Skeletal System

Bones may be classified according to their various traits, such as shape, origin, and texture. Four types are recognized based on shape. These are long bones, short bones, flat bones and irregular bones. Long bones have a long central shaft, called the diaphysis, and two knobby ends, called the epiphysis. In growing long bones, the diaphysis and epiphysis are separated by a thin sheet of cartilage. Examples of long bones include bones of the arms and legs, the metacarpals of the hand, metatarsals of the foot, and the clavicle. Short bones are about as long as wide. The patella, carpels of the wrist and tarsals of the ankle are short bones. Flat bones take several shapes, but are characterized by being relatively thin and flat. Examples include the sternum, ribs, hip bones, scapula and cranial bones. Irregular bones are the oddshaped bones of the skull, such as the sphenoid, the sacrum and the vertebrae. The common characteristic of irregular bones is not that they are similar to each other in appearance, but that they can't be placed in any of the other bone categories.

Bones may also be classified based on their origin. All bone (as well as muscles and connective tissue) originates from an embryonic connective tissue called mesenchyme, which makes mesoderm, also an embryonic tissue. Some mesoderm forms the cartilaginous skeleton of the fetus, the precursor for the bony skeleton. However, some bones, such as the clavicle and some of the facial and cranial bones of the skull, develop directly from mesenchyme, thereby bypassing the cartilaginous stage. These types of bone are called membrane bone (or dermal bone). Bone which originates from cartilage is called endochondral bone.

Finally, bones are classified based on texture. Smooth, hard bone called compact bone forms the outer layer of bones. Inside the outer compact bone is cancellous bone, sometimes called the bone marrow. Cancellous bone appears open and spongy, but is actually very strong, like compact bone. Together, the two types of bone produce a light, but strong, skeleton.