248 THE ARTICULATIONS OR JOINTS. a } 
ni 
ossification, and thus the plane of articulation becomes obliterated, so that direct 
structural continuity between the osseous segments takes place. The primary 
features common to all synarthroses are—(@) continuous 
Pa) oe and direct union of the opposing surfaces; (0) no joint 
| cavity ; (¢) no movement. 
Teeacrenine Suture.—This form of synarthrosis is only found in 
“uyaline connexion with the bones of the skull. In a large 
i number of cases the bones which articulate by suture 
present irregular interlocking margins between which 
there is the interposed fibrous membrane to which refer- 
ence has already been made. When these interlocking 
margins present well-defined projections they are said to 
form true sutures—sutura vera; on the other hand, when 
Fic. 194. —Srcrron THrouch the opposed surfaces present ill-defined projections, or 
THE OCCIPITO-SPHENOID SYN- even flat areas, they are described as false sutures— 
CHONDROSIS. a - P Aan k LO 7 
sutura notha. In each of these subdivisions the particular 
characters of the articulating margins are utilised in framing additional descriptive 
terms. Thus true sutures may possess interlocking margins whose projections are 
tooth-like (sutura dentata), e.g. in the interparietal suture ; saw-like (sutura serrata), 
(Fig. 195) eg. in the interfrontal suture; ridge-like, or comparable 
to the parallel ridges on the welt of a boot (sutura lmbosa). 
Similarly false sutures may articulate by margins which are scale- 
like (sutura squamosa), e.g. in the squamoso-parietal suture; or by 
rough opposed surfaces (sutura harmonia), e.g. in the suture between 
the palate plates of the superior maxillary bones. There is one 
variety of synarthrosis which, in the adult, can scarcely be called a 
suture, although the differences are of minor importance, viz. 
schindylesis, which is an articulation between the edge of a plate- 
like bone, such as the rostrum of the sphenoid, and the cleft in 
another, such as the vomer. aie eee 
Synchondrosis.—IIlustrations of this group can only be found 1% eas ae 
in the young growing individual, because as age advances, and 
erowth ceases, the process of ossification affects the hyaline cartilage which con- 
stitutes the uniting medium, and the plane of articulation disappears. Under this 
heading we may include the planes of junction between all epiphyses and the 
shafts or diaphyses to which they severally belong. The occipito-sphenoid (Fig. 
194), and the petro-jugular articulations in the base of the skull provide other 
well-marked examples. 
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MOVABLE JOINTS. 
The leading features of this group are capability of movement and permanence. 
In very few instances do such joints ever become obliterated under normal con- 
ditions. Determining their permanence, and regulating the amount of possible 
movement, there is always more or less of interruption in the continuity of the 
structures which bind the osseous elements together. That is, there is always some 
evidence of a joint cavity, although as a matter of course such interruption can 
never be so extensive as to entirely disassociate the articulating elements. There- 
fore in all movable joints a new class of structures is found, viz. the ligaments, by 
means of which continuity is maintained even when all the other uniting media 
have given place to a joint cavity. The further subdivision of this group is founded 
upon the amount of movement permissible and the extent to which the joint cavity 
takes the place of the original continuous uniting medium. ‘Thus we obtain the 
partly movable or amphiarthroses, and the freely movable or diarthroses. 
An amphiarthrosis (Fig. 198) presents the following characteristics :— (a) 
partial movement; (>) union by ligaments and by an interposed plate or disc of 
fibro-cartilage, in the interior of which there is, (c) an incomplete or partial joint 
cavity which may be lined by a rudimentary synovial membrane whose function it 
is to secrete a lubricating fluid, the synovia or joint-oil; (d) a plate of hyaline 
