The axial and appendicular skeleton
The adult skeleton has 206 bones, organised into a central axis and the limbs attached to it.
Two divisions
The axial skeleton, 80 bones, forms the body's central axis: the skull, vertebral column, ribs, and sternum. The appendicular skeleton, 126 bones, includes the limbs and the girdles that attach them: the shoulder (pectoral) girdle and the hip (pelvic) girdle.
The skull
The skull has cranial bones that enclose the brain, including the frontal, parietal, temporal, and occipital bones, joined by immovable joints called sutures. Facial bones include the maxilla (upper jaw) and the mandible (lower jaw), the only movable skull bone. In infants, soft spots called fontanelles allow the skull to compress during birth and the brain to grow.
The vertebral column
The spine has 7 cervical (neck), 12 thoracic (chest), and 5 lumbar (lower back) vertebrae, plus the sacrum and coccyx (tailbone), formed from fused vertebrae. Its natural S-shaped curves absorb shock. Intervertebral discs cushion the vertebrae; a herniated disc can press on spinal nerves and cause pain.
The thoracic cage
There are 12 pairs of ribs. The first 7 pairs (true ribs) attach directly to the sternum through cartilage; pairs 8-10 attach indirectly; pairs 11-12 (floating ribs) do not attach at the front. The rib cage protects the heart and lungs and moves during breathing.
The upper limb
The pectoral girdle is the clavicle (collarbone) and scapula (shoulder blade). The arm has the humerus; the forearm has the radius (thumb side) and ulna (little finger side); the wrist has 8 carpals; the palm 5 metacarpals; and the fingers 14 phalanges. The shoulder trades stability for a wide range of motion.
The lower limb
The pelvic girdle is formed by the two hip bones, which join the sacrum. The thigh has the femur, the body's longest and strongest bone; the kneecap is the patella; the leg has the tibia (shin, weight-bearing) and fibula; the ankle has 7 tarsals; the foot 5 metatarsals; and the toes 14 phalanges.
Male and female pelvis
The female pelvis is generally wider and shallower, with a larger, rounder opening and a wider angle below the pubic bones, adaptations for childbirth. The male pelvis is narrower and heavier. Forensic scientists use these differences to estimate sex from skeletal remains.
Worked example
A person falls on an outstretched hand. The force travels up the arm. Commonly injured bones are the distal radius at the wrist and the scaphoid, a carpal bone, or the clavicle, the most frequently broken bone in the body. Knowing the bones' arrangement helps predict likely injuries.
Key takeaways
The adult skeleton has 206 bones: 80 axial and 126 appendicular. The skull has cranial bones joined by sutures plus facial bones including the movable mandible. The spine has 7 cervical, 12 thoracic, and 5 lumbar vertebrae plus the sacrum and coccyx. There are 12 pairs of ribs. Limbs attach through the pectoral and pelvic girdles; the femur is the longest bone.
Flashcard
206
Check your understanding
How many lumbar vertebrae does a typical adult have?