Not all the bones of the body have the structure of a long bone, as descried above, belonging instead to the following categories:
Long Bones
These bones consist of typically elongated cylindrical bones with enlarged extremities. The cylindrical part is called shaft or body which is hollow from inside enclosing a cavity called medullary cavity. These are relatively longer and slender in shape e.g. Bones of fore limbs and legs.
Sesamoid Bones
These are small, flat and with shape of sesame seed e.g. patella. These are formed in tendons (and occasionally ligaments) at points where they alter direction or pass over an underlaying bony prominence. The patella (kneecap) lies in the tendon of quadriceps femoris muscle.
Flat Bones
As their name suggests, these are flat in shape, and are composed of two layers of cortical bone separated by a small layer of cancellous bone. They have thin and roughly parallel surfaces for attachment of muscles. Skull, sternum, scapula and pelvis are examples of flat bones. There is no medullary cavity.
Irregular Bones
These have an irregular shape, complex shape with short flat notched surface and make up the vertebral column. All the bones have a central canal to accommodate the spinal cord.
Short Bones
These bones have similar length, breath and thickness and do not contain a medullary cavity. They are composed of an outer layer of cortical bone, with an inner layer of cancellous bone. These bones are small and boxy and diminish the frictions and absorb shock during movement. Examples are sesamoid bones.
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