BIO2-LESSON17: Motor Mechanisms (Skeletal System)
Handout:
The coordinated efforts of muscles and skeletons are what make
animal movements possible. Muscles pull or push, and skeletons
give the muscles something to pull or push against. In this lesson
you will learn different types of skeletons and the fundamentals
of muscle functions.
Locomotion of Animals
Fish swim, dogs run, frogs jump, worms crawl, and birds fly. Each
of these types of locomotion, movement from one place to
another, requires animals to use energy to overcome the forces
of friction and gravity that would otherwise hold them to the
Earth.
Each different animal is adapted for the environment it lives in
and the type of locomotion it performs.
- Swimming animals, such as fish and whales, have bodies that
are shaped to minimize resistance as they move through the
water. They’re often coated with water-resistant mucus and may
have structures that make them more buoyant.
- Birds have hollow bones and wings that are shaped like those
of an airplane to create added lift during flight.
- Animals that walk or run on land, such as lions and elephants,
have a strong skeleton and muscles to support them against the
force of gravity.
- Rabbits, kangaroos, and other animals that jump or hop have
extra large leg muscles and strong tendons to help put some
spring in their hop.
- Animals that slither or crawl, think worms and snakes, have
smooth, tubular bodies to lessen resistance due to friction as
they move over or through soil.
Types of Skeleton
Skeletons support animals, give their bodies shape, and protect
their internal organs, but not all animals have the same type of
skeleton. Following are the three different kinds of skeletons
you may see in your study of biology:
- Hydrostatic skeletons: Found in creatures such as worms and
jellies, hydrostatic skeletons are basically chambers filled with
water. Animals with this skeleton type move and change their
shape by squeezing their water-filled chambers — just like what
happens when you squeeze a water balloon.
- Exoskeletons: These are exactly what they sound like —
skeletons on the outside of the body quite familiar
These hard exterior coverings because they’re found on crabs,
lobsters, and many insects. Exoskeletons are rigid and can’t
expand as animals grow, so animals must molt, or shed, their
exoskeletons periodically. After an animal molts, its new
exoskeleton is soft — as in a soft-shelled crab.
- Endoskeletons: The most familiar of all skeleton types is the
endoskeleton. An endoskeleton exists within an animal’s body.
The human endoskeleton is hard because it’s partially
constructed of the mineral calcium. The endoskeletons of other
animals may be more flexible — for example, the endoskeleton
of a shark is made of cartilage, the same material that makes up
the soft parts of your nose.
The Vertebral Skeleton
All vertebrate skeletons— whether they belong to humans,
snakes, bats, or whales — developed from the same ancestral
skeleton (which explains why you may notice similarities
between your skeleton and that of your pet dog due to
homologous structures — structures that are equivalent to each
other in their origin (see previous lesson for more on
homologous structures and their importance to the study).
All vertebrates’ skeletons, whether a man or an animal has two
main parts:
- The axial skeleton: This part supports the central column, or
axis, of the animal. The axial skeleton includes the skull, the
backbone (also called the vertebral column), and the rib cage.
The skull protects the brain, the backbone protects the spine,
and the rib cage protects the lungs and heart.
- The appendicular skeleton: This part extends from the axial
skeleton out into the arms and legs (which are also known as
appendages). It includes the shoulders, pelvis, and bones of the
arms and legs.
In some vertebrates, such as snakes, the appendicular skeleton
has become extremely reduced or nonexistent.
Bone Structure
Bone is actually a moist, living tissue that contains different
layers and cell types.
- Fibrous connective tissue covers the exterior of bones and
helps heal breaks in an injured bone by forming new bone.
- Bone cells, which are embedded in a bone matrix, give cells
their hard nature. The cells actually make the matrix, which
consists of collagen that has been hardened by the attachment
of calcium and phosphate crystals.
- Cartilage covers the ends of bones and protects them from
damage as they rub against each other. The tissues found within
living bone fall into two categories
✓ Spongy bone tissues are filled with little holes, similar to
those you see in volcanic rocks. These holes are filled with red
bone marrow, which is the tissue that produces your blood cells.
✓ Compact bone tissues are hard and dense. A cavity within
compact bone is filled with yellow bone marrow, which is mostly
stored fat. If the body suddenly loses a large amount of blood, it
converts the yellow bone marrow.
The Joints
Joints are structures where two bones are attached so that
bones can move relative to each other. Bones are held together
at joints by ligaments, which are strong, fibrous, connective
tissues.
Three different types of joints enable the many movements of
animals:
- Ball and socket joints consist of one bone, with a rounded, balllike end, that fits into another bone, which has a smooth, dishlike surface. Your arms and legs fit into your skeleton with ball
and socket joints, which is why you’re able to rotate your arms
and legs in all directions.
- Pivot joints allow you to swivel a bone. When you rotate your
arm so that your palm faces up, then down, then up again, you’re
using a pivot joint.
- Hinge joints allow you to bring two bones close together or
move them farther apart, much like you open and close a book.
Your elbows and knees have pivot joints that allow you to extend
and contract your arms and legs.