126 ARTHROLOGY. 



CLASSES OF JOINTS. 



Joints may be divided into three classes — Immovable or 

 Synartbi'odial, Movahle or Diarthrodial, and Mixed or Amphi- 

 arthrodial. 



SYNARTHROSIS. 



In an immovable joint there is only a thin layer of fibrous or 

 cartilaginous material interposed between the bones, the fibrous 

 layer of the periosteum of both bones uniting to cover and 

 become attached to the connecting material, thus serving as a 

 ligament. If the connection is fibrous, the joint is generally 

 called a suture ; if cartilaginous, a synchondrosis. These joints 

 are found chiefly, but by no means solely, in the skull ; and they 

 tend to become obliterated, in the adult, by ossification of the 

 connecting material. The varieties of synarthrosis are the sutura, 

 synchondrosis, schindylesis, and gomphosis. 



Sutures are true or false. In the sutura vera or true suture, 

 the contiguous margins are united by a series of interlocking 

 jDrocesses and indentations, a thin fibrous layer being interposed, 

 connected externally with the periosteum. Variety of shape has 

 led to the following nomenclature : — sutura dentcda, where the 

 processes are large and tooth-like, as in the interparietal ; sutura 

 serrata, where they are small and fine like the teeth of a saw, 

 as in the interfrontal ; and sutura limhosa, where the contiguous 

 ]3arts are dentated and also bevelled, as in the parieto-occipital — 

 the sutura lamhdoidcdis of the human subject. 



In the false sutures, or sutura notha, the bones are joined 

 by plain rough surfaces, of which there are two forms — sutura 

 squamosa, where the adjacent borders are bevelled, the edge of 

 one bone resting on and overlapping the other, as in the parieto- 

 temporal ; and sutura harmonia, where the articulating surfaces 

 of two bones present no marked irregularity, as the nasal and 

 premaxilla. 



Synchondrosis, as already stated, resembles a suture, but the 

 connecting medium is cartilage instead of fibrous tissue ; examples 

 are found in the joints between the basi-occipital and basi-sphenoid 

 bones, and between the latter and the pre-sphenoid. 



Schindylesis is that form of immovable articulation, where a 

 ridge or plate of one bone is received into a slit or fissure in 

 another, as the orbito-sphenoid into the incisura sphenoidalis of 

 the frontal bone. 



