CLASSIFICATION OF JOINTS. 219 



of the cranium and face, excepting those of the lower jaw. The varieties of synar- 

 throsis are four in number : Sutura, Schindylesis, Gomphosis, and Synchondrosis. 



Sutura (a seam) is that form of articulation where the contiguous margins of 

 flat bones are united by a thin layer of fibrous tissue. It is met with only in the 

 skull. Where the articulating surfaces are connected by a series of processes and 

 indentations interlocked together, it is termed sutura vera, of which there are 

 three varieties : sutura dentata, serrata, and limbosa. The surfaces of the bones 

 are not in direct contact, being separated by a layer of membrane continuous 

 externally with the pericranium, internally with the dura mater. The sutura 

 dentata. (dens, a tooth) is so called from the tooth-like form of the projecting 

 articular processes, as in the suture between the parietal bones. In the sutura 

 serrata (serra, a saw) the edges of the two bones forming the articulation are 

 serrated like the teeth of a fine saw, as between the two portions of the frontal 

 bone. In the sutura limbosa (limbus, a selvage), besides the dentated processes, 

 there is a certain degree of bevelling of the articular surfaces, so that the bones 

 overlap one another, as in the suture between the parietal and frontal bones. 

 When the articulation is formed by roughened surfaces placed in apposition with 

 one another, it is termed the false suture (sutura notha), of which there are two 

 kinds : the sutura squamosa (squama, a scale), formed by the overlapping of two 

 contiguous bones by broad bevelled margins, as in the squamo-parietal (squamous) 

 suture; and the sutura harmoma (bpfiovta, a joining together), where there is 

 simple apposition of two contiguous rough bony surfaces, as in the articulation 

 between the two superior maxillary bones or of the horizontal plates of the palate 

 bones. 



Schindylesis (a%wdutymz, a fissure) is that form of articulation in which a thin 

 plate of bone is received into a cleft or fissure formed by the separation of two 

 laminae in another bone, as in the articulation of the rostrum of the sphenoid and 

 perpendicular plate of the ethmoid with the vomer, or in the reception of the latter 

 in the fissure between the superior maxillary and palate bones. 



Gomphosis (yofjupoz, a nail) is an articulation formed by the insertion of a 

 conical process into a socket, as a nail is driven into a board ; this is not illustrated 

 by any articulation between bones, properly so called, but is seen in the articulation 

 of the teeth with the alveoli of the maxillary bones. 



Synchondrosis. Where the connecting medium is cartilage the joint is termed 

 a Synchondrosis. This is a temporary form of joint, for the cartilage becomes con- 

 verted into bone before adult life. Such a joint is found between the epiphyses 

 and shafts of long bones. 



2. Amphiarthrosis. Mixed Articulations. 



In this form of articulation the contiguous osseous surfaces are either connected 

 together by broad flattened disks of fibro-cartilage, of a more or less com- 

 plex structure, which adhere to the end of each bone, as in the articulation be- 

 tween the bodies of the vertebrae and the pubic symphyses. This is termed 

 Symphysis. Or, secondly, the bony surfaces are united by an interosseous liga- 

 ment, as in the inferior tibio-fibular articulation. To this the term Syndesmosis 

 is applied. 



3. Diarthrosis. Movable Articulations. 



This form of articulation includes the greater number of the joints in the body, 

 mobility being their distinguishing character. They are formed by the approxi- 

 mation of two contiguous bony surfaces covered with cartilage, connected by 

 ligaments and lined by synovial membrane. The varieties of joints in this class 

 have been determined by the kind of motion permitted in each. There are two 

 varieties in which the movement is uniaxial ; that is to say, all movements take 

 place around one axis. In one form, the Ginglymus, this axis is, practically 

 speaking, transverse; in the other, the trochoid or pivot-joint, it is longitudinal. 

 There are two varieties where the movement is biaxial, or around two horizontal 



