If you look at a cutaway view of a bone, you can see that it consists of two main kinds of material : a dense outer material and a spongy, porous inner material. The hard outer material gives a bone its shape and strength, and it is made mostly of compounds of the chemical elements calcium and phosphorus. The soft inner part of the bones is called marrow.
Most marrow is yellowish in colour. It is made up of fat cells and is simply a storage spot of fat. Towards the ends of long bones, like those of the arms and legs, and generally throughout the interior of fat bones, such as those of the skull and the spinal column, there are patches and streaks of reddish tissue. This reddish tissue gets its colour from red blood cells.
Long bones are generally cylindrical in shape. The long,
cylindrical portion of these bones is called the shaft. The ends of the long bones are thicker than the shaft, and are shaped so that they may fit into the ends of adjoining bones. The short bones, such as those of the wrist and ankle, are composed mostly of a thick shaft of elastic, spongy material inside a thin covering of hard bone material. Fat bones, such as the ribs are made up of spongy material between two plates of hard bone.
