Structure of bone

Bone is a beautiful substance with an intricate structure. To simplify quite a bit, the basic unit is the Haversian system, which is a hollow, laminated rod of collagen and calcium phosphate; the hollow core is a nutrient channel, the Haversian canal. Within the shaft of a long bone, many of these Haversian systems are bundled together in parallel, forming a kind of bone called compact bone, which is optimized to handle compressive and bending forces. Near the ends of the bones, where the stresses become more complex, the Haversian systems splay out and branch to form a meshwork of cancellous, or spongy bone.

If you cut a cross-section through a region of compact bone, you will see rings of Haversian systems, each with a hole, the canal, in the center. Cutting through the region of cancellous bone produces a more complex section, because the systems have many different orientations.

(This image scanned from a textbook, Basic Medical Anatomy, by Alexander Spence (Benjamin/Cummings 1990). Any anatomy or histology text can give you this information with greater or lesser detail, and it has been readily available to the public since at least the 18th century.)