he 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.

51. Synovial Membrane. A very delicate connective tissue, called the
synovial membrane, lines the capsules of the joints, and covers the
ligaments connected with them. It secretes the _synovia_, or joint oil, a
thick and glairy fluid, like the white of a raw egg, which thoroughly
lubricates the inner surfaces of the joints. Thus the friction and heat
developed by movement are reduced, and every part of a joint is enabled to
act smoothly.

52. Ligaments. The bones are fastened