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Bones are divided into four types: long, short, flat, and irregular.

Long: Long bones can be found in the arms and legs. Long bones are longer than they are wide.

Short: Short bones are found in the hands and feet. Doesn’t it look like your fingers are longer than they are wide? Well, they aren’t. Those are actually three bones!

Flat: Flat bones are considered to be your skull and your ribs.

Irregular: Irregular bones don’t fit into any of the other three categories. You have three bones in your ears that are irregular. Your vertebra in your backbone are also irregular.

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Bones perform different jobs.

Support: Some bones are responsible for support. Without these bones, we couldn’t stand up. Without these bones, we’d be a pile of mush.

Protection: Some bones are responsible for protection. Their job is to protect soft organs, such as the lungs and the brain.

Movement: Some bones make it possible for us to move. Without these bones, we’d be pretty still, like a plant.

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This picture is a sketch of a long bone. This picture is a photo of a long bone that has been cut in two.

Bones have four layers to them.

1. Periosteum: The outside layer is thin and very hard. This layer is called the periosteum.

2. Compact bone: The next layer is called the compact bone. Sometimes it’s called dense bone. The great majority of an adult’s bone mass is in the compact bone.

3. Spongy bone: In the middle of the bone is the spongy bone. This layer is porous, meaning it has holes in it. It kind of looks like a sponge, but it’s not squishy—it’s hard. The technical name for this type of bone is the trabecular bone tissue. Even though this part of the bone doesn’t weigh nearly as much as the compact bone, it has more surface area. All those little holes give it that surface area.

4. Marrow: Inside the bone is bone marrow. When babies are born, their bone marrow is red. Red bone marrow makes blood. As we age, some marrow turns yellow. A good half of the bone marrow in adults is yellow. Yellow marrow stores fat.

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When two bones meet, they form a joint. There are five main different kinds of joints.

1. Ball and Socket Joint2. Gliding Joint3. Saddle Joint4. Hinge Joints5. Pivot Joints

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• Ball and Socket Joint: A ball and socket joint can move in many more directions than a hinge joint. Ball and socket joints allow movement in circles. An antennae on a TV has a ball-and-socket type joint. Move your arm around and see how it can move in front of you, behind you, up, down, and around in circles.

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• Saddle Joint: A saddle joint allows movement along two different planes. Your thumb has a saddle joint. Move your thumb around and feel which movements seem most natural.

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• Hinge Joints: Hinge joints only move along one plane. Take your knee for instance. This girl’s knee can move up and it can move down. Your leg bones can only bend together and apart. These joints are like the joints in a door, and that’s why they are called hinge joints. The door can open and close because of the hinge joints. Besides your leg, where else on your body can you find hinge joints?

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• Pivot Joints: Pivot joints allow some circular movement. Your neck has a pivot joint that allows you to look to the left and to the right. Your neck also has a hinge joint that allows you to move your head up and down.

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• Gliding Joint: In a gliding joint, two bones glide against each other. The bones can’t go very far. Your wrist has gliding joints in it.

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If two logs rub against each other, they can grind and pieces can fall off.

Two bones that form a joint could grind if they didn’t have some kind of lubricant. Different substances compose lubricants in the body.

Our vertebra have cartilage between them.

Other bones have fluid between them.

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