Skeletons

Connected groups of bones that help shape and support animal bodies.

When some people think of skeletons, they think of Halloween or some mad scientist's display in a laboratory. But we all have a skeleton. It is what gives our bodies shape and form. Without a skeleton, we would be a heap of skin and muscles - like jellyfish! Our skeleton allows us to walk across a room, twist our heads from side to side, toss a ball and type on a keyboard.

The skeleton is made of a series of bones. The key feature of animals with internal skeletons is that they have a set of bones known as vertebrae which are located along the spine. Vertebrates are animals that have a backbone inside their bodies. Animals without a backbone are called invertebrates.

So what are bones? Bones are largely made of calcium and other minerals. They are strong enough to support our weight and our movement. Bones protect our major organs (such as lungs, heart and brain) and give our bodies shape. Our skeleton can be categorized into two parts. The axial skeleton contains the ribs, skull, and spine, and its purpose is to keep us upright. The appendicular skeleton is comprised of the bones in our arms, legs, shoulders and hips, and its purpose is for movement.

Bones in our bodies are very much alive. Most bones have four layers: the outer layer is the periosteum, and the next layer is hard, compact bone. Inside this is a layer of lighter, spongy bone called cancellous bone. The innermost part of the bone is the jelly-like bone marrow, where new red and white blood cells are constantly being produced for our bodies.

In children, bones are continually growing and changing. A baby is born with 300 bones. Over time, some of those bones join to make larger bones. The adult human body has 206 bones which make up the skeleton. Almost half of these are in the hands and feet! In fact, the area of the body with the most bones is the wrist, hand, and fingers, where 54 bones are found. The largest bone in the body is the femur or thigh bone. The smallest is called the staples or stirrup, found in the inner ear. It is only .11 inches (2.8 mm) long.

Although our bones stop growing around the age of 20, new bone cells are constantly rebuilt throughout our lives. Did you know that our bones are stronger than concrete? There are very few substances that can compare with the lightness and strength of bones.

Our bones are not able to move on their own. They require something to pull them in order to be useful. Attached to many of the bones are tough, white bands known as tendons. Tendons do not have any blood vessels and so take a long time to heal if damaged.

Tendons connect bones to muscles. Muscles contract and relax as needed to help our bones move, as well as the arms, legs, feet, and back they support. Learn more about how muscles work at the Science Trek: Muscles page.

The place where two bones meet is called a joint. Some joints, like those in the skull, don't move at all. But other joints allow us to bend, twist, and move different parts of our bodies. One type of moving joint is called a hinge joint, found at our elbows and knees. These joints let us bend and straighten our arms and legs. Another type is the ball and socket joint, found at our shoulders and hips. These joints allow for movement in many directions. Bones are held in place at joints by tissues called ligaments.

The special connections between the bones are covered with a plastic-like material called cartilage. Cartilage protects the end of the bones from rubbing together. Cartilage also makes up our outer ears and the end of the nose, which is why we can bend our ear without a bone breaking. The bones, joints, tendons, ligaments, and cartilage together make up the skeletal system.

Our heads contain a special set of bones and plates known as the skull bones. Some of our skull bones protect our brains, while others make up the structure of our faces. When a baby is born, there are spaces between the bones of the skull. As we grow, the spaces between the bones close up. Without this unique system, the skull bones would collide with each other as they grow. The joints that connect bones in our skulls, known as sutures, are not moving joints. In fact, the jawbone is the only bone in our heads that we can move.

The animals known as vertebrates include mammals, fish, birds, reptiles, and amphibians. Each of these animals has a backbone and a skeleton. Birds have bones that are hollow — this makes their bones lighter for flight.

While vertebrates all have skeletons to give their bodies shape and support, invertebrates have no bones. Many invertebrates have an exoskeleton, or a hard outer shell, to provide protection and support like bones do.

Insects, crabs, lobsters, and shrimp are a few of the creatures with exoskeletons. Other invertebrates are soft like the octopus, squid or earthworm.

Our bones are remarkably strong, but they can be broken or fractured. If that happens, bones will heal on their own. Because bones are made of living cells, when a bone is broken it will produce new cells to rebuild the bone. A doctor will make sure the bone mends correctly by using a cast or sling to keep the bone in place while it heals. It can take a long time for the bone to be back to normal, so it's important to take care of our bones to prevent injuries. Our skeleton supports us every day, so we need to be good to our bones!

• Keep the skeleton strong by drinking milk and eating calcium-rich foods like cheese, yogurt, and dark green vegetables. Calcium helps bones become hard and strong.
• Wear a helmet when bike-riding. Wear protective equipment for sports such as football, soccer, lacrosse, ice hockey, horseback riding, and skateboarding.
• Bones need exercise to stay as healthy as possible. Running, jumping, walking and dancing are all weight-bearing activities that are good for bones.