MECHANICS, ANIMAL. A moment's reflection shows that this subject is exhaust less, the application of forces and the variation in the mechanism being infinite, aud this, without embracing molecular mechanics or kinematics, which would uecessarily be involved in a minute study of the action of the nervous system. We must, therefore; content ourselves with a few brief general illustrations of the more obvious vital mechanical movements. The simplest examples tire the hydromechanics of some of the lower infusoria, where the animal is propelled through its native element by the reaction of water forced out of a siugle orifice by the contraction of the shnple cell which forms the body. Some of these minute animals have cilia which also serve as locomotive organs. Other hollow animals of ft higher organization propel themselves through water in a similar manner, as those belonging to the sub-kingdom ccelenterata (q.v. in art. INvEurEnnAw, ANIMALS). TheSe animals are also provided with filamentar3- tentacles which have contractile properties, and the power of. forming hooks or prehensile organs. The mechanism of the circulation in the eydippe, a melentrate belonging to the order Ctenophora is exceedingly interesting. It consists of a complex canal system, the tubular branches of which are lined by a ciliated endoderm for the purpose of keeping up within them the circulation of water. These animals, although no doubt assisted by the contractions of the body cavity, are propelled by certain organs called ctenophores, or parallel rows of cilia, which are arranged in comb-like plates longitudinally upon their globular or oval bodies. Some infusorial animals, as the rotifera or wheel animal cules, included in a higher sub-kingdom (annnlosa), possess a highly mechanical organization, approaching somewhat, in that. respect, the insects. The characteristic wheel organ consists of a retractile disk carrying numerous cilia which, by their succes sive rapid vibrations, produce the illusory appearance of a rotating wheel. The motions are regarded as having an action similar to that of a screw propeller, and as aiding in locomotion as well as serving to throw currents of water into the mouth. All the move ments in these soft-bodied animals involve as complex mechanical principles as those which are exhibited in the action of muscles and tendons upon framework which serve as levers in the higher animals. The locomotion of fishes involves shnilar applications of force in the oblique manner in which the sides and tail fin are brought to act against tlte water in which they SWIM, and also in the position of the pectoral and other fins, which give direction, and are not—with the exception of ,the dorsal and caudal fins— organs of propulsion in ordinary swimming., as is sometimes supposed. When a fish is comparatively quiet he may change his positiou by the action of all the fins, and a back ward motion is often produced by a paddle action of the pectoral and ventral fins. The oblique action of the sides of a fish against the. water is of the same nature as that of a ship when tacking against the wind, or of the paddles of a screw propeller, or of an oar in sculling. or of a serpent in running through grass, and involves mathematical elements of all orders, from the simplest to the most complex.
The attempted solutions of the application of force in the locomotion of fishes, which represent the whole of the tail and latter part of the body as moving alternately front side to side, and producing alternate periods of retarding and of forward action, are founded upon erroneous views. No fisli, not even the clunt-siest, propels itself in accord ance with such crude mechanical principles. The longitudinal line of tbe latter part of the body presents a number (depending upon the form and flexibility of the fish) of ser pentine curves, of more or less depth, -whose combined action produces (in the most rapid motions) an almost uniform forward propelling force, and in one direction, except when the fish curves its body for the purpose of turning, or altering its course. The body and tail fin do not oscillate in one curve, 'nit the fin is always applied to the water in a direc tion which tends to propel the body forward, and its suppleness and flexibility are qual ities given to it for tltis purpose. The complex mechanism displayed in the hig,her animals and in man is all the more interestino. because of its involving- the simpler prin ciples of the mechanical powers, particularry the lever and pulley, as well as those of obl ique action in fishes, which includes in its elements the principle of the inclined plane. The lines of forre in the action,of the muscles, as applied to the bones, undergoing; as they do, constant variation of direction, present, however, equally difficult mathematical problems if it is required to estimate the expenditure of power. The apparatus forma4 tication and deglutition in various classes of animals furnishes one of the most com plex subjects of inve.stiption, ontliiitcleetl,:avhose elements.are,: in their 'tnal vestilts, insolvable, on account of the constantly variable condition, quality, and quantity of food, involving, as it does, constantly varying applications of muscular force, and constantly varying capacity and form of the mouth and pharynx. _Most of the niovements are produced automatically, but the perfect adaptation of the mechanism to the required functions is none the less wonderful. The masticating apparatus in various animals is as various as the animals themselves, and one is adapted to the other so perfectly that many have adopted the idea that the development of the oronism must have kept pace with the development of an appetite, or a change of circumstances. It is maintained
by others, however, that there are facts in anatomy which render such progressive development hypotheses improbable; as, for instance, the arrangement of the superior oblique muscle of the eye-ball. One end of this muscle is attached to a part of the sphenoid bone at the bottom of the orbit; it then passes forwards to a cartilaginous ring or pulley which is attached to the frontal bone at the inner angle of the orbit, and becoming a rounded tendon it passes through this pulley and is then turned backward, becoming again.muscular. It then expands into a.broad band which is inserted into the sclerotic coat of the posterior and outer surface of the eye-ball. It is difficult to imagine how the force of an impending function, or any physiological want, could cause the development of such a piece of apparatus. It is so niuch of a contrivance, to all appear ances, that the elements of design and of immediate creation cannot well be denied recognition. The internal mechanism of the eye-hall is held to afford as much evi dence of design as that furnished by the superior oblique muscle. For the purpose of accommodating the eye to vision at different distances, among other provisions, the degree of convexity of the crystalline lens requires to be constantly changed. This is effected by the ciliary muscle, a circular organ situated at the outer border of the iris and at the junction of the cornea with the sclerotic coat. As examples of the "mechan ical powers" in the mechanism of the human body, we find the cord and pulley in the arrangeinent of the superior oblique muscle of the eye, instanced above; the first kind of lever, that where the fulcrum is between the resistance and power, in the support of the bead upon the axis (the upper cervical vertebra) and the depression of the occiput aud elevation of the face by the contraction of the extensor muscles of the neck, and also la the arm when the extensor muscles act upon the olecranon process of the ulna. See SKELETON. The arm also affords an example of the third kind of lever when acted upon by the flexor muscles, the power beim, applied between the hand and elbow joint, which is the fulcrum. The raising of the ady upon the toes is usually instanced as an exam ple of the second kind of lever. It is evident, however, that if a person lies upon the back and places his toes against a resisting, but movable, object, and pushes it away, he will virtually be performing the same mechanical operation, as far as the foot is con cerned, as when rising upon his toes, and the relations of the toe, the ank'e joint, and the heel will be precisely the same; that is to say, the ankle joint will be the fulcrum, the application of the toe will be at the point of resistance, and the power will be applied by the tendo-achilles at the heel. In raising one's self upon the toes, therefore, the ankle joint is in reality a movable fulcrum. Moreover, the first and second kinds of lever are convertible into each other by making the resistance in the first kind stationary and causing the fulcrum to move. One of the most celebrated and elegant essays upon ani mal mechanism is the Bridgewater treatise for 1834, by Sir Charles Bell, on The Hand, its Meehankm and Vital Endowments as _Evincing Design. The mechanical contriv ance known as the toggle joint, sometimes spoken of as one of the mechanical powers, but which acts upon the principle of the inclined plane, is exemplified in the knee joint. When the knees are considerably bent it is difficult to raise a heavy weight, but as the leg-s become straighter the power over resistance becomes enormous. Of course the tog gle, or knee joint, in this instance is moved by the application of muscles and tendons to levers whose arms (thighs and legs) ar2 also the arms of the toggle joint. In reality the operation of raising the body from a sitting posture combines the principles of two mechanical powers, the lever and inclined plane, the hip forming a toggle joint as well as the knee. See TOGGLE JOINT.
.The mechanics of aerial motion in birds furnishes one of the most interesting subjects of philosophical inquiry and physical research, and has been ably treated by the present duke of Argyle in a work called " The Reign of Law." See also in this cyclopmdia the article on Butt:is. An examination of the anatomy of a bird is a source of never ending admiration to the student of natural history. It reveals the most perfect adap tation of means to results—and results, too, which would seem impossible if one had never witnessed the phenomenon of aerial flight. To watch a bird—like one of the larger sea-gulls, poise itself without flapping its wing,s for a quarter of an hour or more, and when the wind is blowing, for an indefinite space of time, or as lom, as the bird can be seen, without descending from its altitude of several thousand feet, at floating alofi like a kite held by a cord, now rising with majestic 'notion, and now darting obliquely downward with immense speed—is one of the most fascinating of recreations. Scarcely less wonderful is the flight of insects, and perhaps none of the class possess the power in 'greater perfection than the common fly. See FLYING, ante, and In-sEcrs, ante.