JOINT (OF. joint, joiner, from Lat. junctus, joint. from janyere, to join; connected with Ck. i'etryveivat, zeugnynoi, Skt. yej, to Join, mind ulti mately with Eng. yoke), or AwrieutarioN. In anatomy, the connection existing between any of the denser component parts of the skeleton, whether bone or cartilage. The structures which enter into the formation of the more complex joints are bone, ligament, cartilage, fibro-car tilage, and synovial membrane. Bone is the fundamental part of all joints. Ligament is very generally the bond of union between bony seg ments. Cartilage (either articular, costal, or mamhraoiforro) is found as a layer between sur faces or an incrustation upon the opposing end3 of bones or as an extension of a bone to reach a given point. Fihro-cartilage is either disposed about time circumference of an articular cavity in order to deepen it. or is connective, or separa tive. Synovial membranes, when articular, exist as thin saes containing a. little (humid wherewith the joints are lubricated from their position be tween the opposing bones. Joints vary in their degree of motion. They are arranged in three classes: (1) Synarthrosis, or inuuovahle joint, of which there are four varieties. (2) Amphior throsis. or joint of limited motion, also called •synchondrosis' or 'symphysk.' of which there arc three types. (3) Diarthrosis, or freely' movable joint, of which there are three varieties.
Synarthrosis is divided into: (a) Sutura. in which there is a union of bone by a series of processes and indentations which fit closely to gether. There are three types of sutura: viz. sntura dentata, where the indentations are large and extensive, as in the joints between the parietal bones of the skull; sutura serrata, where the dovetailing is smaller and more regu lar. as in the suture between the frontal bones in youth; and suture limbosa, where there is an overlapping of beveled articular surfaces, as in the joint between the temporal bone and the parietal. (b) Ilarmonia, in which there is a
mere cogitation of two rough bony surfaces, as 1 etween the two halves of the upper jaw. (cm Sehindylesis, in which a thin plate of bone is inserted into a cleft, as where the rostrum of the sphenoid articulates with the vomer. (d) Comphosis, in which a Lone is implanted firmly into a socket of another bone, as where the teeth are inserted into the alveolar processes of the jaw.
The three varieties of diarthrosis are: (a) Enarthrosis, or ball-and-socket joint, as in the ease of the head of the humerus in the glenoid cavity of the scapula ; (b) arthrodia. including all articulations of flat or nearly flat surfaces. as in the joint between the aeromion process of time scapula and the clavicle: and (c) ginglymus. or hinge-joint, as in the articulation of the bones of the knee, elbow, and ankle. The joints admit ting of angular, lateral, or rotary motion are all called *ginglynms.' Some anatomists call articu lation that admits of rotary motion diarthrosis rotatorius.
Amphiarthrosis includes synehondrosis or union cartilage, syndesmosis, or union by ligament, and syssareosis, or union by muscular tissue. As an example of synehondrosis, the union between the two bones of the pelvis, in front. is excellent. This is generally termed the •symphysis pubis.' Joints admit of the following Ni.rielies of motion: Flexion, extension, addue thm, abduction, rotation. cireumduction, and gliding movement. The binge-joints and ball-and socket joints are most. useful in securing flexion and extension; the ball-and-soeket joints in secur ing adduction, abduction, eircumduction, and rotation; while arthrodial joints alone secure gliding movement.