BONES AND MUSCLES OF LOWER EXTREMITY. 485 lower extremities, this framework is much more firmly attached to that of the trunk, than is the case with that which supports the arms. It consists, on each side, of a bone which in the adult state is single, though at an early age it is composed of three distinct pieces; and this is closely connected with the sacrum behind, while it meets with its fellow in front in such a manner as to form a sort of bason termed the Pelvis. The spreading sides of this, formed by the iliac bones (Fig. 213), afford support above to the viscera contained in the abdomen; and they give attachment by both surfaces to large muscles by which the thigh-bone is moved, and by their edges to large expanded muscles that pass upwards to the ribs and sternum, and form the walls of the abdomen. Below this spreading portion, we find the articular cavity of the thighbone, which is so deep as almost to form a hemispheric cup when it is completed by its cartilaginous border. The movements of the thigh-bone are consequently more limited than those of the arm; but it is much less liable to displacement. 646. The thigh, like the arm, contains but a single bone, which is named the Femur. Its upper extremity is bent at an angle; and its rounded head is separated from the rest by a narrow portion which is termed its neck. At the point where this neck joins the shaft of the bone, there are two large projections termed trochanters, one on the outer side and the other on the inner; these serve to give attachment to the muscles by which the thigh is moved. Of these muscles, one descends from the lumbar vertebræ, and passes-down with another that rises from the upper expanded surface of the pelvis, over the front border of the pelvis, to be attached to the smaller and interior of the projections just mentioned; these with the assistance of other muscles raise or draw forwards the thigh,—an action which does not require in Man to be performed with any great force. The muscles which draw back the thigh, on the other hand, arise from the under surface and back of the pelvis, where they form a very thick fleshy mass (11, 25, fig. 226); and they pass to the larger and external projection, and to a ridge which runs from it down the thigh-bone. Other muscles which arise from the lower border of the pelvis, serve to rotate the thigh upon its axis. The lower end of the thigh-bone spreads into two large condyles, on which the principal bone of the leg moves backwards and 486 BONES AND MUSCLES OF THE LOWER EXTREMITY. forwards. The knee is a good example of a pure hinge-joint, all its movements being restricted to one plane. 647. The leg, although containing two bones like the forearm, does not in Man possess the peculiar movement which characterises it. One of these bones, called the Tibia, is much larger than the other which is called the Fibula; and it is the former alone on which the thigh-bone rests, and which itself rests upon the foot, so that no movement of rotation is permitted in the leg. In fact, the fibula, which is a long slender bone running nearly parallel with the tibia (fig. 223), looks like a mere appendage or rudiment, and serves only for the attachment of muscles. The upper end of the tibia is broad, and has two shallow excavations, in which the condyles of the femur are received. Upon the front of the knee-joint we find a small separate bone, the patella or knee-pan; the purpose of this is to change the direction of the tendons that come down from the front of the thigh to be attached to the tibia; in such a manner as to enable them to act more advantageously, upon the principle formerly stated (§. 611). In the elbow-joint, this change was not required; since the ulna projects sufficiently far backwards to afford advantageous attachment to the tendon of the extensor muscle. -The very powerful muscles which tend to straighten the knee-joint, arise from the front of the pelvis and from the femur itself; and they form the fleshy mass of the front of the thigh. On the other hand, those which bend the knee arise from the lower border of the pelvis and from the back of the thigh-bone, and pass downwards to be inserted into the sides of the tibia and fibula a little below the knee, their tendons forming the two strong cords known as the hamstrings. The articulating surface at the lower extremity of the leg, which enters into the ankle-joint, is principally formed by the tibia; but its outer border is formed by the fibula, which there makes a considerable projection that can be felt through the skin. In the Quadrumana, and in a less degree in some other Mammals, the two bones of the leg resemble those of the fore-arm; and are so articulated as to give to the foot a power of rotation corresponding with that of the hand. 648. The Foot is composed, like the hand, of three distinct portions, which are called the tarsus, metatarsus, and phalanges. There are seven bones in the tarsus, all of which are larger BONES AND MUSCLES OF THE FOOT. 487 than those of the carpus, and some of them of considerable size. The articulation with the leg is formed by one of these only, the astragalus, which projects above the rest, and is imbedded between the projecting extremity of the tibia (which forms the inner boundary of the ankle-joint) and that of the fibula. The astragalus rests on the os calcis or bone of the heel, which projects considera bly backwards, and is connected in front with the other bones of the tarsus. In front of the tarsus we find the metatarsus, composed of five long bones, which in man are all attached to each other, but of which one is separate in the Quadrumana, in order to give freer play to the great toe, the action of which resembles that of the thumb. The toes, like the fingers, are composed of three phalanges (with the exception of the great toe, which has only two); these are in Man much shorter than those of the hand, and are evidently not adapted for prehension; but in many of the Quadrumana, their length is nearly equal to that of the fingers, and the great toe is as opposable as the thumb. The foot is far from being thus converted, however, into a perfect hand; but it becomes a very useful instrument for clasping the small branches and twigs of the trees among which these animals live. The foot of Man is distinguished from theirs, by its power of being planted flat upon the ground, and thus affording a firm basis of support. Even the Chimpanzee and the Orang, when they attempt to walk erect, rest upon Fig. 230,-MUSCLES OF THE SOLE OF 1, accessory of the long flexor of the 488 BONES AND MUSCLES OF THE FOOT. the side of the foot; and the absence of a projecting heel causes them to be very deficient in the power of keeping the leg upright upon it. For it is to this projection that the strong muscles of the calf of the leg are fixed, by which the heel is drawn upwards or the leg drawn back upon it. Other muscles at the side and back of the leg, the direction of whose tendons is changed by a sort of pulley at the ankle-joint, aided by the muscles of the foot itself, serve to bend the toes, -an action which gives great assistance in walking, running, leaping, &c. And the toes are straightened by an extensor muscle, which lies on the front of the leg, and of which the tendon runs under an annular ligament that encircles the ankle, and is then divided and spread - out to the toes, over the upper surface of the foot. The great toe is a very important instrument in the act of walking, since much of the spring forwards is given by the bending of its phalanges; and it is provided with two flexor muscles of its own. 649. On the internal side of the foot, the bones of the tarsus and metatarsus form a kind of vault or arch, which serves to lodge and protect the vessels and nerves that descend from the leg towards the toes. This arch further serves the important purpose of deadening the shock that would otherwise be experienced every time that the foot is put to the ground; for, by the elasticity of the ligaments which hold together the bones that compose it, a sort of spring is formed, which yields for a moment to the shock, and then recovers itself. We feel the difference which this makes, when we jump from a height upon our heels; the jar is then propagated directly upwards from the heel to the leg, thence to the thigh, and thence to the spinal column, and if it were not from the peculiar manner in which this is constructed (§ 631), a severe shock of this kind might produce fatal effects by concussion (or shaking) of the brain. In animals which walk upon four extremities, the difference of direction in which the legs are connected with the spine prevents a jar from being propagated along the latter to a similar degree. But in those which are destined to obtain their food by sudden and extensive leaps, such as the animals of the Cat tribe (the Lion, Tiger, &c.), we find an arrangement of the bones of the foot, well adapted to diminish the STANDING POSTURE:-EQUILIBRIUM. 489 shock produced by the sudden descent of the body upon the ground. Of the Attitudes of the body, and the various kinds of Locomotion. 650. A small number of Vertebrated animals,-Serpents, for instance, bear habitually on the whole length of their bodies, which rest entirely on the ground; and their only movements are effected by undulations of the spinal column. But the rest are supported upon their extremities; and we give the name of standing to that position in which the animal rests supported by its limbs upon the ground or on any firm horizontal basis. In maintaining this position, the extensor muscles, by which the joints are straightened, must be in continual action, since the limbs would otherwise bend beneath the weight of the body. Now as the sense of fatigue, in any set of muscles, depends in great degree upon the length of time during which they have been in continuous action, the maintenance of the standing posture for a long period is, in most animals, more fatiguing than walking; since in the latter exercise the action of the flexors alternates with that of the extensors. すす 651. But this condition is not the only one essential to steadiness in the standing posture; for in order that the body may rest firmly upon the members, it must be in equilibrium. It has been shown (MECHAN. PHILOS. Chap. IV.) that equilibrium exists, or in other words, that a body remains at rest in its position,—not only when it bears upon the whole of a broad surface, but also when it is so placed that the tendencies of its different parts to descend or gravitate towards the earth counterbalance each other. This is the case when its centre of gravity is supported,that is, when a line drawn perpendicularly from that centre falls within the base. In order, then, that an animal may rest in equilibrium on its legs, it is necessary that the vertical line from its centre of gravity (or line of direction) should fall within the space. Fig. 231. |