Skeletal System Comprehensive Study Notes

Skeletal System: Global Overview

• Total bones in an average adult human: 206.
  • Axial skeleton: 80 bones (≈ 40\% of total).
  • Appendicular skeleton: 126 bones.

Axial Skeleton

• Forms the longitudinal axis of the body.

• Components & bone counts (adult):

  • Skull & associated bones: 29
  • Cranium: 8
  • Face: 14
  • Auditory ossicles: 6
  • Hyoid: 1
  • Vertebral column: 26
  • Cervical: 7
  • Thoracic: 12
  • Lumbar: 5
  • Sacrum: 1 (fused)
  • Coccyx: 1 (fused)
  • Thoracic cage: 25
  • Sternum: 1
  • Ribs: 24
  • Supplemental cartilages
  • Costal cartilages
  • Intervertebral discs

• Functional highlights:

  • Protects brain, spinal cord & visceral organs of trunk.
  • Provides muscle-attachment sites enabling:
  • Head/neck/trunk positioning.
  • Respiratory movements.
  • Stabilisation of the appendicular skeleton.
  • Joints are strong but allow only limited motion.

Skull

Cranial Bones (8)

• Occipital (1)
• Parietal (2)
• Frontal (1)
• Temporal (2)
• Sphenoid (1)
• Ethmoid (1)

Key concepts:

• Enclose the cranial cavity — a fluid-filled chamber cushioning the brain.
• Inner surface: attachment for vessels, nerves & meninges.
• Outer surface: attachment for eye, jaw & head muscles.
• Calvaria (skull-cap) created by occipital, parietal & frontal bones.

Facial Bones (14)

• Paired: Maxillae, Zygomatic, Nasal, Lacrimal, Palatine, Inferior nasal conchae.
• Single: Vomer, Mandible.
• Functions: framework for face, cavities for senses, openings for air/food, secure teeth, anchor facial muscles.

Sutures (immovable fibrous joints)

• Coronal — frontal ↔ parietal.
• Squamous — temporal ↔ parietal.
• Sagittal — between parietals.
• Lambdoid — occipital ↔ parietals (may host sutural bones).

Orbit (eye socket) – 7-bone composite

• Roof: Frontal.
• Lateral wall: Zygomatic.
• Floor: Maxilla.
• Medial wall: Lacrimal & Ethmoid.
• Posterior wall: Sphenoid & Palatine.

Nasal Complex & Paranasal Sinuses

• Bones contributing: Sphenoid, Ethmoid (ethmoidal air cells), Frontal, Maxillae, Palatine.
• Sinuses = air-filled chambers that lighten skull, amplify voice & offer mucous epithelium.
• Inflammation ⇒ sinusitis.

Special Individual Bones

• Mandible — only movable skull bone; notable parts: body, ramus, condylar & coronoid processes, alveolar process, mental foramen.
• Hyoid — U-shaped, suspended by ligaments/muscles (no joint to other bones); greater & lesser horns.
• Auditory ossicles — malleus, incus, stapes; transmit sound from tympanic membrane to inner ear.

Fontanelles (in infants)

• Large fibrous gaps allowing skull deformation during birth & brain growth.
• Types: anterior (largest), occipital, sphenoidal, mastoid.
• Occipital/sphenoidal/mastoid close ≈ 1−2 months; all close by age \le 5.

Vertebral Column

Composition & Regions (adult)

• Total bones: 26.
• Cervical: 7; Thoracic: 12; Lumbar: 5; Sacrum: 1; Coccyx: 1.
• Average total length: 71\,\text{cm}\, (28\,\text{in.}).

Functions

• Supports head/trunk; transfers weight to lower limbs; protects spinal cord; maintains upright posture.

Curvatures

• Primary (fetal): Thoracic & Sacral — accommodate organs.
• Secondary (postnatal): Cervical (head-lifting) & Lumbar (standing) — balance weight.

Generic Vertebra Anatomy

• Vertebral body — weight transfer.
• Vertebral arch — pedicles (sides) + laminae (roof).
• Spinous & transverse processes — muscle attachment; transverse may bear rib facets (thoracic) or foramina (cervical).
• Articular processes & facets (superior/inferior) — intervertebral joints.
• Vertebral foramen → vertebral canal (houses spinal cord).
• Intervertebral discs (fibrocartilage) & intervertebral foramina (nerve/vessel passage).

Region-Specific Differences

• Cervical (C1–C7)
  • Small body, large foramen; bifid spinous (except C1, C7).
  • Transverse foramina protect vertebral arteries/veins.
  • Atlas (C1): no body/spinous; articulates with occipital condyles; nodding “yes”.
  • Axis (C2): dens (odontoid) pivot; rotation “no”.
  • C7 = vertebra prominens; long palpable spinous; attachment for ligamentum nuchae.
• Thoracic (T1–T12)
  • Heart-shaped body; smaller foramen.
  • Spinous long, inferiorly directed.
  • Costal facets (body ± transverse) for rib articulation.
• Lumbar (L1–L5)
  • Massive oval bodies; smallest foramina.
  • Short, blunt spinous; slender transverse; no costal facets.
• Sacrum
  • Fusion of 5 vertebrae (≈ 15−30 yrs).
  • Concave anterior, convex posterior; more curved in males.
  • Sacral promontory, ala, sacral foramina; articulates with iliac bones (sacroiliac joint).
• Coccyx
  • Fusion of 3−5 rudimentary vertebrae starting ≈ 26 yrs.

Thoracic Cage

• Components: Thoracic vertebrae + Ribs (24) + Sternum.
• Functions: protect heart/lungs/thymus; attachment for breathing, posture & upper-limb muscles.

Ribs

• True (vertebrosternal) ribs 1−7 — direct cartilage to sternum.
• False ribs 8−12
  • Vertebrochondral 8−10 — cartilage merges with rib 7 cartilage.
  • Floating (vertebral) 11−12 — no anterior attachment.
• Parts: head (superior/inferior facets), neck, tubercle (articulates with transverse costal facet), angle, shaft, costal groove.
• Biomechanics: act like bucket handles; elevation ↑ thoracic volume, depression ↓.

Sternum

• Manubrium (jugular notch, clavicular notches) + Body + Xiphoid process.

Appendicular Skeleton

• Pectoral girdles (4) + Upper limbs (60) + Pelvic girdle (2) + Lower limbs (60).

Pectoral (Shoulder) Girdle

• Clavicle (S-shaped) — articulates medially at sternoclavicular & laterally at acromioclavicular joints.
• Scapula — flat triangular; notable landmarks: acromion, coracoid, spine, supraspinous/infraspinous fossae, subscapular fossa, glenoid cavity.

Upper Limb Bones

• Humerus — head (to glenoid), anatomical/surgical necks, greater/lesser tubercles, intertubercular sulcus, deltoid tuberosity, capitulum (radial articulation), trochlea (ulnar), radial/coronoid/olecranon fossae, medial/lateral epicondyles.
• Radius & Ulna — parallel; interosseous membrane.
  • Ulna: olecranon (posterior elbow), trochlear notch, coronoid process, radial notch, styloid.
  • Radius: head (proximal), radial tuberosity, ulnar notch (distal), styloid.
• Carpals 8 per wrist: proximal (Scaphoid, Lunate, Triquetrum, Pisiform); distal (Trapezium, Trapezoid, Capitate, Hamate).
• Metacarpals 5 numbered I–V; Phalanges 14 per hand (proximal, middle, distal).

Pelvic Girdle & Pelvis

• Each hip bone = fusion of Ilium (superior; iliac crest, fossa), Ischium (posteroinferior; tuberosity), Pubis (anterior; pubic symphysis).
• Acetabulum — socket for femoral head; obturator foramen reduces weight.
• Sex differences (female vs male): broader inlet/outlet, pubic angle ≥ 100^{\circ}, less sacral curvature, shallower, wider pelvis.

Lower Limb Bones

• Femur — longest/strongest; head (fovea capitis), neck (common fracture site), greater/lesser trochanters, intertrochanteric line/crest, linea aspera, medial/lateral condyles & epicondyles, patellar surface, intercondylar fossa.
• Patella — sesamoid within quadriceps tendon; improves leverage.
• Tibia (medial, weight-bearing): medial & lateral condyles, intercondylar eminence, tibial tuberosity, anterior crest, medial malleolus.
• Fibula (lateral, slender): head, shaft, lateral malleolus; stabilises ankle, not weight-bearing.
• Tarsals 7 per ankle: Talus (trochlea), Calcaneus (heel, calcaneal tendon), Navicular, Cuboid, Medial/Intermediate/Lateral Cuneiforms.
• Metatarsals 5 I–V; Phalanges 14 (big toe = hallux lacks middle phalanx).

Clinical & Functional Notes

• Ligamentum nuchae — elastic band maintaining cervical lordosis without muscle action.
• Transverse ligament binds atlas to dens; rupture ⇒ atlanto-axial instability.
• Costovertebral & costotransverse joints allow rib bucket-handle motion → crucial for ventilation.
• Intervertebral discs — nucleus pulposus + annulus fibrosus; degeneration/herniation can impinge spinal nerves.
• Osteoporosis often first manifests as compression fractures in vertebral bodies, especially thoracic & lumbar.
• Female pelvis adaptations facilitate childbirth; measurements (pelvimetry) assess obstetric suitability.