ARTICULATIONS IN GENERAL. 119 



between the bones. In some instances, as in that of the cranial bones 

 already referred to, the closeness of the apposition, the unevenness of the 

 fitting surfaces or edges, and the small amount and dense nature of the 

 intervening substance, are such as to admit of little or no perceptible 

 motion. In other instances the extremities of the bones are placed at such 

 a distance, and the intervening substance (ligament or cartilage) possesses so 

 much of a yielding quality, that bending or other motions may take place, 

 even while the bones are thus mediately but continuously united. But in 

 the greater number of the connections between the bones, and in those 

 which may be regarded as more properly deserving the name of joints, the 

 apposed surfaces of bone are not united either directly or mediately with 

 each other, but are free by solution of continuity, and are covered with 

 plates of smooth cartilage, the surfaces of which fit accurately together, 

 and the bones are held together by ligamentous structures placed in the 

 vicinity of the joints. In such articulations the bones are capable of gliding 

 or moving upon each other in various extent and directions, according to 

 the shape of the opposed cartilaginous surfaces, and the form and attach- 

 ments of the ligamentous and other bands which unite them. It is upon 

 distinctions such as those now adverted to that the various kinds of joints or 

 articulations have been brought under the three classes of SYNARTHROSIS, 

 AMPHIARTHROSIS, and DIARTHROSIS. 



Synarthrosis means direct or immediate union, and comprehends the 

 joints with little or no motion. It is found chiefly in various forms of 

 suture by which the bones of the head, excepting the lower jaw, are 

 united. The suture, properly so called, is serrated or dentated when the 

 contiguous margins of the bones are subdivided or broken up into projecting 

 points and recesses by which they fit very closely to one another, as in the 

 borders of most of the tabular bones of the cranium. The squamous or scaly 

 suture is that in which, as in the union of the temporal with the parietal 

 bone, the edges are thinned and bevelled, so that one overlaps the other 

 to a considerable extent. 



The term harmonia has been employed to denote simple apposition of compara- 

 tively smooth surfaces or edges, as in the case of the two superior maxillary bones; 

 and the term schindylesis has been used to express that kind of union in which one 

 bone is received into a groove in another, as occurs between the rostrum of the 

 spheuoid bone and the vomer. The impaction of the roots of the teeth in their 

 sockets has likewise been reckoned among the articulations, though with doubtful 

 propriety, and has been designated by the term gomphosis. 



Amphiarthrosis means the mixed articulation, or that in which there is 

 mediate union by some intervening substance, with partial mobility. The 

 articulations between the bodies of the vertebrae, that between the two ossa 

 pubis at the symphysis, and that between the two first pieces of the sternum, 

 may be taken as examples of this mode of connection. Some of the joints 

 of this kind pass on the one hand into synarthrosis, and on the other into 

 diarthrosis. 



Diarthrosis includes the complete joints with separate surfaces of the bones 

 and synovial cavities, and is attended with considerable yet varying degrees 

 of mobility. In this form of joint, plates of cartilage cover the articular parts 

 of the bones and present within the joint free surfaces of remarkable smooth- 

 ness, and these surfaces are further lubricated by the syuovial fluid secreted 

 from the delicate membrane which lines the fibrous coverings and all other 

 parts of the articulating cavity except the cartilage. This membrane is con- 

 tinuous with the margin of the articular cartilages, and along with the 



