Hinge Joints. Here the opposing surfaces of the bones have elevations and depressions which fit into one another and allow of move ment only in one direction. The elbow, ankle, and knee joints are examples of this kind.
Pivot Joints. The best example of this is the joint between the first and second verte brm, the pivot being formed by the odontoid process of the axis, and the ring in which it is placed being provided by the atlas verte bra. The kind of movement permitted here is rotary.
Shifting Joints. In this last kind the amount of movement is restricted and amounts to only a slight gliding between the ends of the bones. The joints between the bones of the wrist and those between the bones of the ankle are good examples.
Besides the ligaments and muscles, a force tending to keep bones in joint is the .pressure of the outside air. The hip-joint, for example, is so completely surrounded by ligaments as to be airtight; and the union is a very strong one. If the ligaments be pierced and air al lowed to enter the joint, the union becomes at once much less close and the head of the femur falls away as far as the ligaments will allow it.
Joints allow of various kinds of movements, the chief of which are:— Angular movement, as when the bones so move as to forum an angle between them in the same line or plane. Angular movements include
those of flexion and extension, as when we bend or straighten the forearm on the arm at the elbow; adduction, as when the arm is brought to the side from the extended position; and abduction, when it is carried from the side towards the extended position.
Coaptation is the term applied when one surface glides over the other like a wheel rolling on the ground, so as to bring successive surfaces into contact. This is seen in the movement of the knee-pan on the lower end of the femur.
Circumduction occurs when the shaft of a long bone describes a cone, the point of which is in the joint, while the base and sides of the cone are described by the moving part, as ex emplified by the swinging of the arm when we attempt to make a circle in the air on one side of the body. The same movement can be de scribed by the leg, the joint involved being the ball-and-socket joint of the hip.
Rotation. In this case the bone moves round an axis, as seen in the movements of the atlas on the axis. (See p. 61\