Introduction to Bone Anatomy

Bones exist to give the body its structure and support. They provide shape to body parts, protect soft tissues, and work together with muscles to enable movement. An adult human has 206 bones after the fusion process, whereas a newborn starts with over 300 bones, most of which are cartilage that later ossifies.

“The whole purpose of the anatomy of the bone purpose is to provide structure and support.”

Skeletal System Division

The skeleton is organized into two major sections:

• Axial skeleton – 80 bones that form the central axis, including the skull, vertebral column, and rib cage.
• Appendicular skeleton – 126 bones that make up the limbs and pelvic girdle.

Together they total the 206 bones of the adult body. Ligaments attach bone to bone, stabilizing the joints throughout the system.

“Um it's going to give shape to your body parts. It's providing uh support to soft body structures and it assists along with the muscular system and body movement.”

Bone Classification

Bones are grouped by shape and internal structure:

• Long bones – Have an epiphysis (ends), a diaphysis (shaft), and a medullary cavity that houses marrow. Examples: femur, humerus, tibia, fibula, ulna, radius.
• Spongy bone – Found inside long bones; its porous nature absorbs impact and helps prevent fractures.
• Short bones – Small, roughly cube‑shaped, located in the wrists, ankles, hands, and feet (carpals, tarsals). Fluid between them aids smooth movement.
• Flat bones – Broad and thin, such as the sternum and the bones of the skull.
• Irregular bones – Have complex shapes, exemplified by vertebrae and the pelvis.

“And the spongy bone is inside of the bone. And that's what allows us to be able to take on impact, jumping, falling, all of those things.”

Specific Structures and Joints

Skull – Composed of cranial and facial bones. The mandible (lower jaw) connects to the maxilla (upper jaw) via a ball‑and‑socket joint that enables chewing. The temporomandibular joint (TMJ) is the site where the ball can dislocate from its socket.

Spinal column – Arranged as cervical, thoracic, lumbar vertebrae, followed by the sacrum and coccyx. Intervertebral discs act as cushions, allowing flexibility.

Rib cage – Consists of the sternum (breastplate) and ribs that protect thoracic organs.

Shoulder girdle – Formed by the clavicle (collarbone) and scapula (shoulder blade).

Arm – Includes the humerus, radius, and ulna; the radius and ulna rotate around each other for forearm movement.

Hand – Built from carpals (wrist), metacarpals, and phalanges (fingers). Synovial fluid between the short bones permits dexterous motion.

Leg – Contains the femur, patella (kneecap), tibia, and fibula.

Pelvic girdle – Made up of the ilium, sacrum, ischium, pubis, and coccyx. The coccyx region combines bone and cartilage, allowing pelvic expansion during childbirth.

Hip joint – A classic ball‑and‑socket joint, similar to the jaw. Dislocation or replacement involves the ball slipping out of its socket.

Foot – Composed of tarsals, metatarsals, and phalanges.

“And that's what allows for that movement when you're chewing.”
“And that could be where that ball falls out of that socket.”
“And that's what allows for the expansion in the pelvic area for the baby to pass through the canal.”

Joint Mechanisms

Ligaments bind bones together, providing joint stability. Between short bones, a thin layer of fluid reduces friction and supports movement. Ball‑and‑socket joints, such as the hip and TMJ, grant a wide range of motion while maintaining alignment. Cartilage and bone together in the coccyx enable slight expansion, crucial during childbirth.