250
UNIT 2
Covering, Support, and Movement of the Body
8
Sutures
Sutures
, literally “seams,” occur only between bones of the skull
(Figure 8.1a)
. Te wavy articulating bone edges interlock, and
the junction is completely filled by a minimal amount of very
short connective tissue fibers that are continuous with the peri-
osteum. Te result is nearly rigid splices that knit the bones
together, yet allow the skull to expand as the brain grows dur-
ing youth. During middle age, the fibrous tissue ossifies and
the skull bones fuse into a single unit. At this stage, the closed
sutures are more precisely called
synostoses
(sin
0
os-to
9
sēz), lit-
erally, “bony junctions.” Because movement of the cranial bones
would damage the brain, the immovable nature of sutures is a
protective adaptation.
Syndesmoses
In
syndesmoses
(sin
0
des-mo
9
sēz), the bones are connected ex-
clusively by
ligaments
(
syndesmos
5
ligament), cords or bands of
fibrous tissue. Although the connecting fibers are always longer
than those in sutures, they vary quite a bit in length.
Te amount of movement allowed at a syndesmosis de-
pends on the length of the connecting fibers. If the fibers
are short (as in the ligament connecting the distal ends
of the tibia and fibula, Figure 8.1b), little or no movement
is allowed, a characteristic best described as “give.” If the
fibers are long (as in the ligament-like interosseous mem-
brane connecting the radius and ulna, Figure 7.28, p. 231), a
large amount of movement is possible.
Te
functional classification
is based on the amount of move-
ment allowed at the joint. On this basis, there are
synarthroses
(sin
0
ar-thro
9
sēz;
syn
5
together,
arthro
5
joint), which are
immovable joints;
amphiarthroses
(am
0
fe-ar-thro
9
sēz;
am-
phi
5
on both sides), slightly movable joints; and
diarthroses
(di
0
ar-thro
9
sēz;
dia
5
through, apart), or freely movable joints.
Freely movable joints predominate in the limbs. Immovable and
slightly movable joints are largely restricted to the axial skeleton.
Tis localization of functional joint types is understandable be-
cause the less movable the joint, the more stable it is likely to be.
In general, fibrous joints are immovable, and synovial joints
are freely movable. However, cartilaginous joints have both rigid
and slightly movable examples. Since the structural categories are
more clear-cut, we will use the structural classification in this dis-
cussion, indicating functional properties where appropriate.
Fibrous Joints
Describe the general structure of fibrous joints. Name and
give an example of each of the three common types of
fibrous joints.
In
fibrous joints
, the bones are joined by fibrous tissue, namely
dense fibrous connective tissue, and no joint cavity is present.
Te amount of movement allowed depends on the length of the
connective tissue fibers uniting the bones. Most fibrous joints
are immovable, although a few are slightly movable. Te three
types of fibrous joints are
sutures
,
syndesmoses
, and
gomphoses
.
Dense
fibrous
connective
tissue
Suture
line
Root of
tooth
Socket of
alveolar
process
Periodontal
ligament
Fibula
Tibia
Ligament
(a)
Suture
Joint held together with very short,
interconnecting fibers, and bone edges
interlock. Found only in the skull.
(b)
Syndesmosis
Joint held together by a ligament.
Fibrous tissue can vary in length, but
is longer than in sutures.
(c)
Gomphosis
“Peg in socket” fibrous joint. Periodontal
ligament holds tooth in socket.
Figure 8.1
Fibrous joints.
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