Your senses supply the brain with a constant stream of information about your surroundings. There are over 200 bones in the body. They not only anchor the muscles but support skin and other tissue, and protect your heart, brain and other organs from damage. Bones are very tough and light because they are made from a mixture of hard minerals, such as calcium, and stringy elastic collagen . Inside the tough casing of minerals and collagen, bone is like a sponge, with holes containing special cells called o steocytes .
Some bones have a jelly-like centre or marrow , where new blood cells are made.
The muscles of your heart, and around the gut and blood vessels, work automatically. Skeletal muscle owes its power to long, thin fibre-like cells, stretching from one end of the muscle to the other. Muscles are bundles of these stringy fibres bound together in a bag or sheath, and attached to bone at either end by tough cords called tendons . Every muscle fibre is made from thin strands called myofibrils . Under a microscope, the strands look striped, which is why this kind of muscle is called striated or striped.