muscles on the heel-bone, fig. 37, k, the action is that of the second kind of lever, in which the resistance (of the tibia on the astragalus), as in fig. 32, w, is between the fulcrum F (afforded by the ball of the hallux), and the power a (tendo achillis).

In lifting a weight in the hand by motion of the fore-arm only, fig. 33, the elbow-joint is bent; the power (of the flexors of the fore-arm) being applied (as by the biceps, p) at a, between the fulcrum (elbow-joint) f, and the resistance w or b, according to the third kind of lever exemplified in fig. 34.

The mechanism of the pulley is exemplified in the passage of the tendons of the peronei muscles through the groove of the external malleolus of the human ankle-joint, in the tendon of the obturator

33

b

internus gliding through the groove in the os ischii, in the tendon of the circumflexus palati passing through the hamular process of the sphenoid bone, in the tendon of the obliquus superior gliding through the ring attached to the frontal bone, and in several other instances where a change of the directions of the limbs results from tendons passing over joints, through grooves in

bones, or under ligaments, by which the muscles are capable of producing effects on distant organs without disturbing the symmetry of the body, an effect which, owing to the limited power of contraction in the muscles, could be accomplished in no other way.

The joints in the mammalian skeleton are chiefly of two kinds,

'ginglymoid' or hinge-joints, and ́enarthrodial' or ball-and-socket joints. In Man the former are less definitely fitted for motion on one plane than in most brutes.

The

34

A

B

Lever of the third kind.

arm and fore-arm move in concentric planes upon the elbow-joint; the knee-joint allows a certain rocking motion of the leg upon the thigh; the ankle-joint has a greater latitude of motion, and the foot may be directed out of the plane of the leg's motion.

Atmospheric pressure exercises its influence upon joints. Dr. Arnott estimates the amount of that on the knee-joint at 60 lbs. ; Weber of that on the hip-joint at about 26 lbs.: in the hip-joint of the Megatherium the pressure could not have been less than 150 lbs.

A. Swimming.-Quadrupeds with inflated lungs are of less specific gravity than water, and swim by alternate extension and flexion of their legs; the effective stroke being the act of extension, when the limb presents a larger area to the water than in flexion: this is seen in the Horse, which strikes the water with the expanded and subconcave surface of the hoof, but draws the convex conical part through the water in the bending of the limb preparatory to the next effective stroke. In the best water dogs the digits are connected by webs, which are stretched in the back or down-stroke, folded in the return movement. The feet of the Otter are broader, especially the hind ones, and more fully palmated. The Seals and Whales have the limbs fashioned as fins.

Man, with the chest well expanded, is lighter than water: the presence of mind which counteracts the tendency produced by immersion in a cold and dense medium to expel the air from the lungs is the first safeguard against drowning; and next, if the art of swimming has not been learnt, to keep the head immersed to the mouth and nose, and to refrain from the misdirected struggles of terror which tend only to hasten on the catastrophe. In swimming, the hands and feet are employed so as to present the greatest surface to the water in the effective stroke, the least in

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."

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

D

E

B

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

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 trot— that 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