‘k2 ANTHROPOLOGY. 
are two kinds, sgywamosa, when the bevelled edge of one overlaps and rests 
on the other, as in the temporo-parietal suture ; and harmonia, where there 
is simple apposition, as seen in the union of most bones of the face. 
_ 6. Schindylesis. This second form of synarthrosis is where a thin plate 
of bone is received into a space or cleft formed by the separation of two 
laminze of another bone, as seen in the insertion of the azygos process of 
the sphenoid bone into the fissure on the superior margin of the vomer. 
c. Gomphosis. Here one bone is inserted into a cavity in another, just 
as a nail is driven into a board, or a tree implanted by its roots in the 
ground. The sole instance in the human subject is seen in the insertion of 
the teeth in the alveoli or sockets. 
d. Amphiarthrosis. This is an articulation where two plane or mutually 
adapted surfaces are held together by a cartilaginous or fibro-cartilaginous 
lamina of considerable thickness, as well as by external ligaments. By 
reason of the elasticity of the interposed lamina, the joint possesses a mani- 
fest though slight degree of motion. Examples of this form of joint are » 
found in the articulations between the bodies of the vertebz, that between 
the two bones of the pubes, and that between the ilium and sacrum. 
B. Diarrurosis. Evident mobility is the distinguishing characteristic 
of this class of joints; the articular surfaces are contiguous, each covered 
by a lamina of cartilage (diarthrodal cartilage) having either one or two | 
synovial sacs interposed. The integrity of the articulation is maintained 
by ligaments which pass from one bone to the other. The varieties are as 
follows : ) 
a. Arthrodia. Here the surfaces are plane, or nearly so: the motion is 
that of gliding, limited in extent and direction only by the ligament of the 
joint or by processes of the bones. Examples are seen in the articular 
processes of the vertebrz, in the radial, the carpal, the sterno-clavicular, 
and other joints. 
b. Enarthrosis. This, sometimes termed a ball and socket joint, is where 
the globular head of one bone plays in a cup-like cavity of another, or 
others. The ball is kept in place by a capsular ligament. Sometimes there 
passes a straight ligament from the head of the ball to be inserted into the 
bottom of the socket. An instance of this is seen in the attachment of the 
thigh bone to the pelvis. The shoulder affords a second instance of the 
ball and socket joint. 
c. Ginglymus or hinge joint. Here the articular surfaces are marked 
with elevations and depressions, which exactly fit into each other, so as to 
restrict motion in all but one line of direction. They are always provided 
with strong lateral ligaments, which are the chief bonds of union of the 
articular surfaces. Perfect examples of this articulation are furnished by 
the elbow and ankle joints. The knee also, and the phalangeal joints, are 
true ginglymi. 
d. Trecharthrosis. A pivot and a ring constitute the mechanism of this 
form of joint. The ring is generally formed partly of bone and partly of 
ligament; it sometimes moves on the pivot, and sometimes the pivot moves 
in it. The motion is confined to rotation, the axis of which is the axis of 
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