One of the ankle bones, the _os calcis_, projects prominently backwards, forming the heel. An extensive surface is thus afforded for the attachment of the strong tendon of the calf of the leg, called the tendon of Achilles. The large bone above the heel bone, the _astragalus_, articulates with the tibia, forming a hinge joint, and receives the weight of the body. The metatarsal bones, corresponding to the metacarpals of the hand, are five in number, and form the lower instep. The phalanges are the fourteen bones of the toes,—three in each except the great toe, which, like the thumb, has two. They resemble in number and plan the corresponding bones in the hand. The bones of the foot form a double arch,—an arch from before backwards, and an arch from side to side. The former is supported behind by the os calcis, and in front by the ends of the metatarsal bones. The weight of the body falls perpendicularly on the astragalus, which is the key-bone or crown of the arch. The bones of the foot are kept in place by powerful ligaments, combining great strength with elasticity. Illustration: Fig. 24.—Right Tibia and Fibula (Anterior surface.) Illustration: Fig. 25.—Bones of Right Foot. (Dorsal surface.) The Joints. 50. Formation of Joints. The various bones of the skeleton are connected together at different parts of their surfaces by joints, or articulations. Many different kinds of joints have been described, but the same general plan obtains for nearly all. They vary according to the kind and the amount of motion. The principal structures which unite in the formation of a joint are: bone, cartilage, synovial membrane, and ligaments. Bones make the chief element of all the joints, and their adjoining surfaces are shaped to meet the special demands of each joint (Fig. 27). The joint-end of bones is coated with a thin layer of tough, elastic cartilage. This is also used at the edge of joint-cavities, forming a ring to deepen them. The rounded heads of bones which move in them are thus more securely held in their sockets. Besides these structures, the muscles also help to maintain the joint-surfaces in proper relation. Another essential to the action of the joints is the pressure of the outside air. This may be sufficient to keep the articular surfaces in contact even after all the muscles are removed. Thus the hip joint is so completely surrounded by ligaments as to be air-tight; and the union is very strong. But if the ligaments be pierced and air allowed to enter the joint, the union at once becomes much less close, and the head of the thigh bone falls away as far as the ligaments will allow it.