Joint. A joint or articulation is the term applied to the means of connection between two distinct portions of the animal skeleton. The ends of the bones entering into the formation of a joint are covered with cartilage, and are united to one another by ligaments, while, surrounding and enveloping the apposed surfaces in the case of movable joints, there is what is known as the synovial sac containing synovial fluid, which facilitates the gliding of the one surface upon the other. A joint which does not allow of any movement is termed a synarthrosis or synchondrosis. Movable joints are classified as amphiarthrosis (symphysis) and diarthrosis; the mobility in the former being only partial, while in the latter it is considerable. Diarthrosis includes the more familiar forms of joint, such as the hinge joint or ginglymus, e.g. the elbow and ankle; the ball and socket-joint, e.g. the hip and shoulder; the gliding joint, e.g. the articulations of the wrist; the pivot joint, e.g. that between the first two vertebrae; and the condyloid joint, e.g. those between the wrist-bones and the first phalanx of the fingers.
Diseases of Joints. Synovitis is inflammation of the synovial membrane of a joint. It may be acute or chronic. In the acute form the inflammatory effusion may become purulent, leading to the formation of an abscess in the joint. Chronic synovitis may result from injury or be associated with rheumatism, gout, syphilis, etc.
Arthritis is the term applied when inflammation affects all the structures entering into the composition of a joint. An acute form of arthritis is met with in young children. Arthritis may be of tubercular origin (Hip-disease), and a common form of joint inflammation is what is known as chronic rheumatic arthritis or osteo arthritis.
Loose bodies arc sometimes met with in joints, the knee being most frequently affe'.ted; they may consist of pieces of cartilage which have become detached, or may originate in a hypertrophied portion of the synovial membrane. Their treatment usually calls for surgical interference.
For the stiffening of joints see ankylosis.
Excision of a joint is an operation which has been frequently performed of recent years in cases in which amputation of the affected limb would have been formerly deemed necessary. Joints, in Geology, are divisional planes occurring in many rocks, both aqueous and igneous, independent of any original stratification, but of the highest practical importance as facilitating quarrying. Thin beds free from joints are known as flagstones. The joints in sedimentary rocks are generally in two sets, those of each set parallel, but the two sets at right angles to one another and to the bedding-planes of the rock. Two joints of each set and two planes of bedding thus form six sides of a cuboidal block of stone, and a rock so divisible is termed a freestone. Among inclined rocks one set of joints is commonly parallel with the strike (q.v.) and the other with the dip, and they arc known as strike- and dip-joints respectively. One set, commonly the strike-joints, is often more strongly marked, more gaping, than the other, and is known to quarrymen as the master-joints, the others being called cutters. In coal-mining the main galleries are generally carried along the master-joint, face, or elect of the coal, in which direction it has a smooth, polished surface, the cross-galleries being along the less strongly jointed ends of the coal, which appear broken. Even hand specimens of coal exhibit a cuboidal form, four faces formed by these joints and two by the powdery, flaking bedding-planes, sometimes bearing fossil leaves. A thick-bedded limestone in which the joint-planes are not sharply cut is termed a ragstone. Such jointing in aqueous rocks seems to be the result of strains set up during upheaval or folding. The production of rectangular jointing by strain has been experimentally illustrated by M. Daubree by wrenching thick plates of glass. Though known by the same name, it is probable that the joints in igneous rocks are quite distinct in mode of origin, they being apparently entirely the result of shrinkage during cooling. The most remarkable joints among igneous rooks are those in basalts, which divide them into very regular columns perpendicular to their surfaces of cooling. In the sheets of Antrim, Fingal's Cave, or Idaho, for instance, this columnar jointing is vertical; but in dykes the columns arc often horizontal, two sets having clearly originated one from each cooling surface. They are often, but by no means always, six-sided, being seemingly due to the intersection of three sets of joints at angles of about 120°; but they are frequently also intersected by a fourth set, parallel to the surface, and have sometimes an elaborate ball-and-socket articulation at the intersections. The whole of these structures is believed to be explicable as the result of the cooling of a rock not perfectly homogeneous from an extended surface. In addition to their importance in quarrying, joints largely determine the direction of percolating water and consequent weathering. Limestone caverns are often dissolved out along lines of joint: frost, acting along similar lines, detaches masses from cliffs; and the granite tors of Devon and Cornwall are similarly produced. Weather or sea acting mainly along one set of joints may form buttresses, as in the Carboniferous Limestone of Derbyshire dales; or, subsequently acting along the other, may convert such buttresses into pinnacles or sea-stacks, as in the Saxon Switzerland or off the coast of Caithness.