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. The stripes are actually alternating filaments of two substances called actin and myosin , and it is these that are the key to muscle power. At a signal from the brain, the myosin and actin filaments are drawn together, making the muscle shorter. As the muscle shortens, it pulls on the bone, making the movement. Cardiac muscle is the muscle in the heart.