Bones, Part 1: The Axial Skeleton – Study Notes

The Axial Skeleton

Overview of the Human Skeleton

The human skeleton is a structural framework composed of bones, cartilage, joints, and ligaments. It is divided into two main divisions: the axial skeleton and the appendicular skeleton. The axial skeleton consists of 80 bones, including the skull, vertebral column, and thoracic cage, and serves to protect the central nervous system and vital organs.

• Axial Skeleton: Skull, vertebral column, thoracic cage

• Appendicular Skeleton: Upper and lower limbs, pectoral and pelvic girdles (covered in Chapter 8)

• Total Bones: 206 named bones in the adult human body

The Vertebral Column

Structure and Regions

The vertebral column, or spine, is a flexible, curved structure that supports the trunk, protects the spinal cord, and provides attachment points for ribs and muscles. It is divided into five major regions, each with distinct characteristics.

• Cervical Vertebrae (C1–C7): Seven vertebrae in the neck

• Thoracic Vertebrae (T1–T12): Twelve vertebrae in the upper back, each articulating with ribs

• Lumbar Vertebrae (L1–L5): Five vertebrae in the lower back, supporting the most weight

• Sacrum: Five fused vertebrae forming the posterior wall of the pelvis

• Coccyx: Three to five fused vertebrae forming the tailbone

Normal Curvatures

• Cervical and Lumbar Curvatures: Concave posteriorly

• Thoracic and Sacral Curvatures: Convex posteriorly

• These curvatures increase the spine's resilience and flexibility.

Structure of a Typical Vertebra

Each vertebra shares a common structural plan, though regional differences exist.

• Body: Weight-bearing region

• Vertebral Arch: Composed of pedicles and laminae, enclosing the vertebral foramen

• Processes: Spinous, transverse, and articular processes for muscle and ligament attachment

Regional Characteristics of Vertebrae

Cervical Vertebrae (C1–C7)

Cervical vertebrae are the smallest and lightest. They have unique features such as transverse foramina and bifid spinous processes (except C7).

• Atlas (C1): Lacks a body and spinous process; supports the skull and allows nodding "yes"

• Axis (C2): Has a dens (odontoid process) that acts as a pivot for rotation, allowing the head to indicate "no"

• C3–C7: Typical cervical vertebrae with wide bodies and large, triangular vertebral foramina

Thoracic Vertebrae (T1–T12)

Thoracic vertebrae articulate with ribs and have heart-shaped bodies, long spinous processes, and circular vertebral foramina. Their articular facets allow rotation but limit flexion and extension.

• Demifacets: For rib articulation (T1–T10)

• Full Facet: T1 for the first rib; T10–T12 have a single facet

Lumbar Vertebrae (L1–L5)

Lumbar vertebrae are large, with thick bodies and short, hatchet-shaped spinous processes. They allow flexion and extension but prevent rotation.

• Vertebral Foramina: Triangular

• Articular Facets: Oriented medially and laterally

Sacrum and Coccyx

The sacrum forms the posterior wall of the pelvis and is composed of five fused vertebrae. The coccyx, or tailbone, is formed from 3–5 fused vertebrae and provides slight support to pelvic organs.

• Sacral Promontory: Anterior border of the first sacral vertebra

• Sacral Foramina: Openings for sacral spinal nerves

• Ala: Wing-like projections from fused rib elements

The Thoracic Cage

Structure and Function

The thoracic cage protects vital organs (heart, lungs) and supports the shoulder girdles and upper limbs. It consists of the sternum, ribs, and thoracic vertebrae.

• Sternum: Manubrium, body, and xiphoid process

• Ribs: 12 pairs, all attaching posteriorly to thoracic vertebrae

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

• False Ribs (8–12): Attach indirectly or not at all to the sternum; ribs 11–12 are floating ribs

The Skull

Overview and Divisions

The skull is a complex bony structure that protects the brain and forms the framework of the face. It is divided into cranial and facial bones, with several cavities and numerous foramina for nerves and blood vessels.

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

• Facial Bones (14): Mandible, vomer, maxillae (2), zygomatic (2), nasal (2), lacrimal (2), palatine (2), inferior nasal conchae (2)

• Cavities: Cranial, orbits, nasal, middle and inner ear, paranasal sinuses

• Openings: Foramina, canals, and fissures for nerves and vessels

Sutures and Sutural Bones

Sutures are immovable joints connecting the bones of the skull. Four main sutures are present in the cranium, and small, irregular sutural bones may occur within these joints.

• Coronal Suture: Parietal and frontal bones

• Squamous Suture: Parietal and temporal bones

• Sagittal Suture: Right and left parietal bones

• Lambdoid Suture: Parietal and occipital bones

• Sutural Bones: Small, irregular bones within sutures (not present in all individuals)

Developmental Aspects: Skull of a Newborn

The skull of a newborn features fontanelles—soft spots that allow for brain growth and facilitate passage through the birth canal. These ossify with age.

• Anterior Fontanelle: Largest, closes by 18–24 months

• Posterior, Mastoid, and Sphenoidal Fontanelles: Close earlier

Table: Regional Characteristics of Vertebrae

The following table summarizes the main differences between cervical, thoracic, and lumbar vertebrae:

Characteristic	Cervical (C3–C7)	Thoracic	Lumbar
Body	Small, oval, wider side to side	Larger than cervical; heart-shaped	Massive, kidney-shaped
Vertebral Foramen	Large, triangular	Circular	Triangular
Spinous Process	Short, bifid (except C7)	Long, sharp, projects inferiorly	Short, blunt, projects directly posteriorly
Transverse Process	Contain transverse foramina	Bear facets for rib articulation	Thin and tapered
Articular Facets	Superior face superoposteriorly	Superior face posteriorly	Superior face medially
Movements Allowed	Flexion, extension, lateral flexion, rotation	Rotation, limited flexion/extension	Flexion, extension; rotation prevented