490 EQUILIBRIUM OF ANIMALS-BASE OF SUPPORT. which its feet cover and inclose between them; and the wider this space, in proportion to the height of the centre of gravity, the more stable will the equilibrium be, since the body may be more displaced without being upset. Thus in fig. 231 the table a must be upset; because the line of direction e from the centre of gravity c falls outside the base of support d; whilst the table b, although equally inclined, will not be upset but will return to its proper place, because the line of direction e from its centre of gravity c falls within its base d. Hence an animal which is supported upon four legs will stand much more firmly than one which rests on two only; since its real base is the whole space included between its four points of support. And again, an animal is more firm when standing upon two legs, than when resting upon one only. ta t ta ta' C 652. Moreover when an animal rests upon four legs, the extent of its base is but little influenced by the size of the feet; and thus to render them broad would be to increase their weight without adding much to their use as supports. This is easily understood by comparing a quadruped to four-legged table; if the legs are sufficiently strong to support the weight that rests upon them, it matters little in regard to the steadiness of the table, whether they bear upon the ground by mere points or by flat surfaces; since it is the large surface that would be enclosed by lines joining them, which constitutes the real base. Hence we find that, in most quadrupeds, the limbs only touch the ground by slightly-dilated extremities; and the number of fingers is reduced more and more, without diminishing their effect as instruments of locomotion. Thus in Ruminant animals, as the Deer, the number of toes is reduced to two in each foot, as seen in fig. 232, where t represents the tibia, ta the bones of Р Р pi pi pt pt Fig. 232. Fig. 233. FOOT OF DEER. FOOT OF HORSE. EQUILIBRIUM OF ANIMALS-BASE OF SUPPORT. 491 the tarsus, c the bone of the metatarsus termed the canon (in which the trace of a division into two pieces can be seen), and p, pi, pt, the three phalanges of the toes, of which the last is enveloped in the hoof, which is nothing else than a large nail inclosing the whole extremity of the toe. In the Horse this consolidation is carried still further than in the Ruminants, for it has only one toe in each foot (fig. 233); but we see the rudiment of an additional bone in the metatarsus b, which is commonly termed the splint bone. 653. But when an animal is supported upon two feet only, whatever may be their degree of separation from each other, the base of support cannot have sufficient extent, unless the extremities touch the ground by a considerable surface. This is the case with the foot of Man, and still more with that of many Birds which habitually stand upon one leg (fig. 234). In order that an animal may hold itself in equilibrium upon a single limb, it is necessary that the foot should be placed vertically beneath the centre of gravity of the body; and that its muscles should be so arranged as to permit it to keep this limb inflexible and immovable. Man can accomplish this, for the centre of gravity of his body is at about the middle of the pelvis; and to place this vertically over one foot, it is sufficient for him to bend himself a little from the side which is not supported. But the greater number of Quadrupeds are destitute of the power of doing this; and a large part of them cannot even raise themFig. 234. selves on their hind legs, on account of the direction of these members relatively to the trunk; or if they can do so for an instant, they cannot maintain themselves in this position. The reason of this is very simple. The base of support, on account of the smallness of the feet, is very narrow, and the centre of gravity of 492 MUSCULAR EXERTION TO MAINTAIN EQUILIBRIUM. the body is placed near the front; hence the body must be entirely changed in its position by a violent and not sustainable action of the muscles which connect it with the hind legs; and, when thus reared up, it cannot rest with firmness on account of the narrowness of the base. 654. There are some Quadrupeds, however, which are able to raise themselves occasionally into this position; this is the case, not only with the Quadrumana, but also with the Bear, Squirrel, and other animals whose habits require them to ascend and live among trees, as well as in the Kangaroo, and animals constructed upon the same plan, whose peculiar organisation will be presently considered (§ 661). In standing upright, the muscles of the back part of the neck are kept in a contracted state, to retain the head in equilibrium on the vertebral column; and the extensor muscles of that column must also be kept in action, to prevent it from bending forwards under the weight of the head, upper extremities, and viscera of the trunk. The whole weight of the upper part of the body is thus transmitted to the sacrum, and thence to the other bones of the pelvis, by which it is brought to bear on the femur. If left to themselves, the thigh-bones would bend beneath the pelvis, and the trunk would fall forwards; but the contraction of their extensor muscles keeps them firm. In the same manner, the extensor muscles of the knee and ankle keep these joints from yielding beneath the weight of the body, which is thus at last transmitted to the ground. The sitting posture is less fatiguing than the standing position, because the weight of the body is then directly transmitted from the pelvis to the base of support, so that it is not requisite for the extensor muscles of the lower limbs to keep-up a sustained action. But the lying posture is that of the most complete rest; because the weight of every part of the body is at once transmitted to the surface on which it bears, and no muscular movement is requisite to keep it in its position. 655. This difference in muscular effort, is the cause of a well-marked variation in the pulse, according to the position in which the body is at the time. From a considerable number of observations it has been found that the average pulse of an adult man is about 81 when standing, 71 when sitting, and 66 when lying; so that the difference between PULSE IN DIFFERENT POSTURES :-ACT OF WALKING. 493 standing and sitting is 10 beats or 1-8th of the whole, whilst the difference between sitting and lying is 5 beats or 1-13th of the whole. In the female, the pulse is quicker in each position by from 10 to 14 beats per minute; but the differences occasioned by position are nearly the same. It will be observed that the difference between standing and sitting is greater than that between sitting and lying; and this closely corresponds with the relative amounts of muscular exertion required in these positions respectively. At the moment when the posture is changed, the pulse is considerably quickened, in consequence of the muscular effort required for the purpose, which acts especially on the veins, and forces the blood more rapidly back to the heart (§ 279); but this increase in rapidity is temporary only. 656. All that has been said of the positions of Vertebrated animals applies equally well to those of the Invertebrata, which like them have the body raised from the ground upon extremities. This is the case in the higher Articulata, such as Insects, Crustacea, Arachnida, and Myriapoda. But the lower Articulata crawl, like Serpents, upon the whole length of their bodies; or, being aquatic, are buoyed-up by the element they inhabit. And among the Mollusca and Radiata, there are none that have members upon which they can be said to stand. 657. The progressive movements by which the bodies of Man and other animals are made to change their places, are accomplished by means of the alternate contractions and extensions of those limbs, which we have hitherto considered only as supporting them in a rigid position. It is easy to see that when a joint is straightened after being bent, the two ends of the levers which form it must be separated from each other, and that motion must thus be given to the parts against which one or both of them bear. Now in the ordinary movements of progression, one of these levers bears against the ground, which is immovable; and the whole motion produced by straightening the joint must consequently be communicated to the body. In the ordinary act of walking, one of the feet is planted in front, whilst the other is extended or carried backwards beneath the leg, by the action of the muscles of the calf aided by those of the toes (§. 648). Its length is thus increased; and as it bears upon the resisting soil, this 494 ACT OF WALKING:- -OTHER MODES OF LOCOMOTION. elongation acts through the thigh upon the pelvis, and thus carries forward the whole body. At the same time, the pelvis makes a slight turn upon the femur of the other side on which it is resting; and the limb which was at first behind the other, is now drawn forward by a flexion of its joints, and is planted on the ground in front of the other, so as to serve for the support of the body in its turn; whilst the other, by extending itself, gives a fresh forward impulse to the body. Thus each limb is alternately made to support the whole weight of the body, just as it would do in standing on one leg; while at the same time the other is engaged in urging it forwards. Hence the centre of gravity must vibrate a little from side to side in the act of walking, so that it may be brought alternately over each foot; and this movement from side to side is the more obvious, in proportion as the pelvis is wider, and the limbs more separated from each other. Hence it is more seen in women than in men, on account of the greater proportional breadth of the hips in the former. 658. In all the higher animals, as in Man, there are members which serve for locomotion; but the nature of these movements varies greatly; and there is a corresponding difference in the structure of the instruments by which they are performed. The manner in which the Creator has made the same organs answer a variety of different purposes, in accordance with the habits of the animals to which they belong, is a most interesting object of study; for we see the most varied results attained, without the least departure from the general plan which has been adopted in the construction of the various species of the same group; and this solely by slight changes in the forms and proportions of some of the instruments whose union makes-up the entire body. The organs of locomotion in the Mammalia furnish us with obvious examples of this principle. This class includes not only the quadrupeds which run or bound along the surface of the ground, but animals which are destined to live solely in water like fishes, others which sometimes swim through that element and sometimes inhabit the land, others which possess wings that enable them to fly through the air like birds, and others which only employ their anterior members for grasping or feeling; yet in all these animals, these organs are constructed of the same parts. In the paddles of a Seal |