They protect tendons from wear and tear, and they alter the direction of pull of a tendon. Sesamoid bones form in response to stress on joints, so the sesamoid bones vary in size and number in different individuals. See also Figs. Flat bones have two roughly parallel compact bone surfaces, with a layer of spongy bone between them. The breastbone, ribs, shoulder blades, and the cranial bones are flat bones.
Flat bones provide protection to internal organs and they provide large areas for muscle attachment.
The ribs and breastbone protect the thoracic cavity. The shoulder blades provide muscle attachments for many back muscles, while protecting the thoracic cavity. All of the irregular bones consist primarily of spongy bone enclosed by a thin layer of compact bone. The external surfaces of bones often have bulges, depressions and holes. These surface markings are adapted for certain functions.