![]() If you were to look at it in under a microscope, it would look a lot like your kitchen sponge. Unlike compact bone that is mostly solid, spongy bone is full of open sections called pores. About 20% of the bone in your body is spongy. Spongy bone is found mostly at the ends of bones and joints. Osteons are like thick tubes all going the same direction inside the bone, similar to a bundle of straws with blood vessels, veins, and nerves in the center. Together, a canal and the osteocytes that surround it are called osteons. These cells are lined up in rings around the canals. Compact bone is made of special cells called osteocytes. ![]() If you looked at it through a microscope, however, you would see that it's actually filled with many very tiny passages, or canals, for nerves and blood vessels. If you were to look at a piece of compact bone without the help of a microscope, it would seem to be completely solid all the way through. It makes up the outer layer of the bone and also helps protect the more fragile layers inside. About 80% of the bone in your body is compact. It needs to be very strong as it supports your body and muscles as you walk, run, and move throughout the day. ![]() The little black spots are osteocytes.Ĭompact bone is the heaviest, hardest type of bone. The large dark spots are passages for blood vessels and nerves. These bones are attached to many little muscles that give you very exact control over how you move your fingers and feet.Ĭross section showing osteons. More than half of the bones in your body are actually located in your hands and feet. A skeleton has plenty of joints, but without muscles, there is nothing to pull the bones in different directions. This allows you to bend your elbows and knees, or pick up objects. Muscles often attach to two different bones, so that when the muscle flexes and shortens, the bones move. The movement of our bodies is possible because of both joints and muscles. This type of joint lets you rotate your shoulder in many directions, or swing it all the way around in a circle like softball pitchers do. Other bones fit together like a ball and socket, such as the joint between your shoulder and arm. How many different ways can you move your joints? Some bones, like your elbow, fit together like a hinge that lets you bend your arm in one specific direction. The place where two bones meet to allow your body to bend is called a joint. Each bone has a very specific shape which often matches up with neighboring bones. Movement: Many of your bones fit together like the pieces of a puzzle. For this purpose, being "thick-headed" is a very good thing. The thick bone layer of your skull protects your brain. Your brain is another organ that needs a lot of protection. Your rib cage, for example, acts like a shield around your chest to protect important organs inside such as your lungs and heart. Protection: Bones form a strong layer around some of the organs in your body, helping to keep them safe when you fall down or get hurt. Without bones, it would be difficult for your body to keep its shape and to stand upright. Support: Like a house is built around a supportive frame, a strong skeleton is required to support the rest of the human body. Click on the image to see a larger version.
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