Covering, Support, and Movement of the Body
in sites that are subjected to both pressure and stretch, such as
the padlike cartilages (menisci) of the knee and the discs be-
tween vertebrae, colored red in Figure 6.1.
Growth of Cartilage
Unlike bone, which has a hard matrix, cartilage has a ﬂexible
matrix which can accommodate mitosis. It is the ideal tissue to
use to rapidly lay down the embryonic skeleton and to provide
for new skeletal growth.
Cartilage grows in two ways. In
un-al; “growth from outside”), cartilage-forming cells in
the surrounding perichondrium secrete new matrix against
the external face of the existing cartilage tissue. In
al; “growth from inside”), the lacunae-
bound chondrocytes divide and secrete new matrix, expanding
the cartilage from within. Typically, cartilage growth ends dur-
ing adolescence when the skeleton stops growing.
Under certain conditions—during normal bone growth in
youth and during old age, for example—cartilage can become
calciﬁed (hardened due to deposit of calcium salts). Note, how-
ever, that calciﬁed cartilage is not bone; cartilage and bone are
always distinct tissues.
Check Your Understanding
Which type of cartilage is most plentiful in the adult body?
What two body structures contain ﬂexible elastic cartilage?
Cartilage grows by interstitial growth. What does this mean?
For answers, see Appendix H.
Classiﬁcation of Bones
Name the major regions of the skeleton and describe their
Compare and contrast the four bone classes and provide
examples of each class.
±e 206 named bones of the human skeleton are divided into
two groups: axial and appendicular.
forms the long axis of the body and in-
cludes the bones of the skull, vertebral column, and rib cage,
shown in orange in Figure 6.1. Generally speaking these bones
protect, support, or carry other body parts.
u-lar) consists of the
bones of the upper and lower limbs and the girdles (shoulder
bones and hip bones) that attach the limbs to the axial skeleton
(colored gold in Figure 6.1). Bones of the limbs help us move from
place to place (locomotion) and manipulate our environment.
Bones come in many sizes and shapes. For example, the pisi-
form bone of the wrist is the size and shape of a pea, whereas the
femur (thigh bone) is nearly 2 feet long in some people and has
a large, ball-shaped head. ±e unique shape of each bone fulﬁlls
a particular need. ±e femur, for example, withstands great pres-
sure, and its hollow-cylinder design provides maximum strength
with minimum weight to accommodate our upright posture.
±e human skeleton is initially made up of cartilages and ﬁ-
brous membranes, but bone soon replaces most of these early
supports. ±e few cartilages that remain in adults are found
mainly in regions where ﬂexible skeletal tissue is needed.
Basic Structure, Types, and Locations
is made of some variety of
molded to ﬁt its body location and function. Cartilage con-
sists primarily of water, which accounts for its resilience, that
is, its ability to spring back to its original shape a²er being
±e cartilage, which contains no nerves or blood vessels, is
surrounded by a layer of dense irregular connective tissue, the
dre-um; “around the cartilage”). ±e
perichondrium acts like a girdle to resist outward expansion
when the cartilage is compressed. Additionally, the perichon-
drium contains the blood vessels from which nutrients diﬀuse
through the matrix to reach the cartilage cells internally. ±is
mode of nutrient delivery limits cartilage thickness.
As we described in Chapter 4, there are three types of carti-
lage tissue in the body: hyaline, elastic, and ﬁbrocartilage. ±e
skeletal cartilages include examples from all three. All three
types have the same basic components—cells called
, encased in small cavities (lacunae) within an
containing a jellylike ground substance and ﬁbers.
, which look like frosted glass when freshly
exposed, provide support with ﬂexibility and resilience. ±ey
are the most abundant skeletal cartilages. ±eir chondrocytes
are spherical (see Figure 4.8g), and the only ﬁber type in their
matrix is ﬁne collagen ﬁbers (which are undetectable micro-
scopically). Colored blue in
, skeletal hyaline carti-
, which cover the ends of most bones at
, which connect the ribs to the sternum
, which form the skeleton of the
) and reinforce other respiratory passageways
, which support the external nose
resemble hyaline cartilages (see Figure 4.8h),
but they contain more stretchy elastic ﬁbers and so are better
able to stand up to repeated bending. ±ey are found in only
two skeletal locations, shown in green in Figure 6.1—the exter-
nal ear and the epiglottis (the ﬂap that bends to cover the open-
ing of the larynx each time we swallow).
Highly compressible with great tensile strength,
consist of roughly parallel rows of chondrocytes alternating
with thick collagen ﬁbers (see Figure 4.8i). Fibrocartilages occur