[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