Axial Skeleton: Skull, Vertebral Column, and Thoracic Cage Study Notes

Axial Skeleton Overview

Introduction

The axial skeleton forms the central framework of the human body, providing support and protection for the brain, spinal cord, and thoracic organs. It consists of the skull, vertebral column, and thoracic cage.

Skull Anatomy

Cranial and Facial Subdivisions

The skull is divided into cranial bones, which encase the brain, and facial bones, which form the structure of the face.

• Cranial bones: Occipital, parietal, frontal, temporal, sphenoid, ethmoid

• Facial bones: Maxillary, palatine, nasal, vomer, inferior nasal concha, zygomatic, lacrimal, mandible

• Associated bones: Auditory ossicles, hyoid bone

Bone Group	Number of Bones
Cranium	8
Face	14
Associated Bones	7

Major Sutures of the Skull

• Coronal suture: Between frontal and parietal bones

• Sagittal suture: Between parietal bones

• Lambdoid suture: Between parietal and occipital bones

• Squamous suture: Between parietal and temporal bones

Key Features of the Adult Skull

• External occipital protuberance: Prominent projection on the occipital bone

• Mastoid process: Rounded projection of the temporal bone

• Zygomatic arch: Cheekbone structure formed by zygomatic and temporal bones

• Mandible: Lower jawbone, only movable bone of the skull

Sectional Views of the Skull

• Sagittal section: Reveals the cranial cavity, nasal cavity, and oral cavity

• Horizontal section: Shows the base of the skull, including the foramen magnum and sella turcica

Smaller Bones of the Face

• Nasal bone: Forms the bridge of the nose

• Lacrimal bone: Small bone forming part of the eye socket

• Inferior nasal concha: Thin, curved bones in the nasal cavity

• Vomer: Forms part of the nasal septum

The Mandible and Hyoid Bones

• Mandible: Contains the body, ramus, angle, coronoid process, condylar process, mental foramen

• Hyoid bone: U-shaped bone in the neck, supports the tongue and is suspended by ligaments

The Orbit

• Orbit: The bony cavity containing the eye, formed by the frontal, sphenoid, ethmoid, maxillary, zygomatic, palatine, and lacrimal bones

• Key features: Superior and inferior orbital fissures, optic canal, orbital plate

Vertebral Column

Spinal Curvature

The vertebral column consists of four spinal curves that provide balance and flexibility.

• Primary (accommodation) curves: Thoracic and sacral (present at birth)

• Secondary (compensation) curves: Lumbar and cervical (develop after birth)

Abnormal Curvatures

• Kyphosis: Exaggerated thoracic curvature

• Lordosis: Exaggerated lumbar curvature

• Scoliosis: Lateral curvature of the spine

Vertebral Anatomy

• Body: Main weight-bearing portion

• Vertebral arch: Encloses the vertebral foramen

• Processes: Spinous, transverse, superior and inferior articular processes

• Intervertebral discs: Fibrocartilaginous pads between vertebrae

Vertebral Regions

• Cervical vertebrae (C1–C7): Small, with large vertebral foramen and transverse foramina

• Thoracic vertebrae (T1–T12): Heart-shaped body, long spinous processes, rib articulations

• Lumbar vertebrae (L1–L5): Largest, most massive, least mobile, bear greatest stress

• Sacrum and Coccyx: Fused vertebrae forming the base of the spine

Region	Number of Vertebrae	Key Features
Cervical	7	Transverse foramina, bifid spinous process
Thoracic	12	Rib facets, long spinous process
Lumbar	5	Large body, short spinous process
Sacrum	5 (fused)	Sacral foramina, sacral promontory
Coccyx	3-5 (fused)	Small, tailbone

Thoracic Cage

Components of the Thoracic Cage

• Thoracic vertebrae: Posterior anchor for ribs

• Ribs: 12 pairs, protect thoracic organs

• Sternum: Flat bone at the center of the chest

Rib Classification

• True ribs (1–7): Attach directly to sternum via costal cartilage

• False ribs (8–12): Attach indirectly or not at all to sternum

• Floating ribs (11–12): No anterior attachment

Functions of the Thoracic Cage

• Protects heart and lungs

• Supports shoulder girdles and upper limbs

• Provides attachment points for muscles of the back, chest, and shoulders

Summary Table: Axial Skeleton Components

Structure	Main Function	Key Features
Skull	Protects brain, forms face	Cranial and facial bones, sutures
Vertebral Column	Supports body, protects spinal cord	Curvatures, regions, intervertebral discs
Thoracic Cage	Protects thoracic organs	Ribs, sternum, costal cartilages

Additional info:

• Clinical relevance: Abnormal spinal curvatures can lead to pain and impaired movement.

• Developmental context: Secondary spinal curves develop as infants begin to hold up their heads and walk.