194
UNIT TWO
The wall of the diaphysis is mainly composed of tightly
packed tissue called
compact bone
(kom
pakt bo
¯n), or corti-
cal bone. This type of bone has a continuous extracellular
matrix with no gaps
(f g. 7.3
a
). The epiphyses, on the other
hand, are largely composed of
spongy bone
(spunj
e bo
¯n),
or cancellous bone, with thin layers of compact bone on their
surfaces (F g. 7.3
b
). Spongy bone consists of many branching
bony plates called
trabeculae
(trah-bek
u-le). Irregular con-
necting spaces between these plates help reduce the bone’s
weight. The bony plates are most highly developed in the
regions of the epiphyses subjected to compressive forces. Both
compact and spongy bone are strong and resist bending.
A bone usually has compact bone overlying spongy
bone, with the relative amounts of each varying in the differ-
ently shaped bones. Short, fl
at, and irregular bones typically
consist of a mass of spongy bone either covered by a layer
of compact bone or sandwiched between plates of compact
bone (F
g. 7.3
c
).
Compact bone in the diaphysis of a long bone forms a
semirigid tube with a hollow chamber called the
medul-
lary cavity
(med
u-la
¯r
e kav
ı˘-te) that is continuous with the
spaces of the spongy bone. A thin membrane containing bone-
forming cells, called
endosteum
(en-dos
te
¯-um), lines these
Parts of a Long Bone
The femur, the long bone in the thigh, illustrates the struc-
ture of bone
(Fig. 7.2)
. At each end of such a bone is an
expanded portion called an
epiphysis
(e-pif
ı˘-sis) (pl.,
epi-
physes
), which articulates (or forms a joint) with another
bone. One epiphysis, called the proximal epiphysis, is near-
est to the torso. The other, called the distal epiphysis, is far-
thest from the torso. On its outer surface, the articulating
portion of the epiphysis is coated with a layer of hyaline car-
tilage called
articular cartilage
(ar-tik
u-lar kar
tı˘-lij). The
shaft of the bone, between the epiphyses, is called the
dia-
physis
(di-af
ı˘-sis).
A bone is enclosed by a tough, vascular covering of
F
brous tissue called the
periosteum
(per
e-os
te-um), except
for the articular cartilage on its ends. The periosteum is
F rmly attached to the bone, and the periosteal F
bers are con-
tinuous with connected ligaments and tendons. The perios-
teum also helps form and repair bone tissue.
A bone’s shape makes possible its functions. Bony
projections called
processes,
for example, provide sites for
attachment of ligaments and tendons; grooves and openings
are passageways for blood vessels and nerves; and a depres-
sion of one bone might articulate with a process of another.
(a)
(e)
(b)
(c)
(d)
Femur
Periosteum
Yellow marrow
Medullary cavity
Space containing
red marrow
Spongy bone
Compact bone
Articular cartilage
Epiphyseal plates
Proximal
epiphysis
Distal
epiphysis
Diaphysis
Endosteum
FIGURE 7.2
±Major±
parts of a long bone.
FIGURE 7.1
Bones are classiF
ed by shape. (
a
) The femur of the thigh
is a long bone, (
b
) a tarsal bone of the ankle is a short bone, (
c
) a parietal
bone of the skull is a ²
at bone, (
d
) a vertebra of the backbone is an
irregular bone, and (
e
) the patella of the knee is a sesamoid bone. The
whole-skeleton location icon highlights the bones used as examples for
classiF
cation.
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