Axial Skeleton: Structure, Bones, and Key Markings

Axial Skeleton Overview

Introduction

The axial skeleton forms the central axis of the human body and consists of 80 bones. It provides support and protection for the brain, spinal cord, and vital organs within the thorax. The axial skeleton includes the skull, vertebral column, and thoracic cage.

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

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

• Thoracic Cage: Protects the heart and lungs.

Skull

Cranial and Facial Bones

The skull is composed of 22 bones, divided into cranial and facial bones. Cranial bones protect the brain, while facial bones form the structure of the face.

• Cranial Bones (8): Frontal, Left and Right Parietal, Occipital, Left and Right Temporal, Sphenoid, Ethmoid

• Facial Bones (14): Left and Right Maxilla, Left and Right Palatine, Left and Right Zygomatic, Left and Right Lacrimal, Left and Right Nasal, Left and Right Inferior Conchae, Vomer, Mandible

Bones of the Eye Socket (Orbit)

The orbit is formed by several bones that protect the eye and support its function.

• Key bones: Frontal, Zygomatic, Sphenoid, Ethmoid, Lacrimal, Maxilla, Palatine

Hyoid Bone

The hyoid is a small U-shaped bone located in the neck, not directly attached to other bones. It supports the tongue and is important for swallowing.

Ear Ossicles

There are six bones in the middle ear (three per ear: malleus, incus, stapes), but their names are not required for this exam.

Bone Markings and Structures to Learn

Skull Sutures

Sutures are immovable joints between skull bones.

• Surrounding Parietal Bone: Sagittal Suture, Lambdoid Suture, Coronal Suture, Squamosal Suture

Occipital Bone

• Foramen Magnum: Large opening for the spinal cord

Temporal Bone

• External Auditory Meatus: Ear canal opening

• Mastoid Process: Projection behind the ear

• Mandibular Fossa: Depression for jaw articulation

• Styloid Process: Slender pointed projection

Ethmoid Bone

• Cribriform Plate: Contains olfactory foramina

• Crista Galli: Attachment for brain membranes

• Perpendicular Plate: Forms part of nasal septum

• Middle Nasal Concha: Increases surface area in nasal cavity

Sphenoid Bone

• Sella Turcica: Houses the pituitary gland

• Greater Wing: Forms part of the cranial floor

• Lesser Wing: Forms part of the cranial floor

• Optic Foramen: Passage for optic nerve

Mandible

• Mental Foramen: Opening for nerves and blood vessels

• Mandibular Condyles: Articulate with temporal bone

Maxilla

• Alveolar Process: Contains tooth sockets

• Infraorbital Foramen: Passage for nerves and vessels

Zygomatic and Temporal Bones

• Zygomatic Arch: Cheekbone structure

Nasal Cavity

• Nasal Septum: Formed by perpendicular plate of ethmoid and vomer

Spine

Structures of a Generalized Vertebra

Vertebrae are the bones that make up the vertebral column. Each vertebra has several key structures:

• Spinous Process

• Transverse Process

• Vertebral Body

• Vertebral Foramen

• Facet

• Vertebral Arch

• Lamina

• Pedicle

Regions of the Vertebral Column

• Cervical Vertebrae (C1–C7): Includes Atlas (C1) and Axis (C2, which has the dens)

• Thoracic Vertebrae (T1–T12): Articulate with ribs

• Lumbar Vertebrae (L1–L5): Largest vertebrae, support lower back

• Sacrum (S1–S5): Fused vertebrae forming the back of the pelvis

• Coccyx: Tailbone, composed of fused vertebrae

Thoracic Cage

Ribs, Sternum, and Thoracic Vertebrae

The thoracic cage protects vital organs and supports breathing.

• 24 Ribs: 7 pairs of true ribs, 5 pairs of false ribs (last 2 pairs are floating ribs)

• Costal Cartilage: Connects ribs to sternum

Sternum

• Clavicular Notch: Articulation for clavicle

• Suprasternal Notch: Superior border of sternum

• Manubrium: Upper part of sternum

• Sternal Angle: Junction between manubrium and body

• Body: Main part of sternum

• Xiphoid Process: Inferior tip of sternum

Summary Table: Major Bones of the Axial Skeleton

Region	Main Bones	Key Features
Skull	Cranial (8), Facial (14)	Protects brain, forms face
Vertebral Column	Cervical (7), Thoracic (12), Lumbar (5), Sacrum, Coccyx	Supports body, protects spinal cord
Thoracic Cage	Ribs (24), Sternum	Protects heart and lungs
Hyoid	Hyoid bone	Supports tongue
Ear Ossicles	Malleus, Incus, Stapes	Transmit sound (not required for exam)

Additional info:

• Some bones, such as the kneecap (patella), are classified as sesamoid bones because they develop within tendons.

• The skull and hip are examples of bones that fuse as you mature.