Section
The Skeletal System
2
When you hear the word skeleton, you may think of the remains
of something that has died. But your skeleton is not dead; it
is very much alive. Your bones are not dry and brittle. They
are just as alive and active as the muscles that are attached to
them. Bones, cartilage, and the special structures that connect
them make up your skeletal system.
Twaøsl~Lea~
skeletal system
compact bone
spongy bone
• ~iI~
cartilage
joint
ligament
What va v~
• Identify the major organs of the
skeletal system.
•Describe the functions of bones.
• Illustrate the internal structure of
bones.
• Compare three types of joints.
• Discuss how bones function as
levers.
The Burden of Being a Bone
Bones do a lot more than just hold you up. Your bones perform
several important functions inside your body. The names of
some of your bones are identified in Figure 3.
Skull
Protection Your heart and lungs are
shielded by your ribs, your spinal cord
is protected by your vertebrae, and
your brain is protected by your skull.
Ribs
Storage Bones store minerals that
help the nerves and muscles function
properly. Your arm and leg bones also
store fat that can be used for energy.
Radius
Ulna
Humerus
Movement Skeletal muscles pull on
the bones to produce movement.
Without bones, you would not be
able to sit, stand, walk, or run.
Patella
Pelvic girdle
/
Blood Cell Formation
Some of your bones are
filled with a special
material that makes
blood cells.
A
Femur
Tibia
Fibula
r
Vertebral
column
7-.
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Figure 3 The adult human skeleton has approximately 206
bones. Several major bones are identified in this skeleton.
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a
Chapter 22
r
* What’s in a Bone?
A bone may seem lifeless, but it is a living organ made of sev
eral different tissues. Bone is composed of connective tissue
and minerals that are deposited by living cells called osteoblasts.
If you look inside a bone, you will notice there are two
different kinds of bone tissue. If the tissue does not have any
visible open spaces, it is balled compact bone. Bone tissue that
has many open spaces is called spongy bone.
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Spongy bone provides most of the strength and
~‘ç~ support for a bone. It acts like the trusses of
a bridge.
Down to the Marrow Bones contain a soft
tissue called marrow. Red marrow, sometimes
found in spongy bone, produces red blood cells.
Yellow marrow, found in the central cavity of
long bones, stores fat. Tiny canals within the
compact bone conthin small blood vessels.
Figure 4 shows a cross section of a femur.
Marrow
Blood
vessels
—
Compact bone
Quick Lab
Pickled Bones
This activity lets you
see how a bone
changes when it is
exposed to an acid,
such as vinegar. Place
a clean chicken bone in a
jar of vinegar. After 1 week,
remove the bone and rinse
it with water. Make a list of
changes that you can see or
feel. How has the bone’s
strength changed? What did
the vinegar remove?
BRA
FOW
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A giraffe has the same
number of neck bones
as a human.
Figure 4 In children, red
marrow fills the center of
long bones. It is replaced
with yellow marrow by
adulthood.
J’..
Spongy bone
Cartilage
Body Organization and Structure
527
G:cowi~i~j ~Bne~
Did you know that most of your skeleton used to
be soft and rubbery? Most bones start out as a soft,
flexible tissue called cartilage. When you were born,
you had little true bone. But as you grew, the car
tilage was replaced by bone. During childhood,
growth plates of cartilage remain in most bones,
providing a place for those bones to continue to
grow.
Feel the end of your nose, or bend the top of
your ear. As shown in Figurt 3, some areas, like
these, never become bone. The flexible material
beneath your skin in these areas is cartilage.
wa~i~ ~ ~ ~4i
The skull and neck bones in this
computer-colored X ray are shown mostly
in blue.
The place where two or more bones connect is
called a joint. Your joints have special designs that
allow your body to move when your muscles con
tract. Some joints allow a lot of movement, while
other joints are fixed, which means they allow lit
tle or no movement. For example, the joints in the
skull are fixed. Joints that have a wide range of
movement tend to be more susceptible to injury
than those that are less flexible. Some examples of
movable joints are shown in ~Ogane
~.
g Joints are shaped according to their function in the body.
Sliding joint
Ball-and-socket joint
Hinge joint
.—
S.
A
Sliding joints allow bones in
the hand to glide over one
another, giving some flexibility
to the area.
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Chapter 22
Like a joystick on a computer
Like a hinge on a door, the
game, the shoulder enables
your arm to move freely in all
directions.
knee enables you to flex and
extend your lower leg.
Bone to Bone Joints are kept together with
strong elastic bands of connective tissue called
ligaments. If a ligament is stretched too far, it
becomes strained. A strained ligament will usu
ally heal with time, but a torn ligament will
not. A torn ligament must be repaired surgi
cally. Cartilage helps cushion the area where
two bones meet. If cartilage wears away, the
joint becomes arthritic.
First-class lever
The fulcrum lies between the load and
the effort.
Effort
Load
I
Fulcrum
Load
I
Cart Levers Le~en i&~ L©ad?
You may not think of your limbs as being
machines, but they are. The action of a muscle
pulling on a bone often works like a type of
simple machine called a lever. A lever is a rigid
bar that moves on a fixed point known as a
fulcrum. Any force applied to the lever is called
the effort. A force that resists the motion of the
lever, such as the downward force exerted by a
weight on the bar, is called the load or the resis
tance. Figut’e 7 shows how three types of levers
are used in the human body.
Effort
Second-class lever
The load lies between the fulcrum and
the effort.
Load Effort
I
L
Fulcrum
FigMre 7 There are three classes of
levers, based on the location of the
fulcrum, the load, and the effort
Effort
Load
I
Third-class lever
I.Describe four important functions of
bones.
2. Draw a bone, and label the inside and out
side structures. Use colored pencils to color
and label spongy bone, blood vessels, mar
row cavity, compact bone, and cartilage.
The effort lies between the fulcrum and
the load.
Effort
Load
I
Fulcrum
3. List three hinge joints in your body.
4. Interpreting Models Study the models of
levers pictured in Figure 7. Use a small
box (load), a ruler (bar), and a pencil (ful
crum) to create models of each type of
lever.
Load
Effort
I
Body Organization and Structure
529