The skeletal system is made up of 206 bones in the average adult, along with cartilage, ligaments, tendons, and the joints that connect everything together. These components split into two major divisions: the axial skeleton running down the center of your body, and the appendicular skeleton forming your limbs and the structures that attach them to your trunk.
The Two Divisions of the Skeleton
Every bone in your body belongs to one of two groups. The axial skeleton consists of 80 bones that form the central axis of your body: your skull, the tiny bones inside your ears, a small horseshoe-shaped bone in your neck called the hyoid, your spinal column, and your ribcage. Its primary job is protecting vital organs. Your skull shields your brain, your vertebrae encase the spinal cord, and your ribs wrap around your heart and lungs.
The appendicular skeleton accounts for the remaining 126 bones. These are the bones of your arms, legs, hands, feet, and the girdles that anchor them to the axial skeleton. The shoulder girdle (two collarbones and two shoulder blades) connects your upper limbs, while the pelvic girdle (two hip bones) connects your lower limbs. This division is built for movement rather than protection.
Axial Skeleton: The Central Framework
Your skull is a collection of cranial and facial bones. The cranial bones form the top and back of the skull, creating a hard shell around the brain. The facial bones give structure to your eyes, nose, cheeks, and jaw. Tucked inside each ear, three tiny bones called auditory ossicles (the smallest bones in your body) transfer vibrations from the eardrum to the inner ear, making hearing possible.
Below the skull, the hyoid bone sits at the front of the neck. It doesn’t attach directly to any other bone, which is unusual. Instead, it’s suspended by muscles and ligaments, and it plays a role in breathing, speaking, and swallowing.
The vertebral column, or spine, runs from the base of the skull to the pelvis. It includes 24 individual vertebrae stacked on top of each other, plus the sacrum and the tailbone (coccyx) at the bottom. The spine gives your torso its upright shape while also housing and protecting the spinal cord. The ribcage attaches to the upper portion of the spine and wraps forward to the sternum (breastbone), forming a protective cage around the heart and lungs. It also anchors muscles involved in breathing and arm movement.
Appendicular Skeleton: Limbs and Girdles
Your upper limbs start at the shoulder girdle. Each arm contains one humerus in the upper arm, then a radius and ulna in the forearm. Your wrists hold 16 small carpal bones (eight per hand), followed by 10 metacarpals forming the palm and 28 phalanges making up the fingers. That’s a lot of small bones packed into a tight space, which is what gives your hands such fine, precise movement.
The lower limbs mirror this pattern on a larger scale. The femur (thighbone) is the longest and strongest bone in the body. Below the knee, the tibia and fibula run side by side in the lower leg. Each knee also contains a patella (kneecap), a small round bone embedded in the tendon that protects the joint. Your ankles have 14 tarsal bones, followed by 10 metatarsals and 28 phalanges in the toes.
The pelvic girdle anchors these lower limbs to the spine. Made of two large hip bones, it bears the weight of the upper body and transfers it to the legs when you stand, walk, or run.
Five Types of Bones
Not all bones look alike, and their shape reflects their function. Bones fall into five structural categories:
- Long bones are cylindrical and longer than they are wide. Your arm bones, leg bones, and even your finger and toe bones qualify. They act as levers, moving when muscles contract.
- Short bones are roughly cube-shaped. They exist only in the wrists (carpals) and ankles (tarsals), where they provide stability and limited motion.
- Flat bones are thin and often curved. Skull bones, shoulder blades, the sternum, and ribs all fall into this group. They protect internal organs and serve as attachment points for muscles.
- Irregular bones don’t fit neatly into any other category. The vertebrae and many facial bones have complex, unique shapes dictated by the specific job they perform.
- Sesamoid bones are small, round bones shaped like sesame seeds. They form inside tendons at points of high pressure, protecting the tendon from compressive force. The kneecap is the most well-known example.
What’s Inside a Bone
Bones aren’t solid all the way through. The outer layer is dense and hard, while the interior contains a spongy, lattice-like tissue that reduces weight without sacrificing strength. Nestled within that spongy tissue, especially at the ends of long bones, is red bone marrow. This is where your body manufactures blood cells: red blood cells that carry oxygen, white blood cells that fight infection, and platelets that stop bleeding by forming clots.
The central cavities of bones contain yellow bone marrow, which is mostly fat. Yellow marrow serves as an energy reserve, but it has a remarkable backup function. In cases of severe blood loss, yellow marrow can convert back into red marrow to help restore blood volume.
Three types of specialized cells keep bone tissue alive and constantly renewing. One type builds new bone, another breaks down old bone, and a third (the most abundant) maintains the existing structure. The balance between building and breaking down is what allows bones to repair fractures, adapt to stress, and stay strong over time.
Cartilage, Ligaments, and Tendons
Bones alone can’t form a functional skeleton. Cartilage is a firm but flexible tissue that acts as a shock absorber wherever bones meet. There are three types throughout your body. Hyaline cartilage, the most common, caps the ends of bones inside joints and sits between your ribs. Fibrocartilage is tougher and found in high-stress areas like the discs between your vertebrae and the meniscus in your knee. Elastic cartilage is the most flexible, shaping your outer ears, voice box, and the tubes that carry sound into your head.
Ligaments are bands of tissue that connect bones to other bones, holding joints together and preventing them from moving in directions they shouldn’t. Tendons, by contrast, connect muscles to bones, transmitting the force of a muscle contraction into actual movement.
Joints: Where Bones Meet
Joints are classified by how much movement they allow. Fibrous joints, like those between the plates of your skull, are barely flexible or completely immobile. Cartilaginous joints, such as the connections between vertebrae, allow some movement but in a limited range. Synovial joints have the greatest freedom of movement. Your knees, shoulders, hips, elbows, and knuckles are all synovial joints, lined with a fluid-filled membrane that reduces friction during motion.
How Bone Count Changes With Age
A newborn enters the world with 275 to 300 bones. Many of these are softer and partially made of cartilage, which is why babies are more flexible and less prone to fractures. As a child grows, separate bones gradually fuse together and harden through a process called ossification. By early adulthood, the count settles to the standard 206, though some people have a few extra sesamoid or other small bones in their hands and feet.