The 206 bones in the human body can roughly be divided into four categories: long, short, flat and irregular.
- Long bones. Not all long bones are actually long (some bones in the fingers and toes are long bones), but most are (such as the leg and arm bones). Long bones are identified by the shape and structure of the bone: a slightly curved shaft capped on both sides with hyaline cartilage, longer than it is thick. They're made mostly of compact bone, allowing them to support great amounts of weight and withstand pressure.
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The femur provides an excellent example of the strength of long bones. Its hollow cylindrical design allows it to provide the most possible strength out of the materials provided (the substance of the bone itself), without making it too heavy. The hollow internal portion of many long bones is where the marrow is located. Long bones grow from both ends, and have a cartilage plate (also known as epiphyseal plates) between the bone shaft and each bone end. These plates continue growing through adolescence.
- Short bones. Short bones consist mainly of spongy bone with a protective covering of compact bone. Short bones are neither long nor thick, but rather cubelike. Your kneecaps (patellae), wrists (carpals) and some of the bones in your feet and ankles (tarsals) are short bones. Short bones aren't designed for a great deal of movement, but they're sturdy, compact and durable.
The short bones in the wrist and ankle are also known as sesamoid bones. Sesamoid bones (usually classified as short or irregular bones) are placed within tendons in parts of the body where a tendon must cross a joint. These bones hold the tendon slightly away from the joint to provide better range of motion when the tendon tightens. For instance, your kneecap connects two pieces of tendon that cross over the joint between your femur (upper leg bone) and the tibia of your lower leg.
- Flat bones. These bones are thin and flat. Flat bones have a middle layer of spongy bone located between two protective layers of compact bone. These bones are generally protective in nature. Flat bones make up most of the 29 bones that fuse together to form the skull and protect the brain, and also protect the major internal organs by forming the 24 ribs (12 on each side) of the rib cage, as well as the sternum. The shoulder blade (scapula) is also a flat bone. Flat bones contain marrow, but aren't large enough to have marrow cavities -- marrow is found instead in the spongy bone. The marrow in flat bones produces more red blood cells than any other adult bone type.
- Irregular bones. The bones that don't fit in the other three categories are irregular bones. Vertebrae in the spine and the jawbone (mandible) are irregular bones. This type of bone usually has a very specialized function and is made up of mostly spongy bone with a thin layer of compact bone around it.
In the next section, we'll learn that even the laziest among us is really a bone-making workaholic.