holds true of one wing does
not necessarily hold true of another. To see the flexing of the wing
properly, the observer should be either immediately above the bird or
directly beneath it. If the bird be contemplated from before, behind,
or from the side, the up and down strokes of the pinion distract the
attention and complicate the movement to such an extent as to render
the observation of little value. In watching rooks proceeding leisurely
against a slight breeze, I have over and over again satisfied myself
that the wings are flexed during the up stroke, the mere extension and
flexion, with very little of a down stroke, in such instances sufficing
for propulsion. I have also observed it in the pigeon in full flight,
and likewise in the starling, sparrow, and kingfisher (fig. 102,
p. 183).
It occurs principally at the wrist-joint, and gives to the wing the
peculiar quiver or tremor so apparent in rapid flight, and in young
birds at feeding-time. The object to be attained is manifest. By the
flexing of the wing in flight, the “_remiges_,” or rowing feathers,
are opened up or thrown out of position, and the air permitted
to escape--advantage being thus taken of the peculiar action of
the individual feathers and the higher degree of differentiation
perceptible in the wing of the bird as compared with that of the bat
and insect.
In order to corroborate the above opinion, I extended the wings of
several birds as in rapid flight, and fixed them in the outspread
position by lashing them to light unyielding reeds. In these
experiments the shoulder and elbow-joints were left quite free--the
wrist or carpal and the metacarpal joints only being bound. I took
care, moreover, to interfere as little as possible with the action of
the elastic ligament or alar membrane which, in ordinary circumstances,
recovers or flexes the wing, the reeds being attached for the most
part to the primary and secondary feathers. When the wings of a pigeon
were so tied up, the bird could not rise, although it made vigorous
efforts to do so. When dropped from the hand, it fell violently upon
the ground, notwithstanding the strenuous exertions which it made with
its pinions to save itself. When thrown into the air, it fluttered
energetically in its endeavours to reach the dove-cot, which was close
at hand; in every instance, however, it fell, more or less heavily, the
distance attained varying with the altitude to which it was projected.
Thinking that probably the novelty of the situation and the
strangeness of the appliances confused the bird, I allowed it to walk
about and to rest without removing the reeds. I repeated the experiment
at intervals, but with no better results. The same phenomena, I may
remark, were witnessed in the sparrow; so that I think there can be no
doubt that a certain degree of flexion in the wings is indispensable
to the flight of all birds--the amount varying according to the length
and form of the pinions, and being greatest in the short broad-winged
birds, as the partridge and kingfisher, less in those whose wings are
moderately long and narrow, as the gulls, and many of the oceanic
birds, and least in the heavy-bodied long and narrow-winged sailing or
gliding birds, the best example of which is the albatross. The degree
of flexion, moreover, varies according as the bird is rising, falling,
or progressing in a horizontal direction, it being greatest in the two
former, and least in the latter.
It is true that in insects, unless perhaps in those which fold or
close the wing during repose, no flexion of the pinion takes place in
flight; but this is no argument against this mode of diminishing the
wing-area during the up stroke where the joints exist; and it is more
than probable that when joints are present they are added to augment
the power of the wing during its active state, _i.e._ during flight,
quite as much as to assist in arranging the pinion on the back or
side of the body when the wing is passive and the animal is reposing.
The flexion of the wing is most obvious when the bird is exerting
itself, and may be detected in birds which skim or glide when they are
rising, or when they are vigorously flapping their wings to secure the
impetus necessary to the gliding movement. It is less marked at the
elbow-joint than at the wrist; and it may be stated generally that,
as flexion _decreases_, the twisting flail-like movement of the wing
at the shoulder _increases_, and _vice versâ_,--the great difference
between sailing birds and those which do not sail amounting to this,
that in the sailing birds the wing is worked from the shoulder by
being alternately rolled on and off the wind, as in insects; whereas,
in birds which do not glide, the spiral movement travels along the
arm as in bats, and manifests itself during flexion and extension in
the bending of the joints and in the rotation of the bones of the
wing on their axes. The spiral conformation of the pinions, to which
allusion has been so frequently made, is best seen in the heavy-bodied
birds, as the turkey, capercailzie, pheasant, and partridge; and here
also the concavo-convex form of the wing is most perceptible. In the
light-bodied, ample-winged birds, the amount of twisting is diminished,
and, as a result, the wing is more or less flattened, as in the
sea-gull (fig. 103).
[Illustration:
FIG. 103.--Shows the twisted levers or screws formed by the wings of
the gull. Compare with fig. 53, p. 107; with figs. 76, 77, and 78,
p. 147, and with figs. 82 and 83, p. 158.---_Original._]
_Consideration of the Forces which propel the Wings of Insects._--In
the thorax of insects the muscles are arranged in two principal sets
in the form of a cross--_i.e._ there is a powerful vertical set which
runs from above downwards, and a powerful antero-posterior set which
runs from before backwards. There are likewise a few slender muscles
which proceed in a more or less oblique direction. The antero-posterior
and vertical sets of muscles are quite distinct, as are likewise the
oblique muscles. Portions, however, of the vertical and oblique muscles
terminate at the root of the wing in jelly-looking points which greatly
resemble rudimentary tendons, so that I am inclined to believe that
the vertical and oblique muscles exercise a direct influence on the
movements of the wing. The shortening of the antero-posterior set
of muscles (indirectly assisted by the oblique ones) elevates the
dorsum of the thorax by causing its anterior extremity to approach
its posterior extremity, and by causing the thorax to bulge out or
expand laterally. This change in the thorax necessitates the descent
of the wing. The shortening of the vertical set (aided by the oblique
ones) has a precisely opposite effect, and necessitates its ascent.
While the wing is ascending and descending the oblique muscles cause
it to rotate on its long axis, the bipartite division of the wing at
its root, the spiral configuration of the joint, and the arrangement
of the elastic and other structures which connect the pinion with
the body, together with the resistance it experiences from the air,
conferring on it the various angles which characterize the down and
up strokes. The wing may therefore be said to be depressed by the
shortening of the antero-posterior set of muscles, aided by the oblique
muscles, and elevated by the shortening of the vertical and oblique
muscles, aided by the elastic ligaments, and the reaction of the air.
If we adopt this view we have a perfect physiological explanation
of the phenomenon, as we have a complete circle or cycle of motion,
the antero-posterior set of muscles shortening when the vertical set
of muscles are elongating, and _vice versâ_. This, I may add, is in
conformity with all other muscular arrangements, where we have what are
usually denominated extensors and flexors, pronators and supinators,
abductors and adductors, etc., but which, as I have already explained
(pp. 24 to 34), are simply the two halves of a circle of muscle and of
motion, an arrangement for securing diametrically opposite movements in
the travelling surfaces of all animals.
Chabrier’s account, which I subjoin, virtually supports this
hypothesis:--
“It is generally through the intervention of the proper motions of
the dorsum, which are very considerable during flight, that the wings
or the elytra are moved equally and simultaneously. Thus, when it is
elevated, it carries with it the internal side of the base of the wings
with which it is articulated, from which ensues the depression of the
external side of the wing; and when it approaches the sternal portion
of the trunk, the contrary takes place. During the depression of the
wings, the dorsum is curved from before backwards, or in such a manner
that its anterior extremity is brought nearer to its posterior, that
its middle is elevated, and its lateral portions removed further from
each other. The reverse takes place in the elevation of the wings; the
anterior extremity of the dorsum being removed to a greater distance
from the posterior, its middle being depressed, and its sides brought
nearer to each other. Thus its bending in one direction produces a
diminution of its curve in the direction normally opposed to it; and
by the alternations of this motion, assisted by other means, the body
is alternately compressed and dilated, and the wings are raised and
depressed by turns.”[86]
[86] Chabrier, as rendered by E. F. Bennett, F.L.S., etc.
In the _libellulæ_ or dragon-flies, the muscles are inserted into
the roots of the wings as in the bat and bird, the only difference
being that in the latter the muscles creep along the wings to their
extremities.
In all the wings which I have examined, whether in the insect, bat,
or bird, the wings are recovered, flexed, or drawn towards the body
by the action of elastic ligaments, these structures, by their mere
contraction, causing the wings, when fully extended and presenting
their maximum of surface, to resume their position of rest, and plane
of least resistance. The principal effort required in flight would
therefore seem to be made during extension and the down stroke. The
elastic ligaments are variously formed, and the amount of contraction
which they undergo is in all cases accurately adapted to the size and
form of the wings, and the rapidity with which they are worked--the
contraction being greatest in the short-winged and heavy-bodied insects
and birds, and least in the light-bodied and ample-winged ones,
particularly in such as skim or glide. The mechanical action of the
elastic ligaments, I need scarcely remark, insures a certain period
of repose to the wings at each stroke, and this is a point of some
importance, as showing that the lengthened and laborious flights of
insects and birds are not without their stated intervals of rest.