The Axial Skeleton

The Axial Skeleton

Overview of the Axial Skeleton

  • The axial skeleton consists of the bones that make up the longitudinal axis of the body.

Parts of the Axial Skeleton

  1. Skull: Comprises 22 bones (8 cranial bones and 14 facial bones).

  2. Auditory Ossicles: Consists of 6 bones.

  3. Hyoid Bone: A singular bone.

  4. Vertebral Column: Comprises 24 vertebrae, plus the sacrum and coccyx.

  5. Thoracic Cage: Consists of the sternum and 24 ribs.

Functions of the Axial Skeleton

  1. Protection

    • Protects the brain, internal organs, and spinal cord.

  2. Attachments for Muscles

    • Provides attachment points for muscles of the neck, head, and trunk.

  3. Respiratory Movements

    • Facilitates movements involved in respiration.

  4. Stabilization

    • Stabilizes portions of the appendicular skeleton.

The Skull

Skull Features

  • Sinuses: Air-filled chambers containing mucous membranes, which help reduce weight of the skull.

  • Sutures: Immovable joints connecting the skull bones, except where the mandible contacts the cranium.

The Cranium

  • Comprises 8 bones, known as the braincase:

    1. Occipital Bone

    2. Frontal Bone

    3. Sphenoid Bone

    4. Ethmoid Bone

    5. Left Parietal Bone

    6. Right Parietal Bone

    7. Left Temporal Bone

    8. Right Temporal Bone

  • Together, these bones enclose the cranial cavity, which houses the brain.

Facial Bones

  • Support entrances to the digestive system and respiratory tract:

    • Paired maxillae bones

    • Paired lacrimal bones

    • Paired nasal bones

    • Paired zygomatic bones

    • Paired palatine bones

    • Paired inferior nasal conchae

    • Vomer

    • Mandible

  • Total of 14 facial bones.

Individual Cranial Bones

Occipital Bone

  • Functions: Forms posterior and inferior surfaces of the cranium.

Features:

  • External Occipital Protuberance: Midline on the inferior surface.

  • Occipital Condyles: Articulate with the atlas.

  • External Occipital Crest: Attachment for the nuchal ligament.

  • Foramen Magnum: Connects cranial and spinal cavities.

  • Jugular Foramen: Passageway for internal jugular veins.

  • Hypoglossal Canals: Passageways for hypoglossal nerves.

Parietal Bones

  • Functions: Form the lateral and superior portions of the cranium.

Features:

  • Superior and Inferior Temporal Lines: Attachment points for the temporalis muscle.

  • Grooves: Indicate the pathways of blood vessels within.

Frontal Bone

  • Functions: Forms anterior cranium and superior portion of the eye sockets.

Features:

  • Frontal Sinuses: Flush the nasal cavities.

  • Supra-orbital Margin: Helps protect the eyes.

  • Lacrimal Fossa: Location for the lacrimal gland, found in the superior and lateral orbit.

  • Supra-orbital Foramen: Passageway for blood vessels to eyelids and eyebrows.

  • Supra-orbital Notch: Passageway for the supra-orbital nerve.

Temporal Bones

  • Functions: Form lateral walls of the cranium.

Features:

  • Zygomatic Arches: Only articulations with the mandible.

  • Zygomatic Process: Articulates with the temporal process.

  • Mastoid Process: Muscle attachment; contains air cells.

  • Styloid Process: Attachment for ligaments and tendons.

  • Carotid Canals: Passageway for internal carotid arteries.

  • External Acoustic Meatus: Canal from outer ear to middle ear.

Sphenoid Bone

  • Functions: Forms a portion of the cranial floor; connects cranial and facial bones; strengthens lateral skull.

Features:

  • Sphenoid Sinus: Helps cleanse the nasal cavity.

  • Sella Turcica: Encloses the pituitary gland.

  • Optic Canals: Passageways for optic nerves.

  • Foramen Rotundum, Ovale, Spinosum: Contain important neurovascular structures.

Ethmoid Bone

  • Functions: Forms anterolateral floor of the cranium; roof of the nasal cavity; part of the nasal septum.

Features:

  • Ethmoid Sinuses: Network of air cells opening into nasal cavities.

  • Olfactory Foramina: For passage of olfactory nerves.

Maxillae

  • Functions: Support upper teeth, form upper jaw and hard palate.

Features:

  • Maxillary Sinuses: Produce mucous associated with nasal cavities.

  • Alveolar Processes: Support upper teeth.

Mandible

  • Functions: Form lower jaw.

Features:

  • Alveolar Process: Supports lower teeth.

  • Condylar Process: Articulates with temporal bone for temporomandibular joint.

Infant Skull

  • Contains many ossification centers that form the adult skull.

  • Fontanelles: Fibrous areas between cranial bones allowing distortion of the skull during early development.

Vertebral Column

Overview

  • Consists of 24 vertebrae, sacrum, and coccyx.

  • Provides structural support; bears weight of head/neck; transfers weight to lower limbs.

Curvatures

  • Primary Curves: Present in fetal development.

  • Secondary Curves: Develop after birth.

  • Intervertebral Discs: Pads of fibrocartilage between vertebrae.

Cervical Vertebrae

  • Comprises 7 cervical vertebrae: C1 (Atlas), C2 (Axis), Typical cervical (C3-C6), and C7 (Vertebra Prominens).

Atlas and Axis

  • Atlas (C1): Unique in shape/function, allows “yes” motion.

  • Axis (C2): Unique shape; dens articulates with atlas, allows “no” motion.

Thoracic Vertebrae

  • Comprises 12 vertebrae, resembling lumbar as they descend. Each thoracic vertebra contains costal facets for rib articulation.

Lumbar Vertebrae

  • Comprises 5 vertebrae, characterized by absence of costal facets and broad spinous processes.

Sacrum

  • Comprises 5 fused sacral vertebrae, protects organs and houses nerve passages. Contains: Sacral foramina, auricular surface, and muscle attachment areas.

Coccyx

  • Formed by fusion of 3-5 coccygeal vertebrae, serves as an attachment point for ligaments.

Ribs

Overview

  • Ribs are elongated, curved, flattened bones that articulate with thoracic vertebrae.

Types of Ribs

  1. True Ribs (1-7): Connect directly to sternum via costal cartilages.

  2. False Ribs (8-12): Do not connect directly to sternum.

    • Vertebrochondral Ribs (8-10): Connect to costal cartilage of rib 7.

    • Floating Ribs (11-12): No connection to sternum.

Sternum

  • Forms the anterior midline of the thoracic wall, consisting of:

    • Manubrium: Articulates with clavicles and first ribs.

    • Body: Attaches to manubrium, xiphoid process, and costal cartilages.

    • Xiphoid Process: Last to ossify; connects to diaphragm.

Hyoid Bone

  • Supports larynx, serves as an attachment for muscles associated with laryngeal, pharyngeal, and lingual movement.

    • Greater Horns: Attachment for tongue movement muscles.

    • Lesser Horns: Attach to stylohyoid ligaments.

References

  • Clemente, Carmine D. Anatomy: A Regional Atlas of the Human Body. Philadelphia: Wolters Kluwer/Lippincott Williams & Wilkins Health, 2011.

  • Martini, Frederic, Nath, Judi, and Bartholomew. Fundamentals of Anatomy and Physiology. Boston, MA: Benjamin Cummings, 2012.

  • Mescher, Anthony L., and Luiz Carlos Uchôa Junqueira. Junqueira's Basic Histology: Text and Atlas. Print.