Bone Classification
The 206 bones that compose the adult skeleton are divided into five categories based on their shapes (Figure 3). Their shapes and their functions are related such that each categorical shape of bone has a distinct function.
Figure 3. Bone Shape. This figure displays the skeleton as a whole with various bone shape examples enlarged to emphasize their form. These examples include the flat bone of the sternum, the irregular bones of the vertebrae, the long bone of the femur, the short bones only found in the feet and wrists, and the sesamoid (also called a round bone) patellar bone of the knee cap.
Long Bone
A long bone is longer than it is wide and is found in the appendages, such as the arm and hand. The long bones are not always obvious the hands and feet, however. Remember that they are longer than they are wide, and can be found in the metacarpals of the palm and phalanges which are the finger bones.
Short Bone
A short bone is one that is cube-like in shape, being approximately equal in length, width, and thickness. The ONLY short bones in the human skeleton are in the carpals of the wrists and the tarsals of the ankles. Short bones provide stability and support as well as some limited motion.
Flat Bone
The term "flat bone" is somewhat of a misnomer because, although a flat bone is typically thin, it is also often curved. Examples include the cranial (skull) bones, the scapulae (shoulder blades), the sternum (breastbone), and the ribs. Flat bones serve as points of attachment for muscles and often protect internal organs.
Irregular Bone
An irregular bone is one that does not have any easily characterized shape and therefore does not fit any other classification. These bones tend to have more complex shapes, like the vertebrae that support the spinal cord and protect it from compressive forces. Many facial bones, particularly the ones containing sinuses, are classified as irregular bones.
Sesamoid Bone
A sesamoid bone is a small, round bone that, as the name suggests, is shaped like a sesame seed. These bones form in tendons (the sheaths of tissue that connect bones to muscles) where a great deal of pressure is generated in a joint. The sesamoid bones protect tendons by helping them overcome compressive forces. The patellar bones of the knees are the only round bones found in common with every person.
| Bone Classification | Features | Functions | Examples |
|---|---|---|---|
|
Long |
Longer than wide |
Leverage |
Femur, metatarsals, ulna, phalanges |
|
Short |
Cube-like shape |
Provide stability while allowing for some movement |
Carpals, tarsals |
|
Flat |
Thin, curved |
Protects internal organs |
Sternum, ribs, cranial bones |
|
Irregular |
Unusual shape |
Protect organs |
Vertebrae, facial bones |
|
Sesamoid |
Small and round; inside tendons |
Protect tendons from compressed forces |
Patellar bones |
