Because bone is so hard that it cannot be sectioned in the usual way. In this method, the mineral content is left intact. A thick section of the bone is ground on a grinding wheel until it is nearly transparent. These sections are called "ground bone." The image in HA5 is a cross-sectional fragment of a mature long bone. There are numerous osteons of different sizes. The larger canals are evidence of recent remodeling events that had not yet been filled in by concentric lamellae.