WHY DO WE HAVE BONES You cannot see them but you can feel them You cannot stand or move without them but you hardly ever notice them Bones are your body’s hidden frame Bones provide a structure for your other body parts They are connected to Bones and muscles work together to make your body parts move 4
your muscles and they make it possible for you to walk and run Some bones protect the organs inside your body such as your lungs and heart From the outside a bone looks hard and still But inside it is always working Bones store nutrients or things your body needs Bones also make blood cells What would happen if you didn’t have any bones Would you be able to move the same ways you do now THINK ABOUT IT This climber’s bones keep her body stable while she plans her next move 5
BONE STRUCTURE Bones are made of calcium and other minerals fibers of protein and water If you look at a bone that has been cut you might be surprised to see that it has several layers The outer layer of a bone is compact bone It is firm and dense The inner layer is spongy bone It looks like a sponge with many holes but it is not soft or flexible The holes in spongy bone are filled with bone marrow Marrow is soft like jelly and can be red A bone may look solid but inside it has many holes 6
or yellow Most blood cells are created by red marrow while fat is stored in yellow marrow All bones are covered with a membrane or skin The membrane contains nerves and blood vessels that are attached to the bone The blood vessels carry nutrients to the bone They also carry blood cells from the bone to the rest of the body How is the inside of a bone different from the outside COMPARE AND CONTRAST This slice of bone shows compact bone and spongy bone 7
