Study Guide: The Axial Skeleton (Chapter 7) – Anatomy & Physiology I

The Axial Skeleton

Overview

The axial skeleton forms the central axis of the human body and consists of the skull, vertebral column, and thoracic cage. It provides support, protection for vital organs, and attachment points for muscles.

Skull

General Structure

• The skull is composed of 29 bones, including cranial and facial bones, as well as associated bones such as the hyoid and auditory ossicles.

Sutures

Sutures are immovable joints found only between the bones of the skull. They are made of dense fibrous connective tissue and allow for growth during childhood.

• Lambdoid Suture: Between the parietal bones and occipital bone.

• Coronal Suture: Between the frontal bone and parietal bones.

• Sagittal Suture: Between the two parietal bones.

• Squamous Suture: Between the parietal and temporal bones.

Cranial Bones

There are 8 cranial bones, classified as flat bones. They protect the brain and provide attachment sites for head and neck muscles.

• Occipital Bone: Forms the posterior and base of the skull.

  • Occipital condyles: Articulate with the first cervical vertebra (atlas).

  • Foramen magnum: Large opening for the spinal cord.

  • Jugular foramen: Passage for jugular vein and cranial nerves.

• Parietal Bones: Form the superior and lateral aspects of the skull.

  • Superior & inferior temporal lines: Attachment sites for the temporalis muscle, which closes the mouth.

• Frontal Bone: Forms the forehead and roofs of the orbits.

  • Supra-orbital margin: Protects the eyes.

  • Glabella: Area between the supra-orbital margins.

  • Lacrimal fossa: Houses the lacrimal gland.

  • Frontal sinuses: Air-filled spaces that lighten the skull.

• Temporal Bones: Form the sides and base of the skull.

  • Zygomatic process: Articulates with the zygomatic bone to form the zygomatic arch.

  • Mastoid process: Attachment for neck muscles.

  • Styloid process: Attachment for tongue and neck muscles.

  • Auditory ossicles: Small bones involved in hearing.

  • External & internal acoustic meatus: Canals for sound transmission and nerves.

• Sphenoid Bone: Spans the width of the skull and forms part of the cranial floor.

  • Sella turcica: Houses the pituitary gland.

  • Pterygoid processes: Attachment for jaw muscles.

  • Optic canal: Passage for the optic nerve.

• Ethmoid Bone: Forms part of the nasal cavity and the orbits.

  • Cribiform plate: Contains the crista galli, an attachment for brain membranes.

  • Perpendicular plate: Forms part of the nasal septum.

  • Olfactory foramina: Openings for olfactory nerves.

Facial Bones

There are 14 facial bones, classified as irregular bones. They form the framework of the face, house the special sense organs, and provide openings for air and food passage.

• Maxilla: Forms the upper jaw and part of the hard palate.

  • Alveolar processes: Support the upper teeth.

  • Palatine process: Forms most of the hard palate.

  • Maxillary sinuses: Largest paranasal sinuses.

• Zygomatic Bone: Forms the cheekbones.

  • Temporal process: Articulates with the temporal bone to form the zygomatic arch.

• Lacrimal Bone: Smallest facial bone; forms part of the medial wall of the orbit.

• Mandible: Forms the lower jaw.

  • Alveolar part: Supports the lower teeth.

  • Condylar process: Articulates with the temporal bone at the temporomandibular joint (TMJ).

  • Coronoid process: Attachment for jaw muscles.

  • TMJ (Temporomandibular Joint): The only movable joint of the skull.

Hyoid Bone

• Location: In the neck, between the mandible and larynx.

• Function: Supports the tongue and serves as an attachment for muscles of swallowing and speech.

• Joints: Does not articulate directly with any other bone.

Orbits

The orbital complex is formed by seven bones: frontal, sphenoid, zygomatic, maxilla, palatine, lacrimal, and ethmoid. These bones create the eye sockets, protecting the eyes and supporting associated structures.

Paranasal Sinuses

• Definition: Air-filled cavities within certain cranial and facial bones.

• Function: Lighten the skull, warm and moisten air, and enhance voice resonance.

• Main Sinuses: Frontal, sphenoid, ethmoid, and maxillary.

Fontanelles

• Definition: Soft spots on a baby's skull where the bones have not yet fused.

• Function: Allow for brain growth and facilitate passage through the birth canal.

• The Four Fontanelles: Anterior, posterior, sphenoidal (anterolateral), and mastoid (posterolateral).

Vertebral Column

General Structure

• Composed of 24 vertebrae, 1 sacrum, and 1 coccyx.

• Functions: Protects the spinal cord, supports the head, and serves as an attachment for ribs and muscles.

Vertebral Regions

• Cervical: 7 vertebrae (C1–C7)

• Thoracic: 12 vertebrae (T1–T12)

• Lumbar: 5 vertebrae (L1–L5)

• Sacrum: 1 bone (fusion of 5 sacral vertebrae)

• Coccyx: 1 bone (fusion of 3–5 coccygeal vertebrae)

Spinal Curvatures

• Primary Curves: Thoracic and sacral; present at birth; accommodate internal organs.

• Secondary Curves: Cervical and lumbar; develop after birth; help position the body for upright posture.

Typical Vertebrae Parts

• Body (Centrum): Weight-bearing portion.

• Vertebral Arch: Encloses the vertebral foramen.

• Articular Processes: Form joints with adjacent vertebrae.

• Spinous Process: Projects posteriorly; attachment for muscles and ligaments.

• Transverse Processes: Project laterally; attachment for muscles and ligaments.

• Vertebral Foramen: Opening for the spinal cord.

• Vertebral Canal: Formed by the alignment of vertebral foramina.

• Intervertebral Disc: Fibrocartilaginous pad between vertebral bodies; absorbs shock.

Regional Vertebral Characteristics

• Cervical Vertebrae (7):

  • C1 (Atlas): No body; ring-like; articulates with occipital condyles.

  • C2 (Axis): Has the dens (odontoid process) for rotation with atlas.

  • C3–C6: Typical cervical vertebrae; small body; bifid spinous process.

  • C7: Vertebra prominens; prominent spinous process.

• Thoracic Vertebrae (12):

  • Articulate with ribs via costal facets.

  • Larger body than cervical; spinous process points inferiorly.

  • Long, slender spinous process.

• Lumbar Vertebrae (5):

  • Largest bodies; support most weight.

  • Spinous process projects straight out; attachment for lower back muscles.

• Sacrum (1):

  • Fusion of 5 sacral vertebrae.

  • L5 articulates with the sacrum.

  • Transverse fusion lines and sacral foramina for nerve passage.

• Coccyx (1):

  • Fusion of 3–5 coccygeal vertebrae.

  • Sacrum articulates with the coccyx.

Thoracic Cage

General Structure

• Consists of thoracic vertebrae, ribs and costal cartilages, and the sternum.

• Protects the heart, lungs, and major blood vessels; supports the shoulder girdles and upper limbs.

Ribs

• 12 pairs of ribs:

  • Posterior end articulates with thoracic vertebrae.

  • Anterior end articulates with the sternum (via costal cartilage) or remains free.

• Classification:

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

  • False ribs (8–12): Do not attach directly to the sternum. Ribs 8–10 attach via cartilage of rib 7; ribs 11–12 are floating ribs (no anterior attachment).

Sternum

• A flat bone forming the anterior midline of the thoracic cage.

• Parts: Manubrium (superior), body (middle), and xiphoid process (inferior).

Summary Table: Major Bones of the Axial Skeleton

Region	Main Bones	Key Features
Skull	Cranial (8), Facial (14), Hyoid, Auditory Ossicles	Protects brain, forms face, supports sensory organs
Vertebral Column	Cervical (7), Thoracic (12), Lumbar (5), Sacrum, Coccyx	Supports body, protects spinal cord
Thoracic Cage	Sternum, Ribs (12 pairs), Costal Cartilages	Protects thoracic organs, supports respiration

Example: The atlas (C1) and axis (C2) allow for the nodding and rotation movements of the head, respectively.

Additional info: The axial skeleton is essential for maintaining posture, protecting the central nervous system, and providing a framework for muscle attachment. Disorders of the axial skeleton, such as scoliosis or herniated discs, can significantly impact health and mobility.