Musculoskeletal Systems in the Animal Kingdom

Animals differ from plants in that animals can move from one place to another. Two systems that work together make this possible. These are the muscular and skeletal systems. When combined, we call it the musculoskeletal system.

Exoskeletons

Animals have different types of musculoskeletal systems. Arthropods, or jointed animals, include insects and spiders as well as crustaceans such as crabs and lobsters. These animals have an exoskeleton. This means that some form of hardened shell, or an exterior (exo) skeleton, covers their interior parts. Specialized skin cells produce these shells in a similar way that our nails grow. Exoskeletons are made of chitin, a type of complex sugar.

Molluscs, such as snails and clams, also have an exoskeleton. It’s their shell! The shells of molluscs contain calcium carbonate (CaCO₃). This helps to reinforce the shells. The disadvantage of using calcium carbonate is that it can dissolve in warm or acidic waters. This is the reason why coral reefs and molluscs are sensitive to ocean acidification.

One of the major drawbacks of exoskeletons is that they need to be replaced as the animal grows. This is done through the process of moulting. During moulting the animal is not well protected because the new exoskeleton is not hard like the old skeleton. The skeleton hardens over time.

Hydrostatic Skeletons

A second type of musculoskeletal system is a hydrostatic skeleton. This is a system used by jellyfish, octopuses, and earthworms. In this system, water pressure known as hydrostatic pressure is used to make different parts of the body become harder or softer. They do this by pumping water into a compartment called the coelom using muscular “water pumps.” These water pumps are also what move the suction cups of octopuses!

Did you know?

Hydrostatic pressure is not just for worms and octopuses. Your own tongue also uses hydrostatic pressure to move! 

The third and final type of musculoskeletal system is the one you should be familiar with. Endoskeletons are located inside the body. The word “endo” means inside. Scientists consider that a few animals, like starfish, also have an endoskeleton because skin covers skeletal elements. However, most animals bearing endoskeletons are vertebrates, animals with a backbone.

Vertebrate Endoskeletons

Vertebrate animals are a group that include fish, amphibians, reptiles, mammals and birds. All of these animals have skeletons made from bone, except for cartilaginous fishes like sharks and rays, whose skeletons are made from cartilage.

The skeleton has several important jobs. One job is to provide protection for internal organs. A good example of this is your rib cage. The bones of your rib cage protect your heart and lungs as well as part of your liver and stomach. Can you think of other bones that protect important body parts?

Another important job of the skeleton is to provide support for the body and allow for movement with the help of muscles attached to bones.

Skeletons and Evolution

If you have ever been to a natural history museum, you have probably seen animal skeletons. Have you ever wondered why people are so interested in skeletons? Studying animal skeletons is a big part of understanding the evolution of species. It also helps scientists to classify different groups of animals. For some extinct species, like dinosaurs, fossilized skeletons are all that remain of them. Comparing fossil skeletons to the skeletons of living animals helps us to understand some pretty interesting things, such as the evolution of birds from dinosaurs!

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Photograph of a display of primate skeletons at the Museum of Osteology in Oklahoma City. On a model of a tree are skeletons of many small tree-climbing primates posed as if they were climbing, sitting or hanging from the tree.

Parts of a Vertebrate Skeleton

Even though vertebrae animals come in many shapes and sizes, their skeletons have parts in common.

Vertebrates all have bony skulls to protect their brains. Most also have jaws that work together with bones in the skull to help them get and process food. Skulls and jaws sometimes, but not always, contain teeth.

The bottom of the skull connects with the first bone in the spine.

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Drawing of a human skull. The cranium and facial bones are show separately. The adult cranium, facial bones and mandible are made up of 22 bones.

 Did you know?

The smallest bone in the human body is the staple, which is found in the inner ear.

All vertebrates have a column of bones along their backs called vertebrae. This is where the name of this group of animals comes from! We call this group of vertebrae the vertebral column or spine. Vertebrae help to protect the spinal cord, which is part of the nervous system.

The vertebrae can be classified according to where they are located.

Cervical vertebrae are located in the neck area.

Below the cervical vertebrae are the thoracic vertebrae. Ribs as well as the bones of the shoulder connect to these vertebrae.

Below these are the lumbar vertebrae. We call this the lower back.

Below this is the sacrum. In humans, this is a triangular bone that is formed when five vertebrae fuse together during early adulthood. The bones of the pelvis connect to the sacrum.

Forming the end of the spine are the tail bones. Humans have a coccyx that is formed from four fused bones. Instead of having a coccyx, animals with tails have more vertebrae called caudal vertebrae. 

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Drawing of the human vertebral coumn showing the locations of the cervical vertebrae, thoracic vertebrae, lumbar vertebrae, sacrum and coccyx. Humans have 33 bones in their vertebral column.

 Did you know?

Mammals, including giraffes and whales, all have 7 vertebrae in their necks. Manatees and sloths are the exception. They tend to have one or two more neck vertebrae.

As you learned earlier, rib bones help to protect important internal organs. They are also an attachment point for many muscles.

Humans have 12 rib bones on each side of the body. The 7 bones closed to the head are called true ribs. These bones connect directly to a bone called the sternum. We usually call this the breast bone. The other 5 ribs are called false ribs because they only indirectly connect to the sternum. Ribs 11 and 12 are called floating ribs because they do not connect with the sternum at all.

 Most, but not all vertebrates have limbs.

Can you think of a vertebrate that does not have limbs? If you thought snakes, you would be right!

Forelimbs, also known as front legs or arms, are connected to the upper part of the spine.

All vertebrates have the same bones in their forelimbs. These are the humerus, radius, ulna, carpals, metacarpals and phalanges. They are the bones of the arm, wrist and hand.

Even though they have the same bones, their forelimbs can look very different. These differences allow the animals to move in different environments such as on land, in the water and in the air.

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Drawing of the human shoulder blades and forelimbs showing the locations of the scapula, clavicle, humerus, radius, ulna, carpals, metacarpals and phalanges. There are 64 bones in the human shoulders and arms.

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Drawings of the front limb bones of different animals including a human, horse, cat, mole, whale, frog, bat and bird. For each type of animal, the femur is coloured blue, teh radius and ulna are coloured yellow, and the carpals, metacarpals and phalanges are coloured red to show homolgous bones.

Animals that have forelegs also tend to have hindlegs. The exception to this rule are whales, dolphins, porpoises and siren salamanders, which all have only forelimbs.

Hip bones, together with sacrum and coccyx, form the pelvis. Connected to the top of each hip bone is the femur.

Connected to the femur is the knee bone, known as the patella, as well as the tops of the tibia and fibula. Similar to the forearms, at the ends of these bones are the tarsals, metatarsals and phalanges - better known as your ankles and feet.

 Did you know?

The largest bone in the human body is the femur. The biggest bone ever found was a femur from a sauropod dinosaur that was 6.5 feet long!

The number of bones that you will have as an adult, is NOT the same number of bones that you were born with. A newborn's skeleton is made up of 270 bones. During growth, some bones, such as the ones in the skull, fuse together. Adults typically have 206 bones.

Try This!

See if you can identify the animal that belongs to each of the skeletons below BEFORE reading the image caption.

Bones

As you learned earlier, skeletons are made of bone. Let’s first look at the structure of bones.

Bones are made of four different types of tissue:

1. Periosteum
2. Compact bone
3. Spongy bone
4. Bone marrow

The periosteum is the tough outer layer of the bone. This layer contains both blood vessels, like arteries, as well as nerves. Under the periosteum is compact bone. This hard layer protects the softer bones beneath it. Under the compact bone is spongy bone. As the name suggests, this type of bone is full of holes. This allows the bone to be light yet strong. Inside some, but not all bones, is bone marrow. Bone marrow is a jelly-like tissue found within the medullary cavity.

Image - Text Version

This is a drawing of a bone shown partly in cross-section. The outer part of the bone, called the periosteum, is shown as a thin beige layer. Within that, shown as a dark outline is the compact bone. Within the outline of the compact bone is the spongy bone which looks like it is full of little holes. Running up the middle of the spongy bone is the medullary cavity. Within this space is the bone marrow, which is shown as a yellowish cylinder.

Did you know?

The bone marrow in an adult human weighs about 2.7 kg (6 pounds)! 

Bone marrow produces all red blood cells and platelets and around 60–70% of lymphocytes in human adults. The production of blood cells is called hematopoiesis. Red bone marrow produces all blood cell types. Yellow bone marrow produces red blood cells during emergencies and also stores fat.

Did you know?

Your bone marrow produces 200 billion new blood cells every day.

Bird Bones

It was long thought that birds had more air spaces in their bones to make them lighter for flight. But now we know that is only part of the story. Bird bones are pneumatic bones or “breathing bones.” This is because air flows through them. Birds pneumatic bones are attached to their air sacs. It is thought that having these bones connected to the air sacs allows air pressure to strengthen the bones. 

So are bird skeletons lighter than similar-sized mammals skeletons? No! In fact, bird bones are more dense than mammal bones. Even though they have air spaces, they also have lots of internal struts - kind of like rungs on a ladder.

Image - Text Version

In this drawing of a cross-section of a bird bone, the interior structure resembles a hollow tube criss-crossed with jagged, angular struts.

Did you know?

The internal structure of bird bones causes them to shatter more easily than mammals bones. This is why you should never give chicken or turkey bones to a dog. They can break and create shards that can get stuck in the animal’s throat.

 Bone tissue are made of two main things:

1. Connective tissue (see below) that is strengthened with calcium; and
2. Specialised bone cells.

You have probably heard that you should drink milk because it is good for your bones. That is true! Bones contain a lot of calcium phosphate (Ca₃(PO₄)₂). It is what makes bones strong. In addition to calcium phosphate, bones also contain water and small amounts of different minerals such as magnesium and sodium.

Did you know?

One of the roles of the bones in the skeleton is to store calcium. When other body functions need calcium, they can get it from bones. 

Most vertebrates have living bones. Bone tissues are made of three main types of cells:

1. Osteoblasts build new bone tissue.
2. Osteocytes maintain bone tissue health.
3. Osteoclasts break down old bone tissue.

Did you know?

Your bone cells are constantly being replaced by new bone cells. It takes about 10 years to renew your entire skeleton.

To maintain healthy bones, you need to eat a balanced diet and exercise regularly. Exercise encourages bone regeneration. In microgravity, bones tend to lose mass. This is why astronauts need to exercise many hours a week.

Connective Tissues and the Musculoskeletal System

Connective tissues are found throughout our bodies. These tissues help keep parts separate from each other, as well as connect them together.

There are three main types of connective tissue:

1. Loose connective tissue
2. Dense connective tissue
3. Specialized connective tissue.

Loose connective tissue holds your organs in place. It also is what connects your skin to the tissues under it. Dense connective tissue helps to attach muscles to bone as well as link bones together at your joints. Specialized connective tissue includes tissues with specialized cells that connect parts together. Bone, cartilage, blood and lymph are all examples of these specialized tissues.

Dense connective tissue is very important for movement. For example, bones are connected to muscles using connective tissue bundled into tendons. Tendons are strong, yet flexible enough to prevent muscles from extending too much. The easiest tendon to see in your body is the Achilles’ tendon. It connects the calf muscles to the heel bone and without it, you cannot walk.

When connective tissues hold two bones together, they are called ligaments. Ligaments serve as an extra protection. They prevent bones from moving too far apart or twisting dangerously.

Cartilage is another form of connective tissue. It is what forms our skeleton when we are still in the womb. As we grow, some cartilage transforms into bone tissues by a process of mineralization. This is when minerals, such as calcium, are added to the bone. Some cartilage remains as we grow. You can feel it as the flexible part of your ears and nose. Our rib cage also gets its flexibility from cartilage. Cartilage is what joins the ribs to the sternum.

A more rigid form of cartilage covers the ends of bones and is found between the vertebrae in our spine. This form of cartilage protects bones from impacts, like when you wear knee or elbow pads!

Joints, also called articulations, are where bones meet. Not all joints are movable joints. Some joints are immovable. This includes where bone meets bone in your cranium as well as what holds your teeth in your jaw. These joints are also known as fibrous joints since fibrous tissue connects them.

Other joints can have some, but not a lot, of movement. This includes the joints between vertebrae as well as between your hip bones. These joints are connected by cartilage. This is why they are known as cartilaginous joints.

Finally, the joints that allow the most movement of bones are synovial joints. There are several types of these joints including:

• Hinge joints found in elbows and knees
• Ball & socket joints found in hips and shoulders
• Saddle joints found in wrists and ankles
• Pivot joints found in the neck

All of these joints have some things in common. In each joint there is a space between the two connecting bones called the synovial space. Within this space is synovial fluid. This fluid acts as alubricantlubricant to reduce friction between bones. The fluid is contained within a membrane called the synovial membrane that is located just inside the joint capsule. The joint capsule keeps everything in place as well as provides some structural support to the joint.

Check out this video for a summary of joints and how they work, as well as learn about some of the different types of joint injuries.

Muscles

Humans have three types of muscles:

1. Cardiac muscles
2. Smooth muscles
3. Skeletal muscles

You may have heard the word “cardiac” before. Cardiac refers to the heart and cardiac muscles make up most of the heart. These are the muscles that pump blood to the body.

Smooth muscles are involved in automatic movements of body parts. In the digestive tract, those muscles slowly relax and contract to move food along. They are also present in blood vessels and are the muscles that control the amount of light that your iris lets into your eye.

Skeletal muscles are involved in movement. They make up 30 - 40% of your total body mass. Skeletal muscles are found over most of the body. The biggest muscle in your body is the gluteus, the main muscle in your rear end. The strongest muscle is the masseter muscle. It connects your cranium and mandible. It helps you to open and close your jaw.

Did you know?

Most sources state that there are over 650 named skeletal muscles in the human body, although some sources have up to as many as 840! To learn some of their names, check out the video in the Learn More section.

Motion - Teamwork in Action

A very important function of the vertebrate skeleton is movement. But how does it work? For movement you need a team of tissues and systems to work together.

Let’s say that you want to lift up your arm. Watch the video below to see what would happen.

First your brain needs to send a signal down through your nervous system to your arm. This is the white signal in the animation. The signal then triggers the muscles in the arm. Most of our muscles work in pairs. While one muscle contracts, the other relaxes. You can see this as the arm flexes. The muscle on top shortens and the one below extends. When you want to do the opposite movement, they exchange roles. The muscles pull on tendons, which in turn pull on the bones. The joint at the elbow also helps out by acting as a lever.

So the next time you move, you can thank your musculoskeletal system for such amazing teamwork!