Support & movement · Topic 5 of 18

Skeletal System

Skeletal Divisions & Bones

Tap any line to see why it matters

Anatomy · skeletal system

Skeletal Divisions & Bones

The adult skeleton has 206 bones: 80 axial (skull, spine, rib cage) and 126 appendicular (limbs plus girdles).

01 · AxialAxial = skull + spine + rib cage (80)

02 · AppendicularAppendicular = limbs + girdles (126)

03 · VertebraeVertebrae: 7 cervical, 12 thoracic, 5 lumbar

What it is

Skeleton jobs: support, protect, move, store minerals, make blood must know

Parts & jobs

Axial skeleton (80) = skull, vertebral column, thoracic cage must know

Appendicular skeleton (126) = limbs + pectoral and pelvic girdles must know

Skull = 22 bones (8 cranial + 14 facial); only the mandible moves

Cranial bones: frontal, 2 parietal, 2 temporal, occipital, sphenoid, ethmoid

12 rib pairs: true 1–7, false 8–12 (floating = 11–12) + sternum

Pectoral girdle = clavicle (collarbone) + scapula (shoulder blade)

Upper limb: humerus → radius + ulna → 8 carpals, 5 metacarpals, 14 phalanges

Hip (coxal) bone = fused ilium + ischium + pubis

Lower limb: femur, patella, tibia + fibula, 7 tarsals, 5 metatarsals, 14 phalanges

Know by heart

Adult skeleton = 206 bones (newborn ~300; some fuse with growth) must know

Spine, top down: 7 cervical, 12 thoracic, 5 lumbar, sacrum, coccyx

7 cervical + 12 thoracic + 5 lumbar = 24 movable vertebrae

Axial or appendicular?

Commonly confused

	Axial (80)	Appendicular (126)
Includes	Skull, vertebral column, thoracic cage	Upper and lower limbs + their girdles
Girdles	None (the sacrum anchors the hips)	Pectoral (clavicle, scapula); pelvic (hip bones)
Main job	Protect brain, spinal cord, heart, lungs	Movement and handling the environment
Examples	Mandible, vertebrae, sternum, ribs	Clavicle, humerus, femur, phalanges

Mix-ups to avoid

33 vertebrae develop; fusion leaves 26 adult bones (24 + sacrum + coccyx)

Radius = thumb side (lateral); ulna = little-finger side (medial)

Femur = longest bone; tibia (medial) bears weight, fibula (lateral) doesn't

More detail

Show 5 more facts

Axial 80 also counts the hyoid and the 6 ear ossicles

C1 atlas carries the skull; C2 axis lets the head rotate

Spinal curves: thoracic + sacral primary; cervical + lumbar secondary

Facial bones: maxilla, zygomatic, nasal, lacrimal, palatine, mandible, vomer

Calcaneus = heel bone, the largest foot bone

Pearl

Axial bones form the body's axis; appendicular bones are the appendages and the girdles that hang them on.

Bone Tissue, Formation & Joints

Tap any line to see why it matters

Anatomy · bone tissue & joints

Bone Tissue, Formation & Joints

Bones come in five shapes; osteoblasts build and osteoclasts break down compact and spongy bone; joints are fibrous, cartilaginous or synovial.

01 · Bone shapes5 shapes: long, short, flat, irregular, sesamoid

02 · Bone cellsOsteoblasts build; osteoclasts break down

03 · Joint typesJoints: fibrous, cartilaginous, synovial

What it is

Minerals make bone hard; collagen makes it flexible

Parts & jobs

5 bone shapes: long, short, flat, irregular, sesamoid must know

Osteoblasts build, osteocytes maintain, osteoclasts break down bone must know

Compact bone = dense outer layer; spongy bone = inner lattice must know

Joints by structure: fibrous, cartilaginous, synovial must know

Long bone: diaphysis = shaft; epiphyses = the two ends

Periosteum covers bone; endosteum lines the marrow cavity

Red marrow makes blood cells; yellow marrow stores fat

Fibrous joints barely move, e.g. skull sutures

Cartilaginous joints move a little: intervertebral discs, pubic symphysis

Synovial = most common, freely movable, fluid-filled capsule

Ball-and-socket: shoulder, hip · hinge: elbow, knee · pivot: C1–C2

Step by step

Intramembranous = from membrane (skull); endochondral = replaces cartilage

Growth in length happens at the epiphyseal (growth) plate

Remodeling: lifelong breakdown + rebuild; stress and exercise add bone

Compact or spongy bone?

Commonly confused

	Compact bone	Spongy (cancellous) bone
Look	Dense and solid	Lattice of thin bony spikes (trabeculae)
Where	Outer layer; walls of the long-bone shaft	Inside bone; fills the epiphyses
Structural unit	Osteon (Haversian system)	Trabeculae, no osteons
Marrow	Shaft cavity holds yellow marrow	Spaces hold red marrow

Mix-ups to avoid

Ligament = bone to bone; tendon = muscle to bone

More detail

Show 6 more facts

Osteon = matrix rings around a central canal; osteocytes sit in lacunae

Also synovial: condyloid (knuckles), saddle (thumb base), plane (gliding)

Osteogenic cells divide → osteoblasts; osteoclasts come from monocytes

Appositional growth widens bone: add under periosteum, remove inside

Calcium builds bone, and vitamin D is needed to absorb it

Syndesmosis = radius–ulna membrane; gomphosis = tooth socket; bursa = cushion

Pearl

Blasts build, clasts break down: osteoblasts lay bone down and osteoclasts dissolve it.

Sources · OpenStax Anatomy & Physiology 2e §6.1 The Functions of the Skeletal System · OpenStax Anatomy & Physiology 2e §6.2 Bone Classification · OpenStax Anatomy & Physiology 2e §6.3 Bone Structure · OpenStax Anatomy & Physiology 2e §6.4 Bone Formation and Development · OpenStax Anatomy & Physiology 2e §6.6 Exercise, Nutrition, Hormones, and Bone Tissue · OpenStax Anatomy & Physiology 2e §9.1 Classification of Joints · OpenStax Anatomy & Physiology 2e §9.2 Fibrous Joints · OpenStax Anatomy & Physiology 2e §9.3 Cartilaginous Joints · OpenStax Anatomy & Physiology 2e §9.4 Synovial Joints · Union Test Prep — HESI A&P study guide, page 2 (skeletal system) · Union Test Prep — HESI A&P study guide, page 1 (bone formation) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

Practice this topic, then test the whole section.

Start HESI prep free

HESI and HESI A2 are registered trademarks of Elsevier. NurseSavvy is not affiliated with or endorsed by Elsevier.