In this method, the calcium salts that make bone so rigid are removed by treating the bone with acid and/or EDTA, a chelating agent. The acid also causes swelling of the type I collagen fibers, so individual fibers are difficult to discern. HA1 is a low magnification light micrograph of a transverse section through a long bone (M = marrow cavity). The endosteum is found on the marrow side of the compact bone and the periosteum is on the outside. The marrow occupies the central area of spongy/trabecular bone. Just outside the bone on one side is skeletal muscle.