Joints may be classified—(_a_) anatomically, according to their mode of development, the nature of the uniting media, and the form of the joint surfaces; (_b_) physiologically, with regard to the amount and kind of movement or the absence of mobility in them; (_c_) by a combination of the foregoing considerations. The classification of joints is still in a very unsatisfactory state, and, unfortunately, the same term is used in various senses by different authors. The two main subdivisions proposed by Hepburn are: (1) Those in which the uniting medium is coextensive with the opposed joint surfaces, and in which a direct union of these surfaces is thereby effected. (2) Those in which the uniting medium has undergone interruption in its structural continuity, and in which a cavity of greater or less extent is thus formed in the interior of the joint. This distinction is of considerable importance clinically. Three chief subdivisions of joints are usually recognized—viz., =synarthroses=, =diarthroses=, and =amphiarthroses=. SYNARTHROSES In this group the segments are united by fibrous tissue or cartilage, or a mixture of the two in such a manner as practically to preclude movement; hence they are often termed fixed or immovable joints. There is no joint cavity. Many of these joints are temporary, the uniting medium being invaded by the process of ossification, with a resulting ankylosis or synostosis. The chief classes in this group of joints are as follows: (1) =Suture.=—This term (Sutura) is applied to those joints in the skull in which the adjacent bones are closely united by fibrous tissue—the sutural ligament. In many cases the edges of the bones have irregular interlocking margins, forming the =sutura serrata=, _e. g._, the frontal suture. In others the edges are beveled and overlap, forming the =sutura squamosa=, _e. g._, the parieto-temporal suture. If the edges are plane or slightly roughened, the term =sutura harmonia= is applied to the joint, _e. g._, the nasal suture. (2) =Synchondrosis.=—In these the two bones are united by cartilage, _e. g._, the joint between the basioccipital and the sphenoid bone. Very few of these joints are permanent. (3) =Symphysis.=—This term is usually limited to a few median joints which connect symmetrical parts of the skeleton, _e. g._, symphysis pelvis, symphysis mandibulæ. The uniting media are cartilage and fibrous tissue. In some cases a cleft-like rudimentary joint cavity occurs. (4) =Gomphosis.=—This term is applied to the implantation of the teeth in the alveoli. DIARTHROSES These joints are characterized by the presence of a joint cavity and by their mobility. They are often called movable or true joints. The structures which enter into their formation are: