Axial Skeleton: Structure, Features, and Identification

Axial Skeleton

Objectives

The axial skeleton forms the central axis of the human body and includes the skull, vertebral column, and thoracic cage. Understanding its structure and features is essential for identifying bones and their functions.

• Identify the bones of the axial skeleton: Includes the skull, vertebral column, and thoracic cage (ribs and sternum).

• Define terms used for bone features: Learn anatomical terminology for bone markings and features.

• Distinguish between cranial and facial bones: Cranial bones protect the brain; facial bones form the structure of the face.

• Identify the major features of the skull and vertebral column: Recognize key landmarks and articulations.

• Identify the bones and features of the thoracic cage: Includes ribs and sternum.

Features of Bones

Bone Markings and Terminology

Bones have specific markings that serve as sites for muscle and ligament attachment, articulation, and passage of blood vessels and nerves. Understanding these terms is crucial for anatomical identification.

• Process: Any bony prominence.

• Tuberosity: Roughened, raised area.

• Tubercle: Small, rounded projection beyond a bone's surface.

• Trochanter: Very large, blunt, irregularly shaped process (unique to femur).

• Crest: Narrow, prominent ridge.

• Spine: Sharp, slender projection.

• Condyle: Rounded articular projection.

• Head: Bony expansion carried on a narrow neck.

• Facet: Smooth, nearly flat articular surface.

• Foramen: Round or oval opening through a bone.

• Fossa: Shallow basin-like depression in a bone, often an articular surface.

• Meatus: Canal-like passageway.

Skull: Cranial Bones

Major Cranial Bones and Their Features

The skull consists of several cranial bones that protect the brain and provide attachment sites for muscles. Key features help distinguish each bone.

• Frontal Bone: Forms the forehead and upper part of the eye sockets.

• Parietal Bones: Paired bones forming the sides and roof of the cranium.

• Temporal Bones: Located at the sides and base of the skull; features include mastoid process, styloid process, zygomatic process, and external acoustic meatus.

• Occipital Bone: Forms the back and base of the skull; features include occipital condyles, foramen magnum, and external occipital protuberance.

• Sphenoid Bone: Butterfly-shaped bone at the base of the skull; features include greater and lesser wings, sella turcica, pterygoid processes.

• Ethmoid Bone: Located between the eyes; features include cribriform plate, perpendicular plate, and middle nasal conchae.

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

Skull: Facial Bones

Major Facial Bones and Their Features

Facial bones form the structure of the face and support sensory organs. Some are paired, others are single.

• Mandible: Lower jawbone; only movable bone of the skull.

• Maxilla: Upper jawbone; forms part of the orbit and hard palate.

• Zygomatic Bones: Cheekbones.

• Nasal Bones: Bridge of the nose.

• Lacrimal Bones: Small bones forming part of the medial orbit wall.

• Palatine Bones: Form posterior part of the hard palate.

• Vomer: Forms part of the nasal septum.

• Inferior Nasal Conchae: Thin, curved bones in the nasal cavity.

Example: The mandible articulates with the temporal bone at the temporomandibular joint (TMJ).

Fontanelles and Sinuses

Fetal Skull and Paranasal Sinuses

Fontanelles are soft spots on a fetal skull where bones have not yet fused, allowing for growth and flexibility during birth. Paranasal sinuses are air-filled cavities that lighten the skull and enhance voice resonance.

• Anterior and Posterior Fontanelles: Membranous areas in the fetal skull that close within the first year.

• Sinuses: Frontal, ethmoid, sphenoid, and maxillary sinuses are present in the skull bones.

Example: The maxillary sinuses are the largest of the paranasal sinuses.

Vertebral Column

Structure and Regional Differences

The vertebral column supports the body and protects the spinal cord. It consists of cervical, thoracic, lumbar, sacral, and coccygeal regions, each with unique features.

• Cervical Vertebrae (C1-C7): Smallest vertebrae; C1 (atlas) and C2 (axis) have unique structures for head movement.

• Thoracic Vertebrae (T1-T12): Articulate with ribs; have costal facets.

• Lumbar Vertebrae (L1-L5): Largest vertebrae; support most body weight.

• Sacrum: Five fused vertebrae forming the posterior pelvic wall.

• Coccyx: Four fused vertebrae forming the tailbone.

Example: The atlas (C1) allows nodding movement of the head, while the axis (C2) allows rotation.

Key Vertebral Features

• Body: Main weight-bearing portion.

• Vertebral Foramen: Opening for the spinal cord.

• Spinous Process: Posterior projection for muscle attachment.

• Transverse Process: Lateral projections for muscle and ligament attachment.

• Superior and Inferior Articular Facets: Articulate with adjacent vertebrae.

• Intervertebral Foramina: Openings between vertebrae for spinal nerves.

Thoracic Cage: Ribs and Sternum

Structure and Function

The thoracic cage protects vital organs and supports breathing. It consists of ribs, sternum, and thoracic vertebrae.

• Ribs: 12 pairs; true ribs (1-7) attach directly to sternum, false ribs (8-12) do not.

• Sternum: Composed of manubrium, body (gladiolus), and xiphoid process.

• Costal Cartilages: Connect ribs to sternum.

Example: The manubrium articulates with the clavicles and first pair of ribs.

Tables

Bone Markings: Classification and Function

Bone Marking	Type	Function
Process	Projection	Muscle/ligament attachment
Condyle	Projection	Forms joint
Foramen	Depression/opening	Passage for nerves/vessels
Fossa	Depression	Articulation surface
Meatus	Opening	Canal-like passageway

Additional info:

• The axial skeleton is essential for protecting the central nervous system and supporting the body’s posture.

• Bone markings are critical for understanding muscle attachment, joint formation, and passage of neurovascular structures.

• Fontanelles in infants allow for brain growth and ease of birth.

• Intervertebral foramina are important for the exit of spinal nerves from the vertebral column.