L2: Axial - Skull

The Axial Skeleton

Overview

The axial skeleton forms the central axis of the human body, consisting of the skull, vertebral column, and thoracic cage. It serves to protect vital organs such as the brain, spinal cord, heart, and lungs, and provides attachment sites for muscles involved in movement and posture.

• Total bones: 80 in adults

• Main divisions: Skull, vertebral column, thoracic cage

• Functions: Protection, support, muscle attachment

Skull

Cranium (Cranial Vault, Braincase)

The cranium encloses and protects the brain and provides attachment points for muscles involved in chewing and head movement. It contains air-filled sinuses that reduce weight and contribute to resonance.

• Single cranial bones: Frontal, occipital, sphenoid, ethmoid

• Paired cranial bones: Parietal, temporal

• Sinuses: Found in frontal, sphenoid, ethmoid, and maxillary bones

• Sutures: Coronal, sagittal, lambdoid, squamous

Key Cranial Bone Features

• Frontal bone: Forms forehead, roof of nasal cavity, roof of orbits; contains frontal sinuses

• Occipital bone: Forms back and base of cranium; features foramen magnum and occipital condyles

• Sphenoid bone: Forms base and sides of cranium; houses sella turcica (pituitary gland); contains two sinuses

• Ethmoid bone: Forms part of nasal cavity roof, cranial floor, orbital walls; features crista galli, cribriform plate, perpendicular plate, and sinuses

Facial Bones

The facial bones protect sensory organs and provide attachment for muscles of mastication, facial expression, and eye movement. There are thirteen immovable and one movable bone (mandible).

• Single facial bones: Mandible (lower jaw), vomer (lower nasal septum)

• Paired facial bones: Maxillae (upper jaw), palatine, zygomatic (cheek bones), lacrimal, nasal, inferior nasal conchae

Key Facial Bone Features

• Mandible: Only movable bone; features include body, ramus, mandibular notch, condyle, coronoid process, mental foramen, alveolar processes

• Maxillae: Form upper jaw, roof of mouth, house largest sinuses, alveolar processes

• Zygomatic bones: Form cheek prominences, floor/lateral walls of orbits

• Palatine bones: Form part of hard palate

Vertebral Column

General Structure

The vertebral column consists of 26 vertebrae in adults, separated by intervertebral discs. It protects the spinal cord and supports the body.

• Regions: Cervical (7), thoracic (12), lumbar (5), sacrum (5 fused), coccyx (4 fused)

• Curvatures: Primary (thoracic, sacral), secondary (cervical, lumbar)

• Abnormalities: Kyphosis (hunchback), lordosis (swayback), scoliosis (lateral curve)

Structure of a Typical Vertebra

• Centrum (body): Large ventral portion

• Vertebral arch: Laminae (dorsal arch), pedicles (lateral portions), vertebral foramen

• Processes: Spinous, transverse, superior/inferior articular

Regional Differences

• Cervical vertebrae: Smallest, bifid spinous processes (C2-6), transverse foramina, atlas (C1), axis (C2 with dens)

• Thoracic vertebrae: Long, pointed spinous processes, articular surfaces for ribs

• Lumbar vertebrae: Largest, horizontal spinous process, large centrum

• Sacrum: 5 fused vertebrae, dorsal/ventral foramina, sacral promontory, sacroiliac joint

• Coccyx: 4 fused vertebrae, shock absorber

Thoracic Cage

Structure and Function

The thoracic cage supports the pectoral girdle and arms, protects thoracic and upper abdominal organs, and aids in breathing.

• Ribs: 12 pairs attached dorsally to thoracic vertebrae

• True ribs: First 7 pairs, attach directly to sternum

• False ribs: Pairs 8-12, do not attach directly to sternum; last 2 pairs are floating ribs

• Sternum: Manubrium (superior), body (middle), xiphoid process (inferior)

Identification and Review

Skull and Thoracic Cage Items for Identification

• Skull: Condylar process, palatine bone, coronal suture, occipital bone, coronoid process, parietal bone, cribriform plate, occipital condyle, crista galli, olfactory foramina, ethmoid bone, perpendicular plate, external auditory meatus, sagittal suture, foramen magnum, sella turcica, frontal bone, sphenoid bone, hyoid bone, squamous suture, incisive foramen, styloid process, lacrimal bone, supraorbital foramen, lambdoid suture, temporal bone, mandible, vomer bone, mastoid process, zygomatic bone, maxillae, zygomatic process, mental foramen, nasal bone, nasal conchae (superior/middle/inferior)

• Thoracic Cage and Spine: Atlas, vertebral column, axis, suprasternal notch, body, sternum, cervical vertebrae, thoracic vertebrae, coccyx, transverse process, intervertebral discs, vertebral ribs (floating), lamina, vertebrochondral ribs (false), lumbar vertebrae, transverse foramina, manubrium (body), vertebral foramen, pedicle, vertebrosternal ribs (true), sacrum, xiphoid process, spinous process

Lab Review and Problem Solving

Sample Questions and Explanations

• External acoustic meatus location: Temporal bone

• Spinous process projection: Thoracic vertebra

• Teeth association: Maxilla and mandible

• Hard palate formation: Maxilla and palatine bones

• Kyphosis: Primary thoracic curvature abnormality

• Pituitary gland location: Sella turcica of sphenoid bone

• Skull exception: Hyoid bone is not part of the skull

• Lambdoid suture: Joins parietal and occipital bones

• Cervical vertebrae: 7 in cervical region

• Spinal cord exit: Foramen magnum

Problem Solving Example

Joe's lumbar scoliosis after polio:

• Accept: Paralysis during polio resulted in unequal muscle pull on the spine, leading to abnormal curvature (scoliosis).

• Reject: Osteoporosis/osteomalacia, kyphosis, obesity, and unrelated post-growth causes are not supported by the clinical history.

Summary Table: Cranial vs. Facial Bones

Bone	Type
Ethmoid	Cranial
Frontal	Cranial
Inferior nasal concha	Facial
Lacrimal	Facial
Mandible	Facial
Maxilla	Facial
Nasal	Facial
Occipital	Cranial
Palatine	Facial
Parietal	Cranial
Sphenoid	Cranial
Temporal	Cranial
Vomer	Facial
Zygomatic	Facial

Additional info:

• Sinuses are air-filled spaces that lighten the skull and enhance voice resonance.

• Intervertebral discs consist of anulus fibrosus (outer ring) and nucleus pulposus (inner core).

• Abnormal spinal curvatures can result from muscle imbalance, congenital defects, or trauma.