The bones of the human skeletal system are of varied shapes, sizes, and functions. They can be classified based on their shape and function into four major classes: long bones, short bones, flat bones, and irregular bones. Some classifications include a fifth type, the sesamoid bones, as a separate class, whereas others categorize them under short bones.
Long and Short Bones
The appendicular skeleton, particularly the upper and lower limbs, is primarily made of long and short bones. The long cylindrical bones include bones in the upper limbs (humerus, radius, and ulna) and those of the lower limbs (tibia, fibula, and femur). Long bones are defined as longer in length than their width, but can also be small in size, such as those in the fingers and toes (phalanges, metacarpals, and metatarsals). In contrast, short bones have a more cube-like appearance, as seen in the wrist and ankle bones. Both long and short bones typically function to bear weight and support movement.
Sesamoid bones are similar to short bones but have a flatter, rounded appearance. They are embedded in tendons and function to distribute force and reduce damage to the tendons. The largest sesamoid bone in humans is the patella (knee cap). Smaller ones, found in the thumb, index finger, wrist, and foot, can vary in number and placement from person to person.
Flat and Irregular Bones
Flat bones are usually found in the axial skeleton. They function as protective structures and provide large areas for muscle attachment. The cranial bones, sternum, ribs, scapulae, and pelvic bones are all categorized as flat bones.
An irregular bone has no 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.
Part of this text is adapted from Openstax, Anatomy and Physiology 2e, Section 6.2: Bone Classification
