Immovable joints, or _synarthroses_, are usually adaptations to growth rather than mobility, and are always between bones. When growth ceases the bones often unite, and the joint is then obliterated by a process known as _synostosis_, though whether the union of the bones is the cause or the effect of the stoppage of growth is obscure. Immovable joints never have a cavity between the two bones; there is simply a layer of the substance in which the bone has been laid down, and this remains unaltered. If the bone is being deposited in cartilage a layer of cartilage intervenes, and the joint is called _synchondrosis_ (fig. 1), but if in membrane a thin layer of fibrous tissue persists, and the joint is then known as a _suture_ (fig. 2). Good examples of synchondroses are the epiphysial lines which separate the epiphyses from the shafts of developing long bones, or the occipito-sphenoid synchondrosis in the base of the skull. Examples of sutures are plentiful in the vault of the skull, and are given special names, such as sutura dentata, s. serrata, s. squamosa, according to the plan of their outline. There are two kinds of fibrous synarthroses, which differ from sutures in that they do not synostose. One of these is a _schindylesis_, in which a thin plate of one bone is received into a slot in another, as in the joint between the sphenoid and vomer. The other is a peg and socket joint, or _gomphosis_, found where the fangs of the teeth fit into the alveoli or tooth sockets in the jaws. [Illustration: FIG. 3.--Vertical section through an amphiarthrodial joint. b, b, the two bones; c, c, the plate of cartilage on the articular surface of each bone; Fc, the intermediate fibro-cartilage; l, l, the external ligaments.]