The human skeletal system is composed of 206 individual bones, each categorized by shape into five main types: long, short, irregular, sesamoid, and flat bones. This classification helps in understanding how different bones are structured to perform their specific roles. Flat bones are generally thin and broad, making them distinct from the cylindrical long bones or the cube-shaped short bones.
The Unique Structure of Flat Bones
Flat bones are thin, often curved structures that are expanded into broad plates. Their unique morphology is characterized by a “sandwich” design, consisting of three distinct layers of bone tissue. The outer and inner surfaces are formed by two parallel layers of dense compact bone, also known as cortical bone. This dense outer shell provides mechanical strength.
Between these two sheets lies a layer of porous spongy bone, which is also called cancellous bone. Within the cranial bones, this middle layer is specifically referred to as the diploë. This internal layer contains red bone marrow, which is involved in the production of blood cells. The layered architecture provides a balance of strength and flexibility, helping to absorb impact without fracturing easily.
Primary Roles in Protection and Movement
The broad, thin shape of these bones is directly related to their two main functional duties. The primary function of flat bones is the extensive protection of underlying soft organs. The curved, shield-like nature of these bones forms a strong barrier against external forces, safeguarding the contents beneath.
The secondary function is to provide wide areas for the attachment of muscles, which helps facilitate movement. Their expansive surface area allows numerous muscle fibers and tendons to anchor securely, generating greater leverage. This muscle attachment is particularly important in regions that require powerful or extensive ranges of motion.
Where Flat Bones Are Found in the Body
Flat bones are distributed across the body in locations where internal organs require robust defense or where large muscles converge. A major concentration of flat bones forms the calvaria, or the dome of the skull, which includes the frontal, parietal, and occipital bones. These curved bony plates encase and protect the brain, one of the body’s most sensitive organs.
The ribs and the sternum, or breastbone, also exemplify flat bones. The twelve pairs of ribs, along with the sternum, form the thoracic cage, which surrounds and shields the heart and lungs from physical trauma. The triangular scapulae, commonly called the shoulder blades, are also classified as flat bones. They do not primarily protect an organ, but instead provide the broad surface necessary for the attachment of the powerful muscles of the back, chest, and shoulder, which control arm movement. Other examples include the coxal bones of the pelvis, which offer surfaces for leg muscle attachment and protection for the pelvic organs.