Axial Skeleton: Structure, Organization, and Key Features

Skeletal System – Axial Skeleton

Introduction

The axial skeleton forms the central axis of the human body and is essential for protection, support, and movement. It includes the skull, vertebral column, and thoracic cage. Understanding the bone markings, shapes, and organization of the axial skeleton is fundamental in Anatomy & Physiology.

Bone Markings and Features

Definition and Function

• Bone markings are specific structural features on bones that serve as attachment sites for muscles, ligaments, and tendons, or as passageways for nerves and blood vessels.

• Common types include foramina (holes for passage), processes (projections for attachment), and fossae (depressions for articulation).

• Example: The foramen magnum in the occipital bone allows passage of the spinal cord.

Bone Shapes (Applies to All of the Skeleton)

Classification of Bones

• Long bones: Longer than they are wide; found in limbs (e.g., femur).

• Flat bones: Thin and broad; provide protection (e.g., skull, sternum).

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

• Short bones: Nearly equal in length and width (e.g., carpals).

• Sesamoid bones: Embedded in tendons (e.g., patella).

Axial Skeleton Organization

Main Components

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

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

• Ribs: Form the thoracic cage, protecting the heart and lungs.

• Sternum: Central bone of the chest, connecting ribs via cartilage.

Cranial (Skull) Bones

Overview and Key Features

• Frontal bone: Forms the forehead; includes the zygomatic process (articulates with cheekbone).

• Parietal bone: Forms the sides and roof of the cranium.

• Occipital bone: Forms the back and base of the skull.

  • Foramen magnum: Large opening for spinal cord.

  • Jugular foramen: Passage for jugular vein.

  • Occipital protuberance: Prominent projection at the back.

• Temporal bone: Forms the lower sides of the skull.

  • Mastoid process: Attachment for neck muscles.

  • Zygomatic process: Articulates with cheekbone.

• Zygomatic bone: Cheekbone; includes frontal, maxillary, and temporal processes.

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

• Nasal bone: Forms the bridge of the nose.

• Maxilla: Upper jaw; includes alveolar, palatine, and zygomatic processes.

• Palatine bone: Forms part of the hard palate.

• Mandible: Lower jaw; includes alveolar process, body, and mental foramen.

• Sphenoid bone: Central skull bone; includes hypophyseal fossa, optic foramen, and superior orbital fissure.

• Ethmoid bone: Forms part of nasal cavity; includes cribriform plate and crista galli.

• Hyoid bone: U-shaped bone in the neck, supports tongue.

• Auditory ossicles: Three small bones in the middle ear (malleus, incus, stapes).

• Sinuses: Air-filled cavities in cranial bones (e.g., ethmoid air cells, frontal sinus).

Vertebral Column

Structure and Terminology

• Vertebra: Basic unit of the vertebral column; consists of body, arch, and various processes.

• Articulating facet: Surface for joint formation between vertebrae.

• Intervertebral discs: Fibrocartilaginous pads between vertebrae for shock absorption.

• Spinous process: Posterior projection for muscle attachment.

• Transverse process: Lateral projection for muscle and ligament attachment.

• Vertebral foramen: Canal for spinal cord.

• Lamina and pedicle: Parts of the vertebral arch.

Regions of the Vertebral Column

• Cervical vertebrae (7): Neck region; includes atlas (C1) and axis (C2).

• Thoracic vertebrae (12): Chest region; articulate with ribs.

• Lumbar vertebrae (5): Lower back; largest vertebrae.

• Sacrum: Five fused vertebrae; forms posterior pelvic wall.

• Coccyx: Four fused vertebrae; tailbone.

Special Features

• Atlas (C1): Supports the skull; allows nodding motion.

• Axis (C2): Has odontoid process (dens); allows rotation of the head.

• Sacral canal and foramina: Passageways for nerves.

Ribs

Classification and Features

• True ribs (vertebrosternal): First 7 pairs; attach directly to sternum.

• False ribs (vertebrochondral): Next 3 pairs; attach indirectly via cartilage.

• Floating ribs (vertebral): Last 2 pairs; do not attach to sternum.

• Key parts: Neck and head of rib.

Sternum

Structure and Function

• Manubrium: Upper portion; articulates with clavicles and first ribs.

• Body: Central portion; attaches to most ribs.

• Xiphoid process: Inferior tip; site for muscle attachment.

Summary Table: Axial Skeleton Components

Component	Main Bones	Key Features
Skull	Frontal, Parietal, Occipital, Temporal, Sphenoid, Ethmoid, Mandible, Maxilla, Zygomatic, Nasal, Lacrimal, Palatine	Foramina, processes, sinuses, sutures
Vertebral Column	Cervical (7), Thoracic (12), Lumbar (5), Sacrum, Coccyx	Body, arch, spinous/transverse processes, intervertebral discs
Thoracic Cage	Ribs (24), Sternum	True/false/floating ribs, manubrium, body, xiphoid process

Additional info: Academic context and definitions have been expanded for clarity and completeness. This guide covers all major structures and features listed in the original notes, with added explanations for study purposes.