Chapter 7 Part B – The Skeleton: Vertebral Column & Thoracic Cage

Vertebral Column: Gross Overview

• Extends from skull to pelvis; aka spine or spinal column.
• Composition: $26$ irregular bones (vertebrae) arranged in a flexible, curved, vertical stack.
• Principal functions:
  • Transmit weight of trunk to lower limbs.
  • Surround & protect spinal cord.
  • Provide attachment points for ribs & back/neck muscles.
• Overall length: ~$28\,\text{in}$ (~$71\,\text{cm}$).

Regions & Curvatures of the Spine

• Five major vertebral regions ("meal-time" mnemonic: $7\,\text{am}$, $12\,\text{pm}$, $5\,\text{pm}$):
  • Cervical – $7$ vertebrae, $C<em>1–C</em>7$
  • Thoracic – $12$ vertebrae, $T<em>1–T</em>{12}$
  • Lumbar – $5$ vertebrae, $L<em>1–L</em>5$
  • Sacrum – one bone from fusion of $5$ vertebrae; articulates with hip bones.
  • Coccyx – terminus; fusion of $3–5$ small vertebrae.
• Four normal curvatures improve resilience & flexibility:
  • Cervical (concave posteriorly)
  • Lumbar (concave posteriorly)
  • Thoracic (convex posteriorly)
  • Sacral (convex posteriorly)

Vertebral Support Structures

Ligaments

• Work with trunk musculature to maintain upright posture.
• Anterior & Posterior Longitudinal Ligaments
  • Continuous bands on anterior & posterior aspects of vertebral bodies (neck ➔ sacrum).
  • Resist hyperextension (anterior) & hyperflexion (posterior).
• Ligamentum Flavum – connects adjacent vertebrae; high elastic-fiber content.
• Short segmental ligaments – unite each vertebra with the ones just above & below.

Intervertebral Discs

• Located between adjacent bodies; act as shock absorbers.
• Make up ~$25\%$ of total spinal height.
• Two parts:
  1. Nucleus pulposus – inner gelatinous core; provides elasticity & compressibility.
  2. Anulus fibrosus – outer collar of collagen + fibrocartilage; limits expansion of nucleus under compression.

Clinical Homeostatic Imbalances (Spine)

• Herniated (Prolapsed) Disc
  • Rupture of anulus fibrosus ➔ nucleus pulposus protrudes & may compress spinal nerves/spinal cord.
  • Symptoms: numbness, excruciating pain.
  • Management: exercise, massage, heat, analgesics; surgery if conservative care fails.
• Abnormal Curvatures
  • Scoliosis – lateral rotation (commonly thoracic); severe cases affect breathing.
  • Kyphosis (hunchback) – exaggerated dorsal thoracic curve. Causes: osteoporosis, TB, rickets, osteomalacia.
  • Lordosis (swayback) – accentuated lumbar curve. Causes: disease, pregnancy, obesity (pot-belly).

General Anatomy of a Typical Vertebra

• Body (centrum) – anterior, weight-bearing.
• Vertebral arch – posterior ring formed by:
  • Pedicles (2) – walls.
  • Laminae (2) – roof.
• Vertebral foramen – opening enclosed by body & arch ➔ forms vertebral canal.
• Intervertebral foramina – lateral notches between adjoining pedicles for spinal nerves.
• Seven common processes:
  • Spinous (1) – projects posteriorly.
  • Transverse (2) – project laterally.
  • Superior articular (2) – upward-facing facets.
  • Inferior articular (2) – downward-facing facets.

Regional Vertebral Characteristics

Cervical (C₁–C₇)

• Smallest, lightest.
• Shared traits $C<em>3–C</em>7$:
  • Oval body.
  • Bifid spinous processes (except $C_7$).
  • Large triangular vertebral foramen.
  • Transverse foramina in transverse processes for vertebral arteries.
  • $C_7$ = vertebra prominens (palpable landmark).
• Atlas (C₁)
  • No body or spinous process.
  • Anterior & posterior arches + lateral masses.
  • Superior articular facets receive occipital condyles ➔ "Yes" nodding motion.
• Axis (C₂)
  • Body bears dens (odontoid process) – the "missing" body of atlas.
  • Dens acts as pivot allowing atlas rotation ➔ "No" gesture.

Thoracic (T₁–T₁₂)

• Gradually increase in size inferiorly; all articulate with ribs.
• Hallmarks:
  • Heart-shaped body with two demifacets per side (single facet on $T<em>{10–T</em>{12}}$).
  • Circular vertebral foramen.
  • Long, inferior-pointing spinous process.
  • Transverse costal facets on transverse processes (except $T<em>{11–T</em>{12}}$) for rib tubercles.
  • Articular facets oriented in coronal plane ➔ permits rotation; limits flex/extension.

Lumbar (L₁–L₅)

• "Small of back"; receives most stress ➔ massive kidney-shaped bodies.
• Short, thick pedicles & laminae.
• Flat, hatchet-shaped spinous processes projecting straight posteriorly (good site for lumbar puncture above $L_3$).
• Triangular vertebral foramen.
• Articular facets oriented medially/laterally ➔ locks vertebrae together, preventing rotation while allowing flexion/extension.

Sacrum (S₁–S₅ fused)

• Triangular bone forming posterior pelvic wall.
• Superior articular process interfaces with $L_5$; inferiorly joins coccyx.
• Auricular surfaces laterally articulate with hip (sacroiliac joints).
• Landmarks:
  • Sacral promontory – anterosuperior margin.
  • Transverse ridges – fusion lines.
  • Anterior/posterior sacral foramina – passage for spinal nerves/vessels.
  • Median sacral crest – fused spinous processes.
  • Sacral canal – continuation of vertebral canal; ends at sacral hiatus.
• Alae – lateral wing-like expansions.

Coccyx

• "Tailbone" of $3–5$ fused tiny vertebrae; articulates with sacrum; virtually vestigial.

Thoracic Cage (Bony Thorax)

• Components:
  • Posterior – thoracic vertebrae.
  • Lateral – ribs.
  • Anterior – sternum + costal cartilages.
• Roles:
  • Protects heart, lungs, great vessels.
  • Supports shoulder girdles & upper limbs.
  • Provides attachment for neck, back, chest, shoulder muscles.

Sternum (Breastbone)

• Fusion of three bones:
  1. Manubrium – articulates with clavicles & ribs $1$ and $2$.
  2. Body – articulates with costal cartilages of ribs $2–7$.
  3. Xiphoid process – muscular attachment; ossifies ~age $40$.
• Important surface landmarks:
  • Jugular notch – central indentation on superior manubrium.
  • Sternal angle – horizontal ridge where manubrium joins body (useful for rib $2$ counting).
  • Xiphisternal joint – junction of body & xiphoid.

Clinical Note (Thoracic Cage)

• Xiphoid process can project posteriorly; blunt trauma or misplaced CPR hands can drive it into liver/heart ➔ fatal hemorrhage. Correct CPR hand placement = midsternum.

Ribs

• $12$ pairs attach posteriorly to thoracic vertebrae.
• Classification:
  1. True (vertebrosternal) ribs – pairs $1–7$; direct costal cartilage ➔ sternum.
  2. False (vertebrochondral) ribs – pairs $8–10$; costal cartilage joins cartilage above.
  3. Floating (vertebral) ribs – pairs $11–12$; no anterior attachment.
• General rib anatomy:
  • Shaft – bulk; costal groove on inner surface houses intercostal nerves & vessels.
  • Head – articulates with bodies (demifacets) of two adjacent vertebrae.
  • Neck – constricted segment beyond head.
  • Tubercle – articulates with transverse costal facet of corresponding vertebra.
  • Angle – point where rib curves sharply anteriorly.
  • Junction with costal cartilage – anterior end linking to sternum (directly or indirectly).

Ethical & Practical Connections

• Understanding spinal curvature deviations guides early screening (e.g., school scoliosis checks) to prevent long-term cardiopulmonary compromise.
• Knowledge of vertebral anatomy critical for safe lumbar puncture, epidural anesthesia (performed below $L_2$ to avoid spinal cord injury).
• CPR training emphasizes sternal landmarks to minimize potential organ damage.

Integrative/Foundational Ties

• The vertebral column’s segmentation reflects embryonic somite organization and underpins dermatomal nerve distribution.
• Intervertebral disc structure (nucleus pulposus, anulus fibrosus) recapitulates notochord and mesenchyme origins—linking development to adult pathology (herniation).
• Biomechanics: curvature arrangement acts as a spring—mathematically modeled as alternating concave/convex arcs absorbing axial load.