Chapter 8
Joints
253
8
minimizing wear and tear on the joint surfaces. Besides the knees,
articular discs occur in the jaw and a few other joints (see nota-
tions in the Structural Type column in Table 8.2).
Bursae and Tendon Sheaths
Bursae and tendon sheaths are not strictly part of synovial joints,
but they are oFen found closely associated with them
(Figure 8.4)
.
Essentially bags of lubricant, they act as “ball bearings” to reduce
friction between adjacent structures during joint activity.
Bur-
sae
(ber
9
se; “purse”) are flattened fibrous sacs lined with synovial
membrane and containing a thin film of synovial fluid. ±ey occur
where ligaments, muscles, skin, tendons, or bones rub together.
A
tendon
sheath
is essentially an elongated bursa that wraps
completely around a tendon subjected to friction, like a bun around
a hot dog. ±ey are common where several tendons are crowded
together within narrow canals (in the wrist region, for example).
these fibers detect pain, as anyone who has suffered joint injury
is aware, but most monitor joint position and stretch. Moni-
toring joint stretch is one of several ways the nervous system
senses our posture and body movements (see p. 487). Synovial
joints are also richly supplied with blood vessels, most of which
supply the synovial membrane. ±ere, extensive capillary beds
produce the blood filtrate that is the basis of synovial fluid.
Besides the basic components described above, certain syno-
vial joints have other structural features. Some, such as the hip
and knee joints, have cushioning
fatty pads
between the fibrous
layer and the synovial membrane or bone. Others have discs or
wedges of fibrocartilage separating the articular surfaces. Where
present, these
articular discs
, or
menisci
(mĕ-nis
9
ki; “crescents”),
extend inward from the articular capsule and partially or com-
pletely divide the synovial cavity in two (see the menisci of the
knee in ²igure 8.8a, b, e, and f). Articular discs improve the fit
between articulating bone ends, making the joint more stable and
Table 8.1
Summary of Joint Classes
STRUCTURAL CLASS
STRUCTURAL CHARACTERISTICS
TYPES
MOBILITY
Fibrous
Bone ends/parts united by collagen fibers
Suture (short fibers)
Immobile (synarthrosis)
 
 
Syndesmosis (longer fibers)
Slightly mobile (amphiarthrosis)
and immobile
 
 
Gomphosis (periodontal ligament)
Immobile
Cartilaginous
Bone ends/parts united by cartilage
Synchondrosis (hyaline cartilage)
Immobile
 
 
Symphysis (fibrocartilage)
Slightly movable
Synovial
Bone ends/parts covered with articular
cartilage and enclosed within an articular
capsule lined with synovial membrane
1. Plane
4.
Condylar
2. Hinge
5.
Saddle
3. Pivot
6.
Ball and socket
Freely movable (diarthrosis;
movements depend on design
of joint)
Acromion
of scapula
Joint cavity
containing
synovial fluid
Synovial
membrane
Fibrous
layer
Humerus
Articular
cartilage
Subacromial
bursa
Fibrous layer of
articular capsule
Tendon
sheath
Tendon of
long head
of biceps
brachii muscle
(a) Frontal section through the right shoulder joint
(b) Enlargement of (a), showing how a bursa eliminates friction
where a ligament (or other structure) would rub against a bone
Humerus moving
Bursa rolls
and lessens
friction.
Humerus head
rolls medially as
arm abducts.
Figure 8.4
Bursae and tendon sheaths.
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