The human skeletal system provides the necessary framework for the body, offering mechanical support, protection for internal organs, and structure for movement. Since the adult human body contains approximately 206 individual bones, scientists require systematic methods to organize and study this complex network. Bones are classified through several distinct lenses, including their physical location, external shape, and internal tissue composition.
Grouping Bones by General Location
One primary way to classify the skeleton is by dividing it into two anatomical regions: the axial and the appendicular components. The axial skeleton forms the vertical, central axis of the body, encompassing 80 bones that primarily protect organs and provide structural support for the head, neck, and torso. This division includes the bones of the skull, the vertebral column, the hyoid bone, and the thoracic cage (ribs and sternum).
The appendicular skeleton consists of 126 bones appended to the main axis, focusing on locomotion and manipulation of the environment. These bones comprise the upper and lower limbs, along with the structures that anchor them to the axial skeleton, known as the girdles. This group includes the shoulder (pectoral) girdle and the hip (pelvic) girdle, providing flexibility and a wide range of motion.
Categorizing Bones by Shape
Classification by shape often dictates a bone’s specific mechanical role within the body. The most numerous are Long Bones, which are cylindrical, possessing a length significantly greater than their width, and function primarily as levers. Examples include the femur and the humerus, but also the smaller bones of the fingers (phalanges) and the hands (metacarpals), since classification is based on proportion, not absolute size.
Next are Short Bones, which are roughly cube-shaped, meaning their length, width, and thickness are approximately equal. These bones are found exclusively in the wrist (carpals) and ankle (tarsals), where they provide stability and support with only limited movement.
Flat Bones are typically thin, flattened, and often curved, serving mainly as broad points for muscle attachment or as shields for underlying soft tissue. The bones of the skull (cranial bones), the scapula (shoulder blade), and the sternum (breastbone) all fall into this category, reflecting their role in protecting organs like the brain and heart.
Bones that do not fit neatly into the long, short, or flat categories are termed Irregular Bones because of their complex and unique shapes. The vertebrae of the spine, which protect the spinal cord, and many of the facial bones are classified this way due to their intricate structure designed to withstand various compressive forces.
Finally, Sesamoid Bones are small, round structures embedded within tendons, resembling a sesame seed. The patella, or kneecap, is the largest and most well-known sesamoid bone, acting to protect the tendon and modify the angle of muscle pull to improve mechanical leverage.
Distinguishing Bones by Internal Composition
Bone tissue is classified into two distinct types based on density and structure. Compact Bone, also known as cortical bone, is a dense, hard tissue that forms the smooth, solid outer layer of all bones. This tissue accounts for approximately 80% of the skeletal mass and is organized into functional units called osteons, providing maximum strength and rigidity.
Beneath this hard exterior lies Spongy Bone, also called cancellous or trabecular bone, which is a lighter, porous tissue with a lattice-like network of bony struts called trabeculae. This internal structure helps reduce the bone’s overall weight while still providing strength and allowing the ends of long bones to absorb stress from multiple directions. The spaces within spongy bone are typically filled with red bone marrow, which is the site of blood cell production.