Bones are divided into three classes, distinguished as long bones, flat bones, and irregular bones.

Long bones make up the extremities, where they give support to the body, and act as so many levers in the function of locomotion. Each long bone is composed of a central portion or shaft and two extremities. The former is the more compact and narrow, the latter is chiefly formed out of spongy tissue, and is broad, and yields an articular surface covered with cartilage.

Flat bones, for the most part, enter into the formation of cavities containing important organs, as the cranium, the chest, and the pelvis.

Irregular bones are distinguished by their many angles and depressions, such as the vertebrae, and the bones of the knee and the hock-joint. They are mainly composed of cancellated tissue enclosed in a dense outer layer of compact structure.

To whatever class a bone may belong it will have upon it eminences and depressions. Some of these will be articular, and by uniting with other bones form joints, while others will be non-articular, and give attachment or lodgment to ligaments, muscles, or tendons.