The skeletal system of the human body contains 206 bones, half of which are in the hands and feet. Bones are typically classified into four main categories: long bones, short bones, flat bones and irregular bones. There are several other categories that are sometimes used as well.
Long bones are greater in length than in width and are fairly curved for strength. They are thin, hollow and light. Long bones have a hard outer surface of compact bone and a spongy inner bone surface which contains bone marrow. Both ends are covered in cartilage to help with shock absorption. These bones work as levers and play a key role in all types of movements. Some examples include the femur, tibia, fibula, humerus, ulna, and radius.
Short bones are relatively the same length as they are in width and are cube-shaped. The outside surface of a short bone is made up of a thin layer of compact bone. The inside of a short bone contains mostly spongy bone. The ankle bones and wrist bones are two examples. The kneecap is also considered a short bone.
Flat bones have a thin shape and structure and are bones that provide mechanical protection. The flat bones are strong, but elastic, allowing room for movement such as in breathing. They protect organs in the body and attachment of muscles. Flat bones have anterior and posterior surfaces formed of compact bone to provide the strength and protection these bones need to protect organs. The highest amounts of red blood cells are formed in flat bones. Some examples include the ribs, cranial bones, and shoulder blades.
Any bone that doesn't fit into the above categories is considered an irregular bone. They vary in size, shape, and surface features. Some examples are bones of the vertebrae and a few bones in the skull including facial bones.
Sesamoid Bones and Sutural Bones
Sesamoid bones are bones that could develop in some tendons where there is an extra amount of friction, tension, or stress. The kneecap is a common place where a sesamoid bone may develop. Sutural bones are very small bones in the sutural joints within the cranial bones. The amount of sutural bones in people vary by the person.