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.