MOTION ANIMAL, ANIMAL DY NAMICS, LOCO-MOTION, on PRO GRESSIVE MOTION OF ANIMALS.— Amongst the infinite number of objects pre sented by the Deity to our contemplation in the sublime spectacle of the universe, there are none, relating to the economy of animal life, more important in their consequences, more calculated to awaken inquiry, or deserving of more profound research, than the phe nomena of progressive motion in man and animals.
Life, in virtue of which animated beings possess sensation and exhibit the play of the vegetative functions, endows the muscular system with contractility, and is the funda mental cause of all the motor power of animals.
The theory of the progressive motion of animals presents a most extensive field for anatomical and physiological inquiry, far too extensive indeed for the space here allotted to this subject; it will therefore be treated only in outline. The automatic, and several of the voluntary motions which belong to the vegetative functions of the animal economy, though de rived from the same source as those of progres sive motion, will not be included in this investi gation.
The theory of locomotion relates to those mechanical functions by which animals are capable of changing their relative positions or distances with respect to surrounding ob jects supposed to be stationary or fixed.
The locomotive organs of the higher animals are composed of a system of levers of various forms, orders, and dimensions, so united or ar ticulated at the joints as to give them the re quisite mobility as well as direction of motion. The fulcra to these levers are the earth, the air, or the water ; the active agents of mo tion are the muscles which constitute a complex system of contractile organs, firmly attached to the levers, whereof the points of connexion, amount of contraction, and direc tion of force, communicate to the levers, to which they are firmly attached, all the move ments necessary for progression.
The progression of some animals, such as the Annelids and Ophidian Reptiles, is effected by the alternate contraction or flexion and elongation, or by undulatory movements of the body ; in others, as Bipeds, Quadrupeds, Fishes, Birds, &c., by the alternate approxima tion and angular separation of the levers which form the organs of progression. These prin ciples apply to animals, whether their levers are represented by wings, fins, or legs, and whether the progression is effected on solids, in water, or in the air.
The various modes of animal progression are swimming, flying, crawling, climbing, leaping, running, walking, &c. The con sideration of these diversified methods of progression involves the theory of the mo tion of bodies in general, of the lever, the pulley, the centre of gravity, specific gravity, and the resistance of fluids, &c.; and, as we shall have occasion for constant refer ence to the mechanical principles connected with these subjects, they will be first dis cussed; but for the convenience of those who are unacquainted with the algebraic method of computation and analysis, the latter will generally be separated from the text.
Fundamental Axioms. — First, every body continues in a state of rest or of uniform mo tion in a right line until a change is effected by the agency of some mechanical force. Secondly, any change effected in the quiescence or mo tion of a body is in the direction of the force impressed, and is proportional to it in quantity. Thirdly, reaction is always equal and con trary to action, or the mutual actions of two bodies upon each other are always equal and in opposite directions.
Thus if M (fig. 203) be a particle of matter free to move in any direction, and if the lines MA, MB, represent the intensity of two forces acting on it in the direction MC, the particle INT will move towards C by the combined action of the two forces, and it will require a force in the direction of CM, equal to MA+ MB to keep it in a state of rest: but if MA and MB (fig. 204) represent the intensities and directions of two forces acting on the par ticle 11I in opposite directions, if MA be greater than MB, the particle M will be moved towards A by the difference of these two forces, and it will require a force equal to that difference to keep it at rest.
The composition and resolution offimces.- In the composition of forces it is proposed to find the resultant, arising from any number or system of forces acting upon a given point. The resolution of forces, which is the converse of the former process, consists in discovering what forces acting in given directions would com bine to produce a given resultant: Thus, if there be two forces F (fig. 205), whose directions and magnitudes are represented by F N, F'N, and if FR, F'R be drawn respectively parallel to F'N, FN, then by the composition of forces we find the magnitude and direction of the equivalent or resultant of these two forces to be RN, and conversely it may be resolved into a pair of forces as RF, RF' represented by the adjacent sides of any parallelogram, of which RN is the diagonal, and consequently into an indefinite number of such pairs.• Misconstruction is called the parallelogram of forces.
The resultant of any number of forces meeting in a common point may be ascertained thus : first, let the resultant of any two forces be found as before, and substitute this one force for the two components pro ducing it ; then combine this new force with one of the remaining forces, and continue this process until all the forces are reduced to a single force, which is the resultant sought. The following geometrical solution will render the subject more apparent: let P, P', P", &c. (jig. 206) represent a number of forces meeting in the common point A, and let A P, A P', A P" be proportional to these forces respectively : through P draw PR equal and parallel to AP', and through R draw RR' equal and parallel to A P", and through draw It' R" equal and parallel to A P"'; join A It", which represents the resultant of the four forces A P,