Every bone in your body is composed primarily of calcium compounds arranged into two distinct patterns. The outer bone wall, the cortical bone, is a very dense stonelike structure. This area is so jam-packed with dense calcium that it appears white on X-rays because low-dose diagnostic radiation cannot pass through it. The cortical bone is the bedrock of the spine. The inner bone, or cancellous, is arranged in a less dense honeycomb pattern that greatly reduces the overall weight of the bone and allows it to evenly distribute heavy loads to the thicker outer walls. You can think of the outer bone as granite and the inner bone as a lightweight porous rock or pumice. In contrast to the concretelike outer bone, the inner bone has a very rich blood supply and acts as a reservoir and factory for new red blood cells.
More Answers from Gerald M. Silverman