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at side be in advance, moves ght fore-leg, third the right pelling the centre of gravity gth of the first step. When ancing, the trunk is supported ced on a triangular instead of sion of movements of the four

the progression by walking lepends on the time occupied h the limbs effect the step. In may move the length of a step may swing during one quarter ters of a second. Rapid walkers al between putting down one leg ppreciable. In quadrupeds with on to the trunk there is a modifithe Camel and Giraffe seem to o right limbs together and alterBut, though in a quick walk the em to be moved forward simulground at the same time through t on close inspection the hind-leg the ground, and after a very brief ne side. In this way of walk the basis of support, alternately transer. In the Giraffe the long neck with the back, giving the animal a but this is lost when they commence ible :

now lifted

nd are

d them

ent.2
called

35

in pairs

E, fig.

as A, D,

of sup

es A, D, B, E; and the undulations from are in the vertical, not as when walking

2 Ib. p. 244.

the preparatory movement; in this the hands are brought near the mesial plane, with the palmar surfaces parallel to each other; they are then thrust forward by the extension of the arm, with the points of the fingers in advance to cut the water with the least resistance; when the hands have nearly reached their greatest distance from the centre of gravity, they are rotated by pronation, so that the palms are directed at an oblique angle outward and downward; they are then forced backward by the abduction of the whole arm through a large arc of a circle, having the shoulderjoint for its centre, and the length of the arm for its radius; the fore-arm is then flexed, and carried into its former position preparatory to making another stroke. During the extension of the arm, the feet are drawn toward the centre of gravity, with their convex surface directed obliquely backward by the extension of the ankle and flexion of the hip and knee joints, and during the abduction of the arm the flat surfaces of the feet are driven forcibly backward and downward by the sudden extension of the leg. From the ratio of the areas of the hands and feet, and the ratio of the difference of their velocities in the two strokes, there results such a preponderance of the force in the vertical direction upward and in the horizontal direction forward as is sufficient to keep the respiratory openings above the surface of the water, and to overcome the resistance which the water opposes to the motion of the body, due to its figure and velocity.

B. Moving on Land.-In mammalian quadrupeds the limbs are usually long, and support the trunk horizontally, uplifted from the ground, as on columns expanded at their base. The uppermost long bone is single, the next two form a pair, side by side, and these rest on more numerous ossicles, transferring the weight upon the base of two, three, four, or five diverging piles : the single hoof of the Horse seems an exception, but it, too, expands to its base. The shafts of the long bones are hollow, agreeably with the principle of combining greatest strength with least weight. According to the lightness and speed of the quadruped, the limb-bones are inclined to each other's axes at a greater angle. In the colossal Elephant and Megathere they rest on each other almost vertically, in supporting the trunk. The horizontal trunk and produced head and neck of quadrupeds cause the largest proportion of the weight to fall upon the front pair of supporting columns, of which, accordingly, the angles of the joints are less, and the direction more vertical than in the hind pair, as is well exemplified in the hoofed kinds (vol. ii. figs. 307, 309, 310).

In walking, the Horse, if the right side be in advance, moves first the left hind-leg, second the right fore-leg, third the right hind-leg, fourth the left fore-leg; propelling the centre of gravity forward over a space equal to the length of the first step. When the left hind-leg is in the act of advancing, the trunk is supported on the other three legs and is balanced on a triangular instead of a parallelogrammical basis. A succession of movements of the four legs, in the above order, constitutes the progression by walking in most quadrupeds; its rapidity depends on the time occupied in the series of movements by which the limbs effect the step. In a large well-made Horse one foot may move the length of a step in a second of time, when each leg may swing during one quarter and rest on the ground three quarters of a second. Rapid walkers do it in less time, and the interval between putting down one leg and lifting another becomes inappreciable. In quadrupeds with limbs unusually long in proportion to the trunk there is a modification of the act of walking: the Camel and Giraffe seem to swing along by moving the two right limbs together and alternately with the two left limbs. But, though in a quick walk the two legs of the same side seem to be moved forward simultaneously, and are both off the ground at the same time through the greater part of the step, yet on close inspection the hind-leg is seen to be first lifted from the ground, and after a very brief interval the fore-leg of the same side. In this way of walk the trunk is balanced on a linear basis of support, alternately transferred from one side to the other. In the Giraffe the long neck is then stretched out in a line with the back, giving the animal a stiff and awkward appearance; but this is lost when they commence their graceful undulating amble :

the motions of the legs are now peculiar; the hind-pair are lifted alternately with the fore, and are carried outside of and beyond them by a kind of swinging movement.2

E

35

In the pace of the Horse called the trot,' the legs move in pairs diagonally, those marked B, E, fig. 35, e.g. being raised as soon as A, D, strike the ground: the bases of support are alternately in the lines A, D, B, E; and the undulations from the projection of the trunk are in the vertical, not as when walking

B

in the horizontal, plane. Moreover, in the rapid trot, each leg rests a short time on the ground and swings a longer time.

6

The gallop includes three combinations of movements of the limbs. When the Horse begins the gallop on the right hind-leg, the left one reaches the ground first; the right hind and left forelegs next, simultaneously, and the right fore-leg last; this is termed the gallop of three beats. In the gallop where the four legs strike the ground successively, the left hind-foot reaches the ground first, the right hind-foot second, the left fore-foot third, and the right fore-foot fourth; this is the 'canter,' or gallop of four beats, but it is not the kind of movement adapted for great speed. The gallop wherein the legs follow the same order as in the trotthat is, the left hind and right fore-feet reaching the ground simultaneously, then the right hind and left fore-feet-is the order in which horses move their feet in racing, where the greatest speed is required, and is called the gallop of two beats. In the amble,' the two legs on one side rest on the ground and propel the centre of gravity forward, whilst those on the opposite side are raised and advanced, and, on taking a new position on the plane of motion, the former pair are raised and advanced in a similar manner : these successive actions are accompanied by considerable lateral motion. This resembles the gallop of the Giraffe, and is a result of special training in the Horse. In the ordinary gallop, the centre of gravity moves in a vertical plane, and describes the path of a projectile. The space passed over on the plane of motion is equal to the horizontal velocity of the centre of gravity multiplied by the time. According to Sainbell, the horse Eclipse, when galloping at liberty and with its greatest speed, passed over the space of twenty-five feet at each stride or leap, which he repeated 23 times in a second, being nearly four miles in six minutes and two seconds. Flying Childers was computed to have passed over eighty-two feet and a half in a second, or nearly a mile in a minute. In both these famous racers the muscular system had been allowed to gain its full developement, as at four years, before being exercised for the course modern impatience strains and spoils the muscles by the chief prizes being allotted to three-year-old horses.

In many Marsupials and Rodents the hind-legs are shorter than the fore-legs, the disproportion being greatest in the Kangaroos and Jerboas. In slow progression the Kangaroo supports the body on the tail and fore-legs, while the hind-legs are simultaneously moved forward outside and in advance of the fore-legs; the base of support being here transferred from a triangle to a transverse line. In full speed the tail is rigidly outstretched to

afford a firm fulcrum to muscles passing from the caudal vertebræ to the pelvis and hind-limbs: the short fore-limbs are tucked up to the chest so as to offer the smallest surface to the air, and the animal progresses in a series of bounds by simultaneous movements of the hind-limbs.

The Rabbit, in moving slowly, advances the fore-feet two or three steps alternately. The body being thus elongated, the hindlegs are suddenly extended and drawn forward simultaneously: it thus, as it were, walks with the fore-legs, and leaps with the hind. The Hare is under disadvantage with its long hind-limbs in running down-hill, owing to the great inclination of the axis of the trunk to the plane of motion, and it usually zigzags as it descends; but it gains proportionally in the ascent, and its speed on level ground, through the size and strength of the chief propelling limbs, is very great. The degree of flexion of the trunk accompanying the movements of these and other quadrupeds is indicated by that in which the neural spines converge toward the single vertical one marking the centre of motion, and it is commonly greatest in the unguiculate quadrupeds.

The verticality of the long and narrow tarsus and metatarsus producing the digitigrade' character of the type Carnivora, combines with the geometrical and physical relations of the other parts of the limbs to give them their superior speed and agility. In the Dogs and Cats the oblique scapula, being unfettered by bony (clavicular) connection with the sternum, enjoys the freedom of rotation which characterises it in the swift Ungulates. The humerus in the Lion (vol. ii. fig. 337) has its axis directed downward and backward, forming with that of the scapula an angle of 110°. The olecranon projects so far behind the axis of rotation in the elbow-joint as to constitute a powerful lever for the extensors of the fore-arm. The hind-limbs are longest, and the bones are inclined more obliquely to each other than in the fore-limbs, subserviently to elasticity and power in springing. The calcaneum is produced on the same principle as the olecranon, but forms the more powerful lever of the two. The last perfection is given to the limbs of Carnivora by the modifications of the toes of Felines, whereby their tread is noiseless, and the claws exempt from the wear and tear of progressive motion. It is effected by a joint allowing the ungual phalanges to be brought in extension above the middle phalanges, elastic ligaments being adjusted to keep the joint so extended, and by a thick cushion of soft elastic substance beneath the joint or parts of the phalanges transmitting the superincumbent weight to the ground.