The Skeletal System
The adult skeletal system is made up of 206 bones. Eight of those bones form the
axial skeleton or trunk, and the remaining 126 bones form the appendicular
skeleton or limbs.
You are born with more than 300 bones, but as you age, some of your bones fuse
together. The skeletal system protects your vital internal organs, gives form and
movement to your body and serves as a point of attachment for your muscles.
Between the bones is a softer elastic material called cartilage. This acts like a
shock absorber, preventing the bones from jarring when we move. Without your
bones, your body would be a blob.
Your body structure and height are determined partly by genetics and partly by diet
during your early years. Calcium is needed for bones to grow and vitamin D helps
calcium be absorbed. Your bones are about 20% water, 20% protein and 60%
minerals like calcium carbonate and calcium phosphate. These minerals give
bones their stiffness and their ability to resist pressing and squeezing.
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The Skeletal System
Bones are made up of an outer layer called
compact bone. Underneath the compact bone
is layer of spongy honeycomb layer of bones
called trabeculae. This layer of spongy bone
is also called cancellous bone.
The
cancellous bone is not hard and strong like the
outer compact bone, but it acts as a shock
absorber and allows your bones to handle
stress from many directions without breaking.
The porous honeycombed formed by the
trabeculae are filled with marrow. There are
two types of marrow – red marrow and yellow marrow. Red marrow contains the
cells that make new blood cells. Yellow marrow is mainly fatty tissue.
A thick
fibrous tissue called the periosteum covers the outer surface of the bones.
The bones of the skeletal system are divided into 4 categories: long bones, short
bones, flat bones and irregular bones.
Long bones are found in your thigh and upper
arm. The femur in your thigh accounts for ¼ of
your body weight.
They are strong and
designed to support the weight of your body.
The medullary cavity in the centre of the bone
is filled with marrow.
Short bones are a lot like long bones only
shorter; they do not have a medullary cavity.
Examples of short bones are the bones found
in your knees, wrists and ankles.
Your ribs, sternum and cranium are made of
flat bones. They are made of a layer of
spongy bone sandwiched between two
compact bone layers.
The vertebrae that form your spine, your pelvic
bones, the bones in your cheeks and your
clavicle or collarbones are irregular shaped
bones.
They are made of spongy bone
covered by a thin layer of compact bone.
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The Skeletal System
In some sporting activities, like basketball, it is good to be tall; however, in weightlifting it is better to have short arms and short legs. Successful marathon runners
are usually thin and often relatively short. Even bone length can affect sporting
performance. For example, in cycling, where your legs provide the power for
pedalling, your thigh bone works like a lever. If it is longer than your shin bone, it
will give more leverage on each pedal stroke.
The bones in your body that are joined by synovial joints form living mechanical
levers with the joints acting as a fulcrum. Levers provide a mechanical advantage
allowing us to lift things that would otherwise be too difficult to lift unassisted. All
three types of levers you have previously learnt about in physics are found in the
human body.
In a first class lever, the effort and the load are on opposite sides and at an equal
distance from the fulcrum. The effort presses down. When you nod your head up
and down, this living lever lifts the weight of your cranium.
The load of your body as you lift it up to take a step is easily lifted by a second
class lever. In a second class lever the load and effort are on the same side of
the fulcrum and the load is closest to the fulcrum. The ball of your foot forms the
fulcrum and your calf muscle provides the effort to lift your heel.
With a third class lever, the effort and load are on the same side of the lever and
the effort is closest to the fulcrum and the load is further away. When you lift
something with your hand, your elbow is the fulcrum and your arm the lever that lifts
the load of your hand.
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The Skeletal System
Label the parts of the skeleton below.
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The Skeletal System
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Down
1. connective tissue that covers the bone
2. a central cavity found in long bones that contains marrow
4. the portion of the skeleton that makes up the trunk of the body
5. your femur is a __________________ bone
7. spongy bone is also called __________________________ bone
10. your sternum is a _______________ bone
12. about 60% of bone is made of this
13. this layer of bone is very hard
Across
3. the soft elastic material between bones
6. the bones of your cranium are held together by this type of joint
8. this prevents friction between bones
9. the part of the skeleton that makes up the limbs
11. connective tissue that connects two bones to each other
14. the condyle of your mandible (jaw) does this with the temporal bone of your cranium
15. your ilium is an _______________________ bone
16. the type of joint found in your knuckles
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