ANIMALIA VERTEBRATA— VERTEBRATED ANIMALS. 



27 



elimbing is assisted by a thumb almost always directed backwards, and forming a kind 

 of heel very powerful in its operation. The animals just mentioned, as well as most 

 of the Quadrumana, are assisted in chmbing by their tail, which is capable of seizing 

 bodies almost as powerfully as a hand. For this purpose, additional force is imparted 

 to the common muscles of the 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, with 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, the 

 nails are protruded by the muscle, ^hich moves the last phalanx of the toe on the 

 preceding one. 



In the Sloth, the ligaments are differently disposed, and the nails being naturally 

 mflected, must be raised when the animal wishes to use them. As the toes are of an 

 inconvenient form, being composed of two phalanges, one of which is very short, and 

 the other entirely covered with the nail, while the metacarpal bones are ossified to- 

 pether, and immoveable, the Sloths perform their movements with constraint and 

 difficulty. 



The climbing birds are enabled by their claws to fix themselves to the inequalities 

 in the bark of trees ; and they perform this action by the assistance chiefly of the 

 hmder-toes, which are used in supporting and preventing them from falling. The 

 greater number of these genera have two hinder-toes, but the Creepers and Nut- 

 hatches have only one. The Woodpeckers, as well as the Creepers, are assisted in 

 climbing by the quills of the tail, which are stiff and capable of being fixed firmly into 

 the inequalities of surfaces. 



Some birds can raise the food to their mouth by means of the one foot, while they 

 stand upon the other. Parrots have their toes conveniently disposed for this purpose, 

 and also the Owls. Without this provision, the latter would frequently fall whenever 

 they attempted to peck, in consequence of the great weight of the head, 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 holding substances except during 

 flight, when the feet are disengaged. The Cormorant and Pelican will sometimes 

 swim with the one foot and carry some substance in the other ; and the Wading Birds 

 frequently stand for a long time on the one leg, which they are enabled to do without 

 much difficulty, while they hold a stone, or somo .heavy substance, in the other as a 

 counterpoise. 



Among the Reptiles, the Chameleon seems to possess adaptations for climbing, 

 enjoyed by no other animals except the Quadrumana. With a prehensile tail, and 

 hands resembling 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 the surface of the wings or fins, when moved 

 by the animal with great rapidity. Leaping, however, takes place on a fixed sur- 

 face, possessing the power of resistance, from its magnitude and firmness. If we sup- 

 pose the ground to bo cither soft or elastic, leaping may still be performed; but thoro 

 arises a diminution in the velocity of the leap, proportional to the resiliency of the 

 support. It is necessary, therefore, that the moving power should be increased in pro- 

 portion, to produce an equal momentum by the extent of the vibrating surfaces, and 

 ^y the rapidity of their vibrations. The velocity with which the wings or fins must 

 be used, depends on the rarity of the medium in which they move. It is less in water, 

 greater in air near the earth's surface, and increases as the animal ascends into the 

 higher regions of the atmosphere. Birds cannot therefore fly above a certain height, 

 dependent on the strength of their muscles ; and they are capable of rising to a greater 

 height when the barometer is high than when it is low. The 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 swimming 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 the surface of its wing or fin, immediately after having struck 

 the fluid. 



Flying and swimming are sometimes performed by the same animal; but the former 

 is executed most perfectly by Birds, and the latter by Fishes. Some birds never fly. 

 The Ostriches, Auks, and Penguins, arc possessed of small rudimental wings, but 

 they seem to have them but for tho purpose of conforming more nearly in external 

 resemblance to other birds. Some jMammalia can fly, although they have no wings. 

 The Bats possess a membraneous expansion, extending to the feet and to both sides 

 of the tail, but supported chiefly by the humerus, the fore-arm, and the four fingers. 

 These bones being greatly elongated, serve to support the membraneous surface, n hich 

 is of firmness and extent sufiicient to raise and maintain these animals in the air, when 

 acted upon by the powerful muscles of the breast. 



The first motion of a bird in attempting to fly is an ordinary leap with the feet. 

 Accordingly, those birds having the wings very large and the feet very short, as we 

 observe in the Booby and the Martens, commence their flight with great difiicultv, as 

 they cannot leap sufficiently high to obtain the space necessai-y for the extension of 

 their wings. 



In flymg, the resistance of the air is in proportion to the mass struck at one time. 

 On this account the short-winged birds must repeat tlieir vibrations very frequently; 

 they are therefore soon fatigued, and unable to continue their flight for a long time. 

 .When a bird attempts to fly, the humerus is first elevated, and then the entire win"- 

 which had hitherto remained folded; while, at the same time, it is extended in a hori- 

 zontal direction by means of the fore-arm and the last division of the wing, corres- 

 ponding to the foot of Quadrupeds. After the whig has thus acquired all the super- 

 ficial extent which it is capable of attaining, the bird suddenly depresses it, until it 

 forms an acute angle with the vertical plane of the body, subtending the ground. 

 The air resists this motion, which is performed with great rapidity, and produces a 

 reaction of part of the force upon the body of the bird. Its centre of gravity then 

 rises in the same manner as in other leaps. The wings may be compared to a lever, 

 of which the pectoral muscles are the moving power, and the body of the bird the 

 weight; while the air resisting, by its inertia, the action of the expanded wing, is tho 



fulcrum. The impulse being once given, the bird refolds the wings by bending their 

 joints, elevates them again, and gives a new stroke to tho air. The force of gravi- 

 tation diminishes the velocity which the body thus acquires in ascendino-, in the 

 same manner as it affects every other projectile. There consequently occurs a mo- 

 ment in which the 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 fai- as that obtained by the first impulse; it will then rise in a, con- 

 tinuous manner, and with a uniform velocity. If the second impulse of the win" 

 commence before the impulse arising from the first has been lost, the bird wiU 

 ascend with an accelerated motion; or, if the bird do not vibrate its wings at tha 

 exact moment when the ascending velocity is lost, it will begin to descend with great 

 rapidity. Yet the bird may keep itself always at the same height by a series of 

 equal vibrations, if a point be seized in the 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 wings. 

 The darting of Birds of Prey is occasioned by their suppressing the vibrations of tho 

 wings altogether, when the bird, being continually acted upon by the force of gravity, 

 falls with an accelerated velocity. When a bird in descending suddenly, breaks its 

 fall, it is called a recover. The resistance of the aur then increases m 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 are observed 

 to fly upwards in a straight line, have the wings placed entirely horizontal ; but in the 

 greater number of birds the wing is inclined, 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 tho resistance of the air to act on their extremities 

 with a mechanical advantage, while they are the more elevated by it, from Iheir fixed 

 points being placed at the base. 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 gravity, and the other in a 

 vertical direction, opposite to that power. In flying horizontally, the bird rises in 

 an oblique du-ection, and does not make a second movement of tho wings until it is on 

 the point of descending below the line of the intended direction of its flight. These par- 

 tial movements, therefore, will not take place in a straight line, but in a series of curves 

 nearly approaching to the straight Une, and in which the horizontal motion greatly 

 prevails over the vertical. In ascending obliquely, the wings move with greater ra- 

 pidity; in descending obliquely, then- vibrations are less frequent; and both of these 

 motions are performed by a series of curves. 



Some birds cannot sufficiently diminish the obliquity of their wings, and with them 

 the horizontal motion is always very considerable. When the wind blows strongly 

 in the same direction with the flight of tliesc birds, they are carried to a very consi- 

 derable distance out of their intended path. For this reason, those birds of prey 

 which the falconers term nohle, are under the necessity of flying against the wind 

 when they wish to rise perpcndiculai-ly upwards. Tho anterior quills of their win<»s 

 being extremely long, and their extremities pressing closely upon each other, the 

 horizontal motion with them is proportionally greater than that of other birds. On 

 the contrary, with the ignohle birds the quills of the wings are separated at their 

 extremities, and permit the air to pass between them, which renders the win'^ less ca- 

 pable of assuming the oblique position. 



Deviations from the rectdineal path to the right and left, are chiefly occasioned 

 by the unequal vibrations of the wings. When the left wing vibrates the more fre- 

 quently, or with the greater force, the left side moves more rapidly, and the body 

 necessarily turns to the right. The rapid movement or greater force of the rin-ht 

 wing produces a corresponding tm-n to the left. The difficulty of suddenly turning 

 increases with the velocity of flight; and this arises partly from the inertia of tho 

 body, which perseveres in its rectihncal course, and partly from the increased diffi- 

 culty of making the one wing to surpass the other in its velocity. For this reason, 

 birds of rapid flight make great chcuits in turrimg. 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 which raises 

 the hinder part of the body and depresses the 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 are many Birds which, 

 perform two motions, reserabUng those more particularly belonging to each class. 

 Aquatic Birds are improperly said to swim; and the poet, describing the swan " sailing 

 with the breeze," is perhaps not aware that his 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, whicu 

 are oily and impervious to moisture. They precisely resemble a boat, and have ni 

 further occasion for the feet than as oars for moving forwards. As the fore part of th 

 body is completely sustained by the water, the legs are situate farther backwards thaL. 

 those of other birds, that their effect may be more direct, as their presence fartljcr 

 in advance would be superfluous. The legs and thighs are short, that the resistance 

 of the water to the muscles may be as slight as possible. The tarsus, or instep, is 

 compressed for cutting the water, while the toes ai-c very much expanded, or even 

 united by a membrane, in order to form an oar of greater breadth, and capable of 

 acting upon a greater sui-facc of water ; and when the bird inflects its foot in order to 

 give a new stroke to the water, it closes the toe» upon each other to diminish the 

 resistance of the fluid. 



In diving, these birds are obliged to compress the breast with much force, in order 

 to expel the air which it contains. The neck is then elongated, that the body may 

 acquire an inclination forwards, while, by striking the feet upwards, it is. foicaj 



