6.2 Skeletal System Anatomy
Key Takeaways
- Bones are classified by shape as long, short, flat, irregular (and often sesamoid); compact bone forms dense outer shells and spongy bone forms trabecular interiors
- The axial skeleton includes skull, vertebral column, ribs, and sternum; the appendicular skeleton includes pectoral and pelvic girdles and limb bones
- Joints are fibrous, cartilaginous, or synovial; synovial joints are freely movable and include hinge, ball-and-socket, pivot, plane, saddle, and condyloid types
- Red bone marrow produces blood cells; yellow marrow stores fat—locations shift with age from widespread red marrow toward axial and proximal limb sites
- Major long-bone anatomy: diaphysis (shaft), epiphyses (ends), metaphysis/growth plate region, periosteum, medullary cavity, articular cartilage
6.2 Skeletal System Anatomy
Quick Answer: The skeleton provides support, protection, levers for muscle, mineral storage, and blood-cell production (marrow). Know bone shapes (long, short, flat, irregular), compact vs spongy bone, axial vs appendicular bone lists, long-bone regions, red vs yellow marrow, and joint classes (fibrous, cartilaginous, synovial) with common synovial types. This is high-yield Human Anatomy structure—muscle contraction physiology comes later.
The adult human skeleton typically has 206 bones (variation exists with sesamoids and fused bones). For NEX Science, prioritize classification systems and landmark bones rather than every named foramen. Pair each major bone with its skeleton division (axial vs appendicular) and know how bones meet at joints.
Functions of the Skeletal System (Structural Context)
| Function | How anatomy supports it |
|---|---|
| Support | Rigid framework for soft tissues |
| Protection | Skull (brain), vertebrae (spinal cord), rib cage (heart/lungs), pelvis (pelvic organs) |
| Movement | Bones act as levers; joints are fulcrums; skeletal muscle attaches via tendons |
| Mineral storage | Calcium and phosphate in bone matrix |
| Hematopoiesis | Blood cell formation in red bone marrow |
| Fat storage | Yellow marrow in medullary cavities |
Bone Classification by Shape
| Type | Description | Examples |
|---|---|---|
| Long bones | Longer than wide; shaft + two ends; act as levers | Femur, humerus, tibia, radius, phalanges |
| Short bones | Roughly cube-like; stability with some motion | Carpals, tarsals |
| Flat bones | Thin, often curved; protection + broad muscle attachment | Sternum, ribs, scapula, most cranial bones |
| Irregular bones | Complex shapes not in other categories | Vertebrae, many facial bones, coxal (hip) bones |
| Sesamoid bones | Form in tendons; reduce friction / alter leverage | Patella (largest); variable small sesamoids in hands/feet |
Long bone ≠ only limb “long” bones in casual speech: even short phalanges are classified as long bones because of their shaft-and-ends architecture.
Gross Anatomy of a Long Bone
| Region / structure | Description |
|---|---|
| Diaphysis | Shaft; thick collar of compact bone surrounding medullary cavity |
| Epiphysis | Expanded end; outer compact bone, interior spongy bone; forms joint surfaces |
| Articular cartilage | Hyaline cartilage covering epiphyseal joint surfaces; reduces friction |
| Metaphysis | Between diaphysis and epiphysis; in growing bone includes epiphyseal (growth) plate of hyaline cartilage |
| Epiphyseal line | Bony remnant of growth plate after closure in adults |
| Periosteum | Outer fibrous membrane (except on articular cartilage); osteogenic layer; sharpey fibers anchor it; rich nerves/vessels |
| Endosteum | Thin membrane lining medullary cavity and trabeculae |
| Medullary cavity | Hollow shaft interior; yellow marrow in adult long bones |
Flat bones use a “sandwich” pattern: compact bone tables with spongy bone (diploë in skull) between—no long medullary cavity like a femur.
Bone Tissue: Compact vs Spongy
Compact (Cortical) Bone
Compact bone is dense and forms the outer shell of all bones and the bulk of diaphyses. Microscopically it is organized into osteons (Haversian systems): concentric lamellae of matrix around a central canal carrying vessels and nerves. Osteocytes live in lacunae connected by canaliculi. Perforating (Volkmann) canals link central canals to periosteum and marrow cavity.
Spongy (Trabecular / Cancellous) Bone
Spongy bone is a lattice of trabeculae with spaces filled by marrow. It is prominent in epiphyses, short bones, flat bone interiors, and irregular bones. Trabeculae align along stress lines to provide strength with less mass. Spongy bone is not organized into complete osteons like compact bone.
| Feature | Compact bone | Spongy bone |
|---|---|---|
| Density | Dense, few spaces | Porous lattice |
| Location emphasis | Diaphyseal walls; outer surfaces | Epiphyses; bone interiors |
| Unit structure | Osteons | Trabeculae |
| Marrow | Borders medullary cavity | Marrow in intertrabecular spaces |
Bone cells (brief anatomy labels): osteogenic cells → osteoblasts (build matrix) → trapped as osteocytes (maintain); osteoclasts (resorb bone). Remodeling keeps structure adaptive; detailed calcium homeostasis is physiology-heavy and covered elsewhere.
Axial vs Appendicular Skeleton
Axial Skeleton (~80 bones)
Central axis of the body:
| Region | Major bones / notes |
|---|---|
| Skull | Cranial bones (protect brain: frontal, parietal, temporal, occipital, sphenoid, ethmoid) + facial bones (e.g., maxilla, mandible, zygomatic, nasal) |
| Hyoid | Anterior neck; no direct bony articulation; muscle/ligament support; speech/swallowing context |
| Vertebral column | Cervical (7), thoracic (12), lumbar (5), sacrum (fused), coccyx (fused); protect spinal cord; intervertebral discs of fibrocartilage between movable vertebrae |
| Thoracic cage | Sternum (manubrium, body, xiphoid) + ribs (12 pairs: true ribs 1–7 direct to sternum via costal cartilage; false ribs 8–10 indirect; floating ribs 11–12 no anterior attachment) |
Vertebra landmarks (intro level): body, vertebral foramen, spinous and transverse processes, superior/inferior articular processes. Atlas (C1) and axis (C2) are specialized for head nodding and rotation (dens of axis).
Appendicular Skeleton (~126 bones)
Limbs and girdles that attach limbs to the axial skeleton:
| Region | Major bones |
|---|---|
| Pectoral (shoulder) girdle | Clavicle and scapula (not the humerus—the humerus is the free upper limb) |
| Upper limb | Humerus; radius (lateral) and ulna (medial); carpals (8); metacarpals (5); phalanges (14 per hand) |
| Pelvic (hip) girdle | Two coxal (hip) bones—each from fused ilium, ischium, pubis; articulate with sacrum at sacroiliac joints; together with sacrum form bony pelvis |
| Lower limb | Femur; patella; tibia (medial, weight-bearing) and fibula (lateral); tarsals (7, including talus and calcaneus); metatarsals (5); phalanges (14 per foot) |
Axial vs Appendicular Quick Sort
| Axial | Appendicular |
|---|---|
| Skull, hyoid, vertebrae, ribs, sternum | Clavicle, scapula, upper limb bones |
| Coxal bones, lower limb bones |
Exam trap: Scapula and clavicle are appendicular (pectoral girdle), not axial. The pelvis includes axial sacrum plus appendicular coxal bones.
Bone Marrow
| Type | Primary role | Typical adult locations |
|---|---|---|
| Red marrow | Hematopoiesis (RBCs, WBCs, platelets) | Axial skeleton (sternum, vertebrae, ribs, skull), pelvic bones, proximal ends of humerus and femur |
| Yellow marrow | Fat storage; can convert toward red if demand is high | Medullary cavities of long bones in adults |
In infants, most marrow is red; with age, many sites convert to yellow. Knowing that blood is made in red marrow links skeletal anatomy to hematology and lab values you will meet in nursing.
Joints (Articulations): Classification
Joints are classified by structure (what binds the bones) and function (how much movement).
Structural Classes
| Structural type | Binding material | Mobility (general) | Examples |
|---|---|---|---|
| Fibrous | Dense fibrous CT | Usually immovable or slight | Sutures of skull; syndesmosis (e.g., distal tibiofibular); gomphosis (tooth in socket) |
| Cartilaginous | Cartilage | Immovable or slight | Synchondrosis (epiphyseal plate; first rib–sternum); symphysis (pubic symphysis; intervertebral discs) |
| Synovial | Joint cavity with synovial fluid; articular cartilage; capsule | Freely movable (diarthroses) | Shoulder, hip, knee, elbow, most limb joints |
Functional Classes (often paired with structure)
- Synarthrosis — immovable (many fibrous)
- Amphiarthrosis — slightly movable (many cartilaginous)
- Diarthrosis — freely movable (all synovial)
Synovial Joint Essentials
Typical synovial joint features:
- Articular (hyaline) cartilage on bone ends
- Joint (articular) cavity with synovial fluid (lubrication, nutrient diffusion to cartilage)
- Articular capsule (fibrous outer + synovial membrane inner)
- Reinforcing ligaments (intrinsic or extrinsic)
- Optional: menisci, bursae, tendon sheaths (e.g., knee menisci)
Major Synovial Joint Types (by shape of surfaces)
| Type | Movement summary | Example |
|---|---|---|
| Plane (gliding) | Flat surfaces slide | Intercarpal; intertarsal; some vertebral articular processes |
| Hinge | Flexion/extension in one plane | Elbow; knee (primarily); interphalangeal |
| Pivot | Rotation around long axis | Atlantoaxial (C1–C2); proximal radioulnar |
| Condylar (ellipsoid) | Flexion/extension + abduction/adduction | Metacarpophalangeal (knuckles) |
| Saddle | Similar to condylar; freer | Carpometacarpal of thumb |
| Ball-and-socket | Multiaxial: flex/extend, ab/aduct, rotation | Shoulder (glenohumeral); hip |
Ligaments connect bone to bone and stabilize joints; tendons connect muscle to bone (muscular system section). Confusing the two is a classic exam error.
Cartilage Types Touching the Skeleton
| Cartilage | Where related to skeleton |
|---|---|
| Hyaline | Articular surfaces; costal cartilages; growth plates; embryonic model for endochondral bones |
| Fibrocartilage | Intervertebral discs; pubic symphysis; menisci—shock absorption |
| Elastic | External ear, epiglottis—more flexibility (not primary joint cartilage) |
Clinical and Nursing Anchors
- Fractures: described by bone, location (e.g., midshaft humerus), and pattern; periosteum and marrow bleeding explain swelling and fat-embolism risk context in long-bone trauma (awareness level).
- Osteoporosis: loss of bone mass/density—architecture still compact/spongy but thinner trabeculae—falls and vertebral compression fractures.
- Arthroplasty and joint disease: osteoarthritis wears articular cartilage of synovial joints.
- Marrow biopsy: often posterior iliac crest—pelvic bone with red marrow access.
- Growth plates: injuries in children can affect length if epiphyseal plate is damaged.
Exam Traps
- Scapula/clavicle: appendicular, not axial.
- Radius vs ulna: radius lateral (thumb side) in anatomical position; ulna medial.
- Tibia vs fibula: tibia is the weight-bearing medial leg bone; fibula is lateral and thinner.
- Patella: sesamoid in the quadriceps tendon.
- Red marrow ≠ yellow: red = blood cell production; yellow = fat.
- All joints movable? No—sutures are fibrous and essentially immovable in adults.
- Knee as pure hinge: primarily hinge-like but has some rotation when flexed—still classified among hinge-type examples on intro exams.
- Compact bone only in long bones? No—every bone has compact outer bone; amounts vary.
Study Map for NEX
- Drill shape classes with two examples each.
- Recite axial list vs appendicular list out loud.
- Label a long-bone diagram: diaphysis, epiphysis, periosteum, marrow cavity, articular cartilage.
- Master the three structural joint classes and the six synovial types with one example each.
- Link red marrow sites to hematopoiesis.
With those five drills, most skeletal anatomy items become recognition rather than guesswork—and you are ready to attach muscles (next section) onto this bony framework.
Which group of bones belongs entirely to the axial skeleton?
Red bone marrow is best described as the tissue that:
The hip joint and shoulder joint are examples of which synovial joint type?