The Skeleton
Skull, vertebral column, thoracic cage, girdles and limbs
By the end of this unit you can…
- ✓Distinguish the axial and appendicular skeletons and count the bones in each
- ✓Identify the cranial and facial bones and their major markings and sutures
- ✓Describe the regions and curvatures of the vertebral column and features of a typical vertebra
- ✓Describe the thoracic cage: sternum, true/false/floating ribs
- ✓Identify the bones of the pectoral girdle and upper limb, pelvic girdle and lower limb
- ✓Compare male and female pelves
Key terms
Practice →1Axial vs appendicular
The adult skeleton has 206 bones in two divisions:
- Axial skeleton (80 bones) — the long axis of the body: skull, vertebral column and thoracic cage. Supports the head, neck and trunk and protects the brain, spinal cord and thoracic organs.
- Appendicular skeleton (126 bones) — the limbs plus the pectoral and pelvic girdles that attach them to the axial skeleton. Built for movement.
2The skull
The skull has 8 cranial bones (form the cranial vault and base that enclose the brain) and 14 facial bones (the face framework, cavities for the senses, openings for air and food, and attachment for facial muscles). Most skull bones are flat bones joined by immovable sutures.
| Cranial bones (8) | Key features |
|---|---|
| Frontal (1) | Forehead, roof of the orbits; supraorbital margins & foramina; frontal sinuses |
| Parietal (2) | Most of the superior and lateral cranial walls |
| Occipital (1) | Posterior and base; foramen magnum (spinal cord passes); occipital condyles articulate with C1 |
| Temporal (2) | Inferolateral walls; external acoustic meatus, mastoid process, styloid process, zygomatic process, mandibular fossa; the petrous part houses the middle and inner ear |
| Sphenoid (1) | Butterfly-shaped keystone — articulates with all other cranial bones; sella turcica holds the pituitary gland |
| Ethmoid (1) | Deepest bone; cribriform plate (olfactory nerves pass), crista galli, perpendicular plate (nasal septum), superior and middle nasal conchae |
| Suture | Joins |
|---|---|
| Coronal | Frontal ↔ parietal bones |
| Sagittal | Right ↔ left parietal bones |
| Lambdoid | Parietal ↔ occipital bones |
| Squamous | Parietal ↔ temporal bone (each side) |
| Facial bones (14) | Key features |
|---|---|
| Mandible (1) | Lower jaw; the only freely movable skull bone (temporomandibular joint); holds lower teeth; mental foramen |
| Maxillae (2) | Upper jaw and central face — the keystone of the face; articulate with all facial bones except the mandible; maxillary sinuses; palatine processes form the anterior hard palate |
| Zygomatic (2) | Cheekbones; lateral orbit walls |
| Nasal (2) | Bridge of the nose |
| Lacrimal (2) | Medial orbit walls; tear duct groove |
| Palatine (2) | Posterior hard palate |
| Vomer (1) | Inferior part of the nasal septum |
| Inferior nasal conchae (2) | Thin curved bones in the lateral nasal cavity walls |
The paranasal sinuses (in the frontal, sphenoid, ethmoid and maxillary bones) are mucosa-lined air cavities that lighten the skull and enhance voice resonance. The hyoid bone (in the neck) is unique: it does not articulate with any other bone — it is held by ligaments and serves as a movable base for the tongue.
3The vertebral column
The vertebral column (spine) has 26 irregular bones in an adult: 7 cervical, 12 thoracic, 5 lumbar, the sacrum (5 fused) and the coccyx (4 fused). It transmits weight to the lower limbs, surrounds the spinal cord, and anchors the ribs and back muscles.
The S-shaped curvatures increase resilience and flexibility. Cervical and lumbar curves are concave posteriorly (secondary curves — develop as a baby lifts its head and walks); thoracic and sacral curves are convex posteriorly (primary curves, present at birth).
Intervertebral discs cushion the vertebrae: a gel-like inner nucleus pulposus surrounded by a tough outer ring of fibrocartilage, the anulus fibrosus.
- Body
- Weight-bearing, disc-shaped anterior part.
- Vertebral arch
- Formed by two pedicles and two laminae; with the body it encloses the vertebral foramen (successive foramina form the vertebral canal for the spinal cord).
- Spinous process
- Single projection posteriorly where laminae meet (the bumps you feel down your back).
- Transverse processes
- Two lateral projections from the pedicle–lamina junction.
- Superior & inferior articular processes
- Form joints with vertebrae above and below.
- Intervertebral foramina
- Notches between vertebrae through which spinal nerves exit.
| Region | Distinctive features |
|---|---|
| Cervical (C1–C7) | Small bodies; transverse foramina (vertebral arteries); bifid spinous processes (C2–C6). C1 atlas — no body, holds the skull (“yes” nod). C2 axis — dens projects up into the atlas (“no” rotation). C7 = vertebra prominens |
| Thoracic (T1–T12) | Heart-shaped bodies; facets for the ribs on bodies and transverse processes; long spinous processes point inferiorly |
| Lumbar (L1–L5) | Massive kidney-shaped bodies (bear most weight); short, hatchet-shaped spinous processes |
| Sacrum | 5 fused vertebrae; forms the posterior pelvic wall; sacroiliac joints; sacral promontory |
| Coccyx | “Tailbone”; 3–5 fused vertebrae |
4The thoracic cage
The thoracic cage = thoracic vertebrae (posterior) + ribs + sternum + costal cartilages. It protects the heart and lungs and supports the shoulder girdle.
- Sternum
- Breastbone: manubrium (top; jugular notch, clavicular notches), body (middle), xiphoid process (tip). The sternal angle — where manubrium meets body — marks the level of the 2nd rib, a landmark for counting ribs and finding heart valves.
- True ribs (1–7)
- Attach directly to the sternum by their own costal cartilage.
- False ribs (8–12)
- Attach indirectly (8–10 join the cartilage above) or not at all.
- Floating ribs (11–12)
- False ribs with no anterior attachment.
5Pectoral girdle & upper limb
Each pectoral (shoulder) girdle has a clavicle (collarbone — a strut holding the arm away from the trunk; the most frequently broken bone) and a scapula (shoulder blade). Features of the scapula: acromion (tip of the shoulder; joins the clavicle), coracoid process, spine, and the glenoid cavity, which receives the head of the humerus. The girdle is very mobile because only the clavicle attaches to the axial skeleton and the glenoid cavity is shallow — but that makes the shoulder easy to dislocate.
| Bone | Key markings |
|---|---|
| Humerus (arm) | Head, anatomical & surgical neck (common fracture site), greater & lesser tubercles, intertubercular sulcus, deltoid tuberosity; distally the trochlea (with ulna), capitulum (with radius), medial & lateral epicondyles, olecranon & coronoid fossae |
| Ulna (forearm, medial — little-finger side) | Olecranon (point of the elbow), coronoid process, trochlear notch grips the humerus; head distally; styloid process |
| Radius (forearm, lateral — thumb side) | Head (proximal) articulates with the capitulum; radial tuberosity; distal end forms most of the wrist joint; styloid process |
The hand has 8 carpals (wrist), 5 metacarpals (palm, numbered 1–5 from the thumb) and 14 phalanges (fingers: proximal, middle and distal in each finger; the thumb, or pollex, lacks a middle phalanx).
6Pelvic girdle & lower limb
The pelvic girdle = the two hip (coxal) bones, which attach securely to the axial skeleton at the sacroiliac joints and to each other anteriorly at the pubic symphysis. With the sacrum and coccyx it forms the bony pelvis. It bears weight, so it is far more stable (and less mobile) than the pectoral girdle.
Each hip bone is three fused bones: ilium (superior, flaring; iliac crest), ischium (posteroinferior; ischial tuberosities bear weight when sitting) and pubis (anterior). All three meet at the acetabulum — the deep socket for the head of the femur.
| Feature | Female | Male |
|---|---|---|
| Overall | Lighter, thinner bones; shallower | Heavier, thicker; deeper |
| Pelvic inlet | Wider, oval (from side to side) | Narrow, heart-shaped |
| Subpubic (pubic) angle | Wide — > 80° (often ~90–100°) | Narrow — < 70° |
| Sacrum | Shorter, wider, less curved | Narrow, longer, more curved |
| Why | Adapted for childbearing | Adapted for heavier build |
| Bone | Key markings |
|---|---|
| Femur (thigh) | Largest, longest, strongest bone. Head (fovea capitis), neck (common fracture site in the elderly), greater & lesser trochanters, gluteal tuberosity, linea aspera; distally medial & lateral condyles and epicondyles |
| Patella | Kneecap — a sesamoid bone in the quadriceps tendon |
| Tibia (shin, medial) | Weight-bearing; medial & lateral condyles, tibial tuberosity (patellar ligament), anterior border (shin), medial malleolus (inner ankle bump) |
| Fibula (lateral) | Thin, non-weight-bearing; lateral malleolus (outer ankle bump) |
The foot has 7 tarsals (the talus articulates with the tibia and fibula; the calcaneus is the heel bone that bears weight and anchors the Achilles tendon), 5 metatarsals and 14 phalanges (the big toe, or hallux, has two). Three arches (medial and lateral longitudinal, and transverse) distribute weight and act as shock absorbers.