joint stock company: see chartered companies.

joint, in anatomy, juncture between two bones. Some joints are immovable, e.g., those that connect the bones of the skull, which are separated merely by short, tough fibers of cartilage. Movable joints are found for the most part in the limbs. Hinge joints provide a forward and backward motion, as at the elbow and knee. Pivot joints permit rotary movement, like the turning of the head from side to side. Ball-and-socket joints, like those at the hip and shoulder, allow the greatest range of movement, as the rounded end of one bone fits into the hollow or socket of another bone, separated by elastic cartilage. Joints can further be classified as fibrous, cartilaginous, and synovial. Collagen fibers connect fibrous joints. Synovial joints ease movement through the use of a lubricating liquid, supplied by the synovial membrane that lines movable joints. In synovial joints, a cushioning sac known as a bursa contains the fluid, which lubricates and nourishes the joint. Those joints which lack synovial fluid are nourished by blood. Holding the joints in place are strong ligaments fastened to the bones above and below the joint. Joints are subject to sprains and dislocations, as well as to infections and disorders caused by such diseases as arthritis. In recent years, the use of artificial joints has become increasingly common, particularly in hip and knee replacement. Many orthopedic surgeons now perform operations of this sort, using metal or plastic replacement joints in order to relieve pain, or to prevent or correct joint deformity.

joint, in geology, fracture in rocks along which no appreciable movement has occurred (see fault). Nearly vertical, or sheet, joints that result from shrinkage during cooling are commonly found in igneous rocks. Similar joints occur in thick beds of sandstone and gneiss, with the sheets resembling the structure of a sliced onion. The prismatic joints of the Palisades of New Jersey and Devil's Tower, Wyoming, are examples of joints caused by contraction during the cooling of fine-grained igneous rock masses. Deep-seated igneous rocks often have joints approximately parallel to the surface, suggesting that they formed by expansion of the rock mass as overlying rocks were eroded away. Some joints in sedimentary rocks may have formed as the result of contraction during compaction and drying of the sediment. In some cases, jointing of the rock may result from the action of the same forces that cause folds and faults. In relatively undisturbed sedimentary rocks, such joints are often in two vertical sets perpendicular to one another. Commonly, streams develop along zones of weakness caused by joints in rocks, and thus the regional pattern of joint orientation often exerts a strong control on the development of drainage patterns.

ball-and-socket joint, in engineering, mechanical connection used between parts that must be allowed some relative angular motion in nearly all directions. As the name implies, the joint consists essentially of a spherical knob at the end of a shaft, with the knob fitting securely into a mating socket. Like other mechanical joints, a ball-and-socket joint must have some provision for lubrication and is normally provided with a seal to prevent loss of the lubricant. Joints of this type are commonly used in mounting the front wheels of automobiles, allowing these wheels movement sufficient for steering. In this application they are usually called ball joints.

ball joint: see ball-and-socket joint.

Joint Chiefs of Staff, U.S. statutory agency, created in 1949 within the Dept. of Defense. The chairman is the principal military adviser to the President, the National Security Council, and the Secretary of Defense. Members include the chairman, appointed by the President with Senate approval; the chief of staff, U.S. army; the chief of naval operations; the chief of staff, U.S. air force; the commandant of the U.S. Marine Corps, and a vice chairman who manages the Joint Staff. The Joint Chiefs prepare military plans and direct unified and other combat commands under the Secretary of Defense.

In architecture, the connection of construction materials. All joints are carefully detailed by the architect with concern for strength, movement, penetration by the elements, and incompatibilities. The term joinery refers especially to carpentry. Common types of joints include the dovetail, used for interlocking two flat members at right angles, as in the sides of a drawer; the doweled joint, in which doweling is employed for mechanical strength; and the mortise and tenon, in which a projecting piece fits into a groove, used to join a horizontal member with the vertical member of a frame.

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or osteoarthrosis or degenerative joint disease

Most common joint disorder, afflicting over 80percnt of those who reach age 70. It does not involve excessive inflammation and may have no symptoms, especially at first. Cartilage softens and wears away, and bone grows in its place, distorting the joint's surface and causing pain, stiffness, and limited movement, usually in weight-bearing joints (vertebrae, knees, hips). Treatment may include analgesics, rest, weight loss, corticosteroids, and/or physical medicine and rehabilitation or an exercise program. Hip or knee replacement or surgical removal of unhealthy tissue may be needed.

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