ANIMALIA VERTEBRATA— VERTEBRATED ANIMALS. 
27 
clrmbing is assisted by a thumb almost always directed backwards, and forming a kind 
of heel very powerful in its operation. The animals just mentioned, ns well as most 
of the Quadrumana, are assisted in climbing by their tail, which is capable of seizing 
bodies almost as powerfully a.s a hand. For this purpose, additional force is imparted 
to the common muscles of tho tail. 
The sharp and hooked claws found in animals of the Cat kind, enable them to climb 
with facility. Their nails are retained between the toes, ^vith the points elevated by 
two elastic ligaments, altogether independent of the will of the animal. When they 
wish to use the nails, either for tearing their prey or seizing moveable objects, tho 
nails are protruded by the muscle, which moves the last phalanx of the toe on the 
preceding one. 
In the Sloth, the ligaments arc differently disposed, and tho nails being naturally 
inflected, must be raised when tho animal wishes to use them. As the toes arc of an 
inconvenient form, being composed of two phalanges, one of which i^ very short, and 
the other entirely covered with tho nail, while the metacarpal bones are ossified to- 
gether, and immoveable, tho Sloths perform their movements with constraint and 
difficulty. 
The climbing birds arc enabled by their claws to fix themselves to tho inequalities 
in the bark of trees; and they perform this action by the assistance chiefly of the 
hindcr-tocs, which are used in supporting and preventing them from falling. Tho 
greater number of these genera have two hinder-toes, but tho Creepers and Nut- 
hatches have only one. The Woodpeckers, as well n.s the Creepers, arc assisted in 
climbing by tho quids of the tail, which arc stiff and capable of being fixed firmly into 
the inequalhics of surfaces. 
Some liirds can raise the food to their mouth by means of tho one foot, while they 
stand upon the other. Parrots have their toes conveniently disposed for this purpose, 
and also the Owls. Without this pronsion, the latter would frequently fall whenever 
they attempted to peck, in consequence of the great weight of the lie.ad, and the cor- 
responding elevation of their centre of gravity. But as most birds require both feet 
in order to stand firmly, they seldom use them for lioldhig .substances except during 
flight, when tho feet are disengaged. The Cormorant and Pelican will sometimes 
sw’im witli the one foot and carry some substance In tho other; an<l the Wading Birds 
frequently stand for a long time on the one leg, which they arc enabled to do ivithout 
much difficulty, while they hold a stone, or some heavy substance, in tlic other as a 
counterpoise. 
Among the Reptiles, tho Chameleon seems to posse.>s adaptations for climbing, 
enjoyed by no other animals except the (Juadrumana. With a prehensile tail, and 
hands resoinbling forceps, he exhibits a degree of agility unusual with the Reptiles. 
Flying and swimming are leaps taking place in fluids, and the motion is produced by 
the resistance which the fluid makes to tho surface of tho wings or fins, when moved 
bj the animal with great rapidity. Leaping, however, takes place on a fixed sur- 
face, possessing the power of resi.stance, from its magnitude and firmness. If wo sup- 
pose tho ground to be either soft or clastic, leaping may still be performed; but there 
arises a diminution in the velocity of the leap, proportional to the resiliency of the 
support. It is necessary, therefore, that tho moving power dioul.l be increased in pro- 
portion, to produce an equal momentum by the extent of the vibrating surfaces, and 
by the rapuUty of their vibrations. The velocity with which the wings or fins must 
be used, depends on the rarity of the metlium in which they move. It is less in water, 
greater in air near the earth's surfuee, and increases as the luiimal ascends into the 
higher regions of the atmosphere. Birds cannot therefore fly above a certain height, 
dependent on tho strength of their muscles; and they are capable of rising to a greater 
height when tho barometer is high than when it is low. Tho muscles moving tho 
wings and fins, but especially the former, require a force vastly superior to that neces- 
sary to produce a simple leap upon a firm surface. 
As a flying or swimmiug body is entirely surrounded by the medium in which it is 
placed, it experiences an equal resistance in front as well as from behind on striking the 
fluid. An animal would be incapable of advancing, if it did not possess the power 
of greatly diminishing tho surface of its wing or fin, immediately after having struck 
the fluid. 
Flying and swimming arc sometimes performed by the same animal ; but the former 
is executed most perfectly by Birds, and tlie latter by Fishes. Some birds never fly. 
The Ostriches, Auks, and Penguins, arc possessed of small rudimcntnl wings, but 
they seem to have tliora but fur the purpose of conforming more nearly in external 
'■osomblanco to other birds. Some Mammalia can fly, although they have no wings. 
I’hc Bats possess a membraneous expansion, extending to the feet and to both sides 
the tail, hut supported chiefly by the humerus, the forc-arra, and the four fingers, 
'rheso bones being greatly clongat(ul, serve to support the membraneous surface, which 
is of firmness and extent sufficient to riiisc and maintain these animals in the air, when 
^ctod upon by the powerful musclcsj of tho breast. 
The first motion of a bird in atiempling to fly is an ordinary leap with the feet, 
Accordin'dv, those birds having the wings very largo and the feet very short, as wo 
observe in the Booby and the Marten-s, commence their flight with great ditliculty, as 
they cannot leap sufficiently high to obtain tho space nocc.ssary for tho extension of 
their wings. 
In flying, the resistance of the nir is in proportion to the mass struck at one time, 
this account the short-winged birds must repeal tlieir vibrations very frequently; 
they are therefore soon fatigued, and unable to continue thoir flight for a long time, 
'^'ben a birri attempts to fly, the humerus U first elevated, and then the entire wing, 
''Inch had hitherto remained folded; while, at the same time, it is extended in a hcri- 
^•onlal direction liy means of tho fore-arm and the last di\ision of the wing, corres- 
ponding to the foot of Quadrupeds. After the wing has thus acquired nil the super- 
bcial extent which it is capable of attaining, tho bird suddenly depresaos it, until it 
forms an acute angle with the vertical plane of the body, Bubtending tho ground. 
The air resists this motion, which is performed with groat rapidity, and produces a 
^■paction of part of the force upon the body of the bird. Its centre of gravity then 
fise.s in the same manner as in other leaps. The wings may bo compared to a lever, 
®f which the pectoral mu.sclca arc the moving power, and the body of the bird the 
"eight; while the air rcBisting, by its inertia, the action of the expanded wing, is the 
fulcrum. The impulse being once given, the bird refolds the wings by bending their 
joints, elevates them again, and gives a new stroke to the air. The force of gravi- 
tation diminishes the velocity which the body thus acquires in ascending, in the 
same manner a.s it affects every otlicr projectile. There consequently occurs a mo- 
ment in which tho bird neither ascends nor descends. If it seize this moment pre- 
cisely, and give a new stroke to the .air, Its body will acquire a new velocity, which 
will carry it as. far as that obtained hy the first impulse; it will then rise in a con- 
tinuous manner, and with a uniform velocity. If the second impulse of the wing 
commence before the impulse arising from tho first has been lost, the bird will 
ascend with an accelerated motion; or, if tho bird do not vibrate its wings at tho 
exact moment when the ascending velocity is lost, it will begin to descend with great 
rapidity. Yet tho bird may keep itself always at the same height by a series of 
equal vibrations, if a point be seized in tho fall so situate that the velocity which 
would have been acquired in descending, and the small space there would have been 
to reascend, reciprocally compensate and destroy each other ; but if it once allow it- 
self to descend to the point from which it departed, it can only rise by a much stronger 
exertion of the wings. 
In descending, the bird has only to repeat less frequently the vibrations of its wing.?. 
The darting of Birds of Prey is occasioned by their suppressing tho vibrations of tlie 
wings altogether, when the bird, being continually acted upon by the force of gi-avity, 
falls with an accelerated velocity. Wlien a bird in descending suddenly, breaks its 
fall, it is called a recover. Tho resistance of the air then increases in proportion to 
the square of the velocity, and the bird rises again. 
The preceding remarks apply only to flight when made in a vertical direction, either 
in ascending or descending. The Quails, Larks, and other birds which arc observed 
to fly upwards in a straight line, have the wings placed entirely horizontal ; but in the 
grc.'iter number of birds the wing is iuclined, and turned backwards. This inclina- 
tion may be further increased at the will of the bird. It is greatly assisted by the 
length of the quills, which enable the resistance of the air to act on their extremities 
with a mechanical advantage, while they arc the more elevated by it, from their fixed 
points being placed at the kisc. By this arrangement, birds are enabled to advance 
in a horizontal, as well as in a vertical direction, by a scries of oblique curves. 
The oblique motion upwards in flight may be resolved into two other motions, the 
one in a horizontal direction, independent of the force of g;!-avity, and the other in a 
vertical direction, opposite to that power. In flying horizontally, the bird rises in 
an oblique direction, and does not make a second movement of the wings until it is on 
the point of dcsccuditig below the line of the intended dirocUon of its flight. These par- 
tial movements, therefore, will not lake plaCe in a straight lino, but in a series of curves, 
nearly approaching to the straight line, and in which tl;e horizontal motion greatly 
prevails over the vertical. In ascending obliquely, tho wings move with greater ra- 
pidity; ill descending obliquely, their vibrations arc less frequent; and both of these 
motions are performed by a scries of curves. 
Some birds cannot sufficiently diininisli the obliquity of their wings, and with them 
the horizontal motion is always very considerable. "When tho wind blows strongly 
in the same direction with the flight of these birds, they arc carried to a very consi- 
derable distance. out of their intended ‘path. For this rcar.mi, those birds of prey 
which the falconers term nohhi are under tho necessity of flying against tho wind 
when they wish to rise perpendicularly upwards. The anterior quills of their wings 
being extiemely long, and their extremities pressing closely upon each other, the 
horizontal motion with them is proportionally greater than that of other birds. On 
the conlrary, w ith the ignohh birds the quills of tbe wings oi’O separated at their 
extremities, and permit the air to pass between them, which renders thu wing loss ca- 
pable of assuming the oblique position. 
De vialioiis from tho rectilineal path to the rigid and left, arc chiefly occasioned 
by tho unequal vibrations of the wings. When the left wing vibrates tho more fre- 
quently, or with the greater force, the left side moves mere rapidly, and the body 
necessarily turns to the right. Tlie rapid movement or groalor force of (he right 
wing produces a corresponding turn to the left. The difliculty of suddenly turning 
increases with the velocity of fliglit; and this arises partly from llie inertia of the 
body, which perseveres in its rectilineal course, and partly from the increased diffi- 
culty of making tho one wing to surpass the other in its velocity. For this reason, 
birds of rapid flight make great circuits in turning. Some will turn on the side, 
and make use of the tail as a rudder, when they wish to change the horizontal direc- 
tion. 
The tail of birds, when expanded, serves to sustain the hinder part of their body. 
When depressed during flight, the resistance of the air forms an obstacle wlildi raises 
tho hinder part of the body and depresses tho anterior; upon turning the tail upwards, 
a contrary effect is produced. 
As all Birds do not fly, so all Fishes do not swim, yet there ai*e many Birds which 
perform two motions, resembling those more particularly belonging to each clar-s. 
Aquatic Birds arc improperly said to swim; and tlie poet, describing tho swan sailing 
with tho breeze,” is perhaps not aware that Ida term is philosophically correct.' 
The bodies of Aquatic Birds are naturally lighter than water, from the great 
quantity of air which they contain within the abdomen, and from the feathers, which 
arc oily and impervious to moisture. They precisely resemble a boat, and have no 
further occasion for tho feet than as oars for moving forwards. As tho fore part of the 
body is completely sustained by the water, the legs are situate farther backw.irds than 
those of other birds, that thoir effect may be more lUrcct, as tlieir presence farther 
in advance would be superfluous. Tlie legs and thighs arc short, that ihc resistance 
of tho water to tho muscles may be as slight as possible. Tho tarsus, or instep, is 
compressed for cutting the water, while tho toes ai’c very much e.xpanded, or even 
united by a menibrauc, in order to form an oar of greater breadth, and capable of 
acting upon a greater surface of water; and when the bird inflects its foot in order to 
give a new stroke to tlie water, it closes the toes upon each other to diinhiish the 
resiatanco of the fluid. 
In diving, those birds arc obliged to compress tho breast with much force, in order 
to expel the air which it contains. Tho neck is then elongated, that the body may 
acquire an inclinatior forwards, while, by striking the feet upwards, it is forced 
