MOTION, ANDIAL. Motion or progression is that function by which an animal is able to transport itself from place to place. It is enjoyed exclusively by animals, there being nothing strictly analogous to it in the vegetable kingdom. The organs employed in locomotion are of two kinds, the passive and the active; the former including all those textures which form the skeleton, and by which its segments are united, as fibrous and areolar tissue, synovial membrane, cartilage, fibro-cartilage, and bone, while the latter includes the muscles with the nerves, which convey to them the mandates of the will.
Before procoeding to notice the different modes of progression of men and animals, it may be expedient to say a few words on standing, or the natural attitude of an animal. This attitude depends upon the form and functions of the limbs. Most of the terrestrial mammals and the reptiles (excepting the serpents), both of which use four feet in walk ing, have the backbone (the vertebral column) nearly horizontal (being only slightly con cave downwards), and resting, at the same time, both on the fore and hind legs. Birds, whose anterior extremities are intended for flight, stand upon the posterior limbs only, although in their case, too, the backbone is generally nearly horizontal when the animal is at rest. Man alone stands erect with his head supported on the summit of the nearly vertical vertebral column: Some other animals (monkeys, hares, kangaroos, etc.) can rise more or less erect, but in theif case the attitude is obviously not the natural one.
In standing it is requisite that ,the !hubs should be so arranged that the center of gravity may fall within the Space included by the feet, If center, of gravity does not fall within this space the animal cannot stand, but must fall to that side to which the center of gravity inclines. On this account certain aquatic birds (the albatross, for example), which have their feet placed very far back, cannot use them for walking. If an animal has four legs, it is not necessary that they should have the broad base, which is essential in bipeds. We see that most quadrupeds have comparatively small f( et, while birds are furnished with long toes, which, when spread out, form large bases of support. Moreover, the flexor muscles of the toes are so arranged that the weight of the body causes them to contract firmly, and hence birds are enabled to sleep standing with out any effort.
liraaing is the most common form of progression. When it is accomplished by two
legs only, as in man, the body is inclined forward, carrying the center 01 gravity in that direction; and while one leg supports the body the other is thrown forward to prevent it from falling, and to sustain it in turn, Hence, walking has been defined to he a con tinual falling forward, interrupted by the projection of the leg. Those writers who hare especially studied the theory of walking (Borelli, the brothers Weber, and Bishop) have divided the time of a step into two portions—i.e., that in which one leg only rests on the ground, and that in which both legs rest on the ground. The period in width both feet are on the ground is shorter than that in which the body is supported by one leg only. During the time the body intervention supported by one leg, the other leg swings from behind for ward, without the active ntervention of its muscles, but by the mere force of gravity— in short, like the pendulum of a clock. When this leg is again placed on the ground the first interval ends, and the other—i.e., that in which the bodyis supported by both legs—begins, and, of souse, terminates with the raising of the other leg. The time that the body is supported by both legs diminishes as the velocity increases, and vanishes as the walk merges into a run; while, on the other hand, it attains its maximum in extremely slow walking, when it is found by experiment to amount to half the time in which only one leg supports the body. The greatest velocity of walking is attained when the time of a step is equal to half the duration of the motion of the swinging leg, and the velocity in walking of any given person depends on the time taken in making each step, and on the length of the steps; and both of these are, again, dependent on the height at which the center of gravity of the body or the heads of the thigh-bones are carried above the ground; for as the height of the latter diminishes, the length of the step is increased, while its time is diminished, and vice versa. The best walkers arc incapable of acquiring a speed of more than 7 in. an hour; and even this speed cannot be kept up for any length of time. The walking of quadrupeds is a similar process to that of bipeds, except that the body always rests on at least two legs. The limbs are raised in a determinate order, and usually in such a manner that the hind-leg of one side succeeds the fore-leg of the opposite side.