The Skeletal System

What the Skeleton Is For

The adult skeleton has 206 bones and does five jobs at once: it gives the body shape, protects the organs, provides levers for muscles to pull against, stores calcium and phosphorus, and produces blood cells in the marrow.

That last one is easy to forget and it explains a lot. Bone is not inert scaffolding. It is living, vascular tissue with a blood supply, a nerve supply and a metabolism, which is why a fracture bleeds, hurts, and heals.

A newborn starts with roughly 270 bones. The count falls through childhood as separate pieces fuse, which is why the infant skull still has soft spots and the sacrum is five bones before it becomes one.

The Two Divisions

Axial skeleton (80 bones). The central column: skull, vertebral column, ribs, sternum and hyoid. Its job is protection.

Appendicular skeleton (126 bones). The limbs and the girdles that attach them: shoulder girdle, arms, pelvic girdle, legs. Its job is movement.

The Skull

The skull divides into the cranium, which encloses the brain, and the face.

The cranial bones are the frontal, two parietal, two temporal, occipital, sphenoid and ethmoid. They are fused at immovable joints called sutures. The foramen magnum is the large opening at the base of the occipital bone where the brainstem passes through to become the spinal cord.

Facial bones worth naming are the mandible (the only movable skull bone), the maxilla, the zygomatic bones, the nasal bones and the orbits.

In an infant the cranial bones have not fused yet, and the gaps between them are the fontanelles. The posterior fontanelle closes around two to three months, the anterior around twelve to eighteen months. A sunken anterior fontanelle suggests dehydration. A bulging one that is not from crying suggests rising pressure inside the skull.

The Vertebral Column

Thirty-three vertebrae, in five regions, top to bottom:

The count runs 7, 12, 5, 5, 4. Cervical vertebrae are the smallest and the most mobile, which is why they are the most commonly injured region of the spine.

The vertebrae stack so that their openings line up into a canal, and the spinal cord runs down inside it. Injury to the column matters because of what the column is carrying.

The Thorax

Twelve pairs of ribs, all articulating with the thoracic spine at the back.

• Ribs 1 through 7 are true ribs, attached directly to the sternum by their own cartilage.
• Ribs 8 through 10 are false ribs, attached to the cartilage of the rib above rather than to the sternum.
• Ribs 11 and 12 are floating ribs, with no anterior attachment at all.

The sternum has three parts: the manubrium at the top, the body, and the xiphoid process at the bottom. The angle of Louis is the ridge where the manubrium meets the body, and it sits at the level of the second rib, which makes it a landmark you can count from.

Rib fractures matter for what is under them. Fractures of ribs 1 and 2 take significant force and suggest injury to the great vessels. Fractures of ribs 9 through 12 sit over the liver on the right and the spleen on the left.

The Limbs and Girdles

Shoulder girdle: clavicle and scapula. The clavicle is one of the most commonly fractured bones in the body, usually from a fall on an outstretched arm.

Arm: humerus in the upper arm, then radius and ulna in the forearm. The radius is on the thumb side. The ulna is on the little finger side. Below that, eight carpals in the wrist, five metacarpals in the hand and fourteen phalanges in the fingers.

Pelvic girdle: two innominate bones, each formed from a fused ilium, ischium and pubis, joined to the sacrum at the back and to each other at the pubic symphysis at the front. The pelvis is a ring, and a ring generally breaks in more than one place.

Leg: femur in the thigh, which is the longest and strongest bone in the body. The patella sits over the front of the knee. Below that, tibia and fibula, with the tibia bearing the weight and the fibula on the lateral side. Then seven tarsals including the calcaneus (the heel), five metatarsals and fourteen phalanges.

Joints

A joint is where two bones meet, and the amount of movement allowed defines the type.

• Immovable (synarthrosis). The skull sutures.
• Slightly movable (amphiarthrosis). The pubic symphysis, the joints between vertebral bodies.
• Freely movable (diarthrosis). Every joint you think of as a joint. These are synovial joints, with a capsule, a lubricating synovial fluid and cartilage on the bone ends.

Synovial joints come in shapes. Ball and socket joints at the shoulder and hip move in every direction. Hinge joints at the elbow and knee move in one plane. Pivot joints let a bone rotate, as C1 does on C2. Gliding joints in the wrist and ankle allow small sliding movements.

Two connective tissues hold this together and students mix them up constantly.

• Ligament connects bone to bone and stabilizes a joint.
• Tendon connects muscle to bone and transmits pull.

Bone Structure

A long bone has a shaft called the diaphysis and two ends called epiphyses. Between them in a growing child sits the epiphyseal plate, the growth plate, which is where the bone lengthens. A fracture through the growth plate in a child can affect how that limb grows, which is why pediatric fractures near a joint are treated seriously even when they look minor.

The outer layer is compact bone, dense and strong. Inside the ends is cancellous bone, spongy and lighter. The center of the shaft is the medullary cavity, which holds marrow. Red marrow makes red cells, white cells and platelets. Yellow marrow is mostly stored fat.

The periosteum is the membrane covering the outside of a bone. It carries the bone's blood vessels and nerve endings, which is the reason a fracture is so painful and why bone bleeds so readily.

Why This Matters in the Field

Fractures bleed, and the big ones bleed a lot. A closed femur fracture can hold one to two liters of blood inside the thigh with nothing visible from the outside. A pelvic fracture can hold considerably more, because the pelvic ring sits over a dense network of veins and the space has no natural limit. Bilateral femur fractures alone can account for a large share of an adult's circulating volume.

That is why a patient with a long bone or pelvic injury gets treated as a bleeding patient, not just a broken one.

The skull does not expand. Bleeding or swelling inside a rigid box raises the pressure on the brain, and the brain is what pays for it. See the nervous system page for what that looks like.

Bone quality changes with age. Older adults lose bone density, so a fall from standing can fracture a hip. Children have more flexible bone, so they get incomplete fractures such as greenstick and buckle patterns, and they can suffer serious internal injury with no fracture at all because the flexible ribs transmit force instead of breaking.

What to Take Away

206 bones, split into axial for protection and appendicular for movement. Spine counts 7, 12, 5, 5, 4. Ligament is bone to bone, tendon is muscle to bone. Bone is living, vascular tissue, so a femur or pelvic fracture is a hemorrhage problem before it is anything else.