The Flat Bones in the Human Body

African doctor and nurse examining x-rays
Two doctors are examining an x-ray. (Image: Patrick Lane/Blend Images/Getty Images)

Five types of bones make up the human skeleton: flat bones, long bones, short bones, irregular bones and sesamoid bones. Flat bones, true to their name, are flat, thin, curved bones. They are made of 2 outer layers of smooth, hard, compact bone plus an inner layer of spongy bone tissue -- known as cancellous bone -- which contains bone marrow. Flat bones act as a shield, protecting the brain, heart and other internal organs from trauma. They also serve as a scaffold for large muscle groups to attach to and, in adults, they are the area where most of the body's red blood cells are produced

Cranial Bones

The cranium, or skull, is made up of pairs of bones that include the frontal, parietal, occipital, nasal and lacrimal bones. The cranium's primary role is to protect the underlying brain from being injured by a fall or blow to the head. Together with the facial bones, the bones of the cranium also form the eye sockets and the nose. In infants and young children, the bones of the cranium are separated by spaces called sutures. Sutures allow the skull to expand as the brain grows. Once the brain is fully grown, the sutures close and the cranial bones become fused to form 1 continuous bone.

Ribs and Sternum

Shaped like a necktie, the sternum -- also called the breastbone -- lies in the center of the chest. It combines with the ribs to form the rib cage. All ribs attach to the spine and some also have attachments in the front of the chest. Seven of the 12 rib pairs attach directly to the sternum. The 8th, 9th and 10th rib pairs are attached to the sternum by connecting cartilage, while the lowest 2 pairs -- the "floating" ribs -- do not have attachments in the front of the body. The rib cage protects the heart, lungs and aorta -- the major artery of the chest. Because the lower rib cage extends over the upper abdomen, it also protects the liver and spleen. When a person breathes, the rib cage moves outward, allowing the lungs to fill with air.

The Scapula

The scapula, commonly known as the shoulder blade, is a flat, slightly curved triangular bone that connects the upper arm to the collar bone. It helps to protect the back of the chest. Part of the scapula forms the socket of the shoulder joint. The scapula takes part in shoulder motions, including arm elevation and arm movement away from or toward the body. It also serves as the point of attachment for each of the muscles of the "rotator cuff," whose job it is to stabilize the shoulder joint.

Pelvic Bones

The pelvic bones, which include the iliac bones, ischial bones and pubic bones, form the pelvic girdle. The pelvic girdle provides structural support for the body and allows people to stand. It is the site of attachment for several muscle groups, including the abdominal wall muscles, the back muscles and the muscles of the upper thigh. The pelvic girdle also contains the socket of the hip joint. As with other flat bones, the pelvic bones provide protection to the underlying internal organs, such as the bladder.

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