The animal or organic materials that compose bone are about one-third of the bulk, the remainder being made up of inorganic constituents or earthy salts. The animal substances impart flexibility, and the earthy salts hardness, to the bone. The bones of children are softer and more elastic than those of older persons, and so are easily bent. In old age there is a preponderance of inorganic compounds, so that the bones are brittle and liable to fracture.

Composition of Bone.

The Femur, or bone of the thigh, is the largest bone in the body, and it transmits the weight of the body to the knees. Fig. 5 gives a sketch of the front view.

The Two Bones of the Leg, the tibia and the fibula, are different in their size, and are placed parallel to each other - the tibia on the inner side and the fibula on the outer (Fig. 6). The tibia, or shin-bone, the larger of the two, is triangular in section, and receives the weight of the body from the thigh-bone and carries it to the foot. Its direction is vertical, and in well-formed legs the two bones (tibiae) are parallel. At the top front portion it is very sharp and easily felt beneath the skin. The lower end is expanded across to form a joint with the astragalus. It is also, at the lower end, more forward than the fibula; so that the inner and outer ankle are not in the same transverse plane. The outer upper edge of the tibia gives origin to the Tibialis .anticus, and behind the Flexor longus digito-rum, and the Tibialis posticus.

The fibula, or clasp-bone, is situated on the outer side of the leg and a little behind the tibia. It is as long as the shin-bone, but more slender and does not sustain weight. At its upper end it is not level with the knee-joint, of which it forms no part, and at its lower end it is considerably below the tibia, forming the outer ankle (Fig. 6). The muscles that are associated with this bone are Peroneus Ion-gus, Extensor pro-prius pollicis, Flexor longus pol-licis.

Fig. 5.

Fig. 5.

The Bones of the Foot are twenty-six in number, and consist of three groups - the tarsal, metatarsal, and phalanges ; also termed ankle, foot, and toes respectively. The advantage of so many bones forming the foot, with a number of joints, is that motion and elasticity are increased, while the chances of dislocation or fracture are lessened. The bones of the tarsal region are short, thick, and compact. In front of them are the longer bones, that diverge a little as they run forward. The bones of the toe are mobile (Fig. 7).

The Tarsal Bones are seven in number - two backward, and the others anterior. Of the two posterior bones, the os calcis extends backwards, and forms the projection of the heel. . Above the heel-bone is the astragalus, which alone is united to the leg-bones. In front of the astragalus is the scaphoid, and still more forward are three cuneiform bones, to the outer side of which is situated the cuboid (Figs. 7, 8, and 9).

The Os Calcis, or calcaneum, is the largest and strongest of the tarsal bones, and transmits the weight of the body to the ground posteriorly. It also affords attachment for the tendon of the muscles of the calf. At its upper forward surface it supports the key-bone of the foot. It is sometimes thought that the negro has a longer heel than the white man; but the apparent lengthening is due to the smallness of the calf, rather than to any diminutive construction of the os calcis. This bone gives origin to the Extensor brevis digitorum. There is a deep groove running along its under surface for the tendon of the Flexor longus pollicis. Other muscles arising from this bone are - Abductor pollicis, Abductor digiti minimi, Flexor brevis digitorum. There are fixed to this bone three strong ligaments, to preserve the arch of the foot (Figs. 8 and 9).

Fig. 6.

Fig. 6.

The Astragalus, or huckle-bone, is the keystone of the arch of the foot The front portion of this important bone is received in the cavity of the tibia and fibula (Fig. 10).

Fig.7

Fig. 7.

Fig. 8.

Fig. 8.

The upper surface is one-fifth of an inch wider in front than behind, and this prevents dislocation backwards when running or jumping. There is an oblique groove which allows the tendon of the Flexor longus pollicis to run downwards and inwards. In front the convex head is received into a socket formed by the scaphoid, and below by the os calcis. On the inner side below a slightly elastic ligament is situated, filling up the gap left. This ligament mainly supports the arch of the foot, and gives its spring; and if it yields more than it should, down goes the arch, and the foot becomes flat (Figs. 8 and 9).

The Cuboid Bone is situated on the outer side of the foot. It has a groove on its under surface intended for the tendon of the long peroneal muscle that passes obliquely to the sole of the foot. It gives origin to two muscles, the Adductor pollicis and the Flexor brevis pollicis. The adductor arises from a sheath which bridges over the Peroneus longus groove.

The Scaphoid Bone is like a boat in form, and is placed on the inner side of the foot. It gives insertion to the tendon of the Tibialis posticus, which turns the foot inwards.

The Cuneiform Bones, three in number, are in front of the scaphoid, and anteriorly are met by the metatarsal bones. The cuneiform that is on the inner side of the foot is larger than the others, and gives, on its under side, insertion to two muscles that turn the foot inwards. It has a prominence that can be easily felt on the foot, and is sought after in taking measures.

The Metatarsal Bones, of which there are five, are long bones that are close together where they join the tarsal bones, but as they descend, separate slightly. The first, or inner bone, is short and strong, and supports the great toe. The second from the inner side is longer than the others, and should be specially noticed. In the skeleton of the foot, the bones just described form an arch that has two concavities - one from front to back, and the other across the foot.

The Phalanges of the toes, fourteen in number, are arranged three on each toe, with the exception of the great toe, which only has two, thereby giving greater power to the first toe. Under the first metatarsal bones are two small bones, termed sesamoid bones, which increase the leverage of the tendons that work the great toe.

The knowledge of the individual bones will be of little value, unless their relation be also studied as a whole.