Axial Skeleton Overview

Definition and Components

The axial skeleton forms the central axis of the human body and includes the skull, vertebral column, and thoracic cage. It provides support, protection for vital organs, and attachment points for muscles.

• Skull: Protects the brain and forms the structure of the face.

• Vertebral column: Supports the body and protects the spinal cord.

• Thoracic cage: Includes ribs and sternum, protecting the heart and lungs.

Bone Basics

Types and Structure of Bones

Bones are classified by shape and internal structure, which relate to their function and location.

• Compact bone: Dense outer layer providing strength.

• Spongy bone: Trabecular inner structure, lighter and found at ends of bones.

• Long bones: Shaft (diaphysis) and two ends (epiphyses), e.g., femur.

• Short bones: Cube-like, e.g., carpals.

• Flat bones: Two layers of compact bone with spongy bone in between, e.g., skull.

• Irregular bones: Complex shapes, e.g., vertebrae.

• Periosteum: Outer covering of bone.

• Endosteum: Inner lining of bone.

Bone Identification Rules

Standardized Nomenclature

Proper identification of bones and their features is essential for anatomical study.

• Bone: Name of the bone.

• Side: Right (R) or Left (L).

• Feature: Specific landmark or structure.

• Paired bones: Feature / Side / Bone.

• Unpaired bones: Side / Feature / Bone.

• Articulation: Joint contact between bones.

• Perforation: Opening in bone.

• Depression: Hollow area.

• Projection: Raised area.

Skull: Structure and Sutures

Skull Composition and Sutures

The skull consists of 22 bones, divided into cranial and facial groups, connected by sutures.

• Cranial bones (8): Frontal, 2 parietal, 2 temporal, occipital, sphenoid, ethmoid.

• Facial bones (14): Form the structure of the face.

• Sutures:

  • Sagittal: Parietal/parietal.

  • Coronal: Frontal/parietal.

  • Squamosal: Temporal/parietal.

  • Lambdoidal: Occipital/parietal.

Cranial Landmarks

Key Features of Cranial Bones

Cranial bones have distinct landmarks important for identification and function.

• Frontal bone: Glabella, supraorbital notch/foramen, frontal sinus, lacrimal fossa.

• Temporal bone: Mastoid, styloid, zygomatic process, mandibular fossa, external auditory meatus, carotid canal, jugular foramen.

• Occipital bone: Foramen magnum, occipital condyles, hypoglossal canal.

Sphenoid and Ethmoid Landmarks

The sphenoid and ethmoid bones are complex and contain several important features.

• Sphenoid: Greater/lesser wings, sella turcica, optic canal, superior orbital fissure, foramina rotundum/ovale/spinosum.

• Ethmoid: Crista galli, cribriform plate, superior/middle nasal conchae, perpendicular plate.

Facial Bones

Major Facial Bones and Features

The facial skeleton supports the face and provides openings for air and food.

• Mandible: Body, ramus, coronoid process, mandibular condyle, mental and mandibular foramina, alveoli.

• Maxillae: Infraorbital foramen, maxillary sinus, palatine process, alveoli.

• Other facial bones: Palatine, zygomatic, lacrimal, nasal, vomer, inferior nasal conchae.

Skull Openings

Summary of Important Skull Openings

Openings in the skull allow passage of nerves and blood vessels.

• Foramen magnum: Passage for spinal cord.

• Optic canal: Passage for optic nerve.

• Jugular foramen: Passage for jugular vein.

• Other foramina: Rotundum, ovale, spinosum, etc.

Orbit and Sinuses

Bony Orbit and Paranasal Sinuses

The orbit is formed by seven bones and houses the eye; sinuses are air-filled spaces in certain bones.

• Bony orbit: Frontal, sphenoid, zygomatic, maxilla, palatine, lacrimal, ethmoid.

• Sinuses: Five bones contain sinuses (frontal, sphenoid, ethmoid, maxilla, and one more).

• Lacrimal duct: Follow instructor guidance for correct identification.

Vertebral Column

Structure and Curvatures

The vertebral column supports the body and protects the spinal cord, with distinct regions and curvatures.

• Regions: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, 4 coccygeal vertebrae.

• Primary curves: Thoracic and sacral (convex posteriorly).

• Secondary curves: Cervical and lumbar (concave posteriorly).

• Abnormal curvatures: Scoliosis (lateral), lordosis (exaggerated lumbar), kyphosis (exaggerated thoracic).

Vertebrae Identification

Features of Typical and Regional Vertebrae

Vertebrae are identified by their features and regional differences.

• Typical vertebra: Body, pedicle, lamina, vertebral foramen, spinous/transverse processes, articular processes.

• Cervical vertebrae: Transverse foramen; C1 (atlas) has dens facet; C2 (axis) has dens.

• Thoracic vertebrae: Costal facets/demifacets for rib articulation.

• Lumbar vertebrae: Thick, weight-bearing bodies.

• Sacrum: Sacral foramina, auricular surface, superior articular facets.

Intervertebral Discs and Ligaments

Structure and Function

Discs and ligaments stabilize the vertebral column and absorb shock.

• Anterior longitudinal ligament: Prevents hyperextension.

• Posterior longitudinal ligament: Prevents hyperflexion.

• Intervertebral discs: Shock absorbers between vertebral bodies.

• Nucleus pulposus: Inner gel-like core.

• Annulus fibrosus: Outer ring of fibrocartilage.

Ribs and Sternum

Thoracic Cage Structure

The thoracic cage protects vital organs and supports respiration.

• Ribs: 12 pairs; true ribs (1–7) attach directly to sternum; false ribs (8–12) do not; floating ribs (11–12) have no anterior attachment.

• Rib features: Head, tubercle, sternal extremity, costal groove, costal angle.

• Sternum: Manubrium, jugular/clavicular notches, body, sternal angle, xiphoid process.

Rapid Review Questions

Key Identification Questions

These questions reinforce important anatomical facts for quick recall.

1. What bone contains the foramen magnum? Occipital bone

2. What is the special opening of cervical vertebrae? Transverse foramen

3. Which bone has the dens? Axis (C2)

4. What is the joint surface of sacrum with ilium? Auricular surface

5. Which ribs are floating? Ribs 11–12

6. Which suture connects the parietals? Sagittal suture

Final Review Checklist

Essential Facts for Exam Preparation

Memorize these key numbers and terms for quick recall.

• Vertebral column: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, 4 coccygeal.

• Cranial bones: Frontal, temporal, occipital, sphenoid, ethmoid.

• Atlas (C1) vs axis (C2).

• True ribs (1–7); floating ribs (11–12).

• Paired bone identification: feature/side/bone.