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Skull

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The skull is a complex bony structure that forms the head, enclosing and protecting the brain, sensory organs, and the upper part of the respiratory and digestive tracts.[2] It consists of 22 bones, divided into two main parts: the cranium, which encases the brain, and the facial bones, which form the structure of the face. These bones are connected by immovable joints called sutures, except for the mandible, which is movable.

Location

The skull is located at the uppermost part of the axial skeleton, sitting atop the vertebral column.[3] It forms the framework of the head and connects with the spine at the atlanto-occipital joint.

Structure and Anatomy

The skull is a bony framework of the head, consisting of 22 bones. It is divided into two main parts: the cranium, which encases the brain, and the facial skeleton, which supports the facial structures. These bones are connected primarily by sutures, except for the mandible, which forms a movable joint with the cranium.

Cranium (Neurocranium)

The cranium forms the protective housing for the brain. It is composed of eight bones arranged to form the cranial vault and the cranial base.[5]

Bones of the Cranium

  • Frontal Bone
    • Forms the forehead and the roof of the orbits (eye sockets).
    • Contains the frontal sinuses.
  • Parietal Bones (2)
    • Form the sides and roof of the cranial vault.
    • Meet at the sagittal suture and articulate with the frontal, temporal, and occipital bones.
  • Occipital Bone
    • Forms the back and base of the skull.
    • Contains the foramen magnum for the passage of the spinal cord and articulates with the atlas (C1).
  • Temporal Bones (2)
    • Form the sides and base of the cranium.
    • House structures of the middle and inner ear.
    • Notable features: mastoid process, styloid process, external acoustic meatus, and zygomatic process.
  • Sphenoid Bone
    • Butterfly-shaped bone at the base of the skull.
    • Contains the sella turcica, which houses the pituitary gland.
    • Articulates with almost every other cranial bone.
  • Ethmoid Bone
    • Located between the orbits and forms part of the nasal cavity.
    • Notable features: cribriform plate (allows olfactory nerves to pass), perpendicular plate, and ethmoid sinuses.

Cranial Sutures

  • Coronal Suture: Between the frontal and parietal bones.
  • Sagittal Suture: Between the parietal bones.
  • Lambdoid Suture: Between the parietal and occipital bones.
  • Squamous Suture: Between the parietal and temporal bones.

Cranial Cavities

  • Anterior Cranial Fossa: Houses the frontal lobes of the brain.
  • Middle Cranial Fossa: Accommodates the temporal lobes.
  • Posterior Cranial Fossa: Contains the cerebellum and brainstem.

Facial Skeleton (Viscerocranium)

The facial skeleton comprises 14 bones that form the structure of the face and support the openings of the respiratory and digestive tracts.[7]

Bones of the Face

  • Nasal Bones (2)
    • Form the bridge of the nose.
  • Maxillae (2)
    • Form the upper jaw, part of the orbit, and most of the hard palate.
    • Contain the maxillary sinuses.
  • Zygomatic Bones (2)
    • Form the cheekbones and part of the lateral walls of the orbits.
  • Lacrimal Bones (2)
    • Small bones located in the medial walls of the orbits, housing the lacrimal sac.
  • Palatine Bones (2)
    • Form the posterior part of the hard palate and part of the nasal cavity and orbit.
  • Inferior Nasal Conchae (2)
    • Curved bones projecting into the nasal cavity, aiding in airflow regulation.
  • Vomer
  • Mandible
    • The lower jawbone, the only movable bone of the skull.
    • Notable features: mandibular condyle, coronoid process, and alveolar process.

Orbits

  • The orbits are bony cavities that house the eyes and their associated structures.
  • Formed by contributions from the frontal, maxilla, zygomatic, sphenoid, ethmoid, lacrimal, and palatine bones.

Nasal Cavity and Sinuses

  • The nasal cavity is formed by the ethmoid, maxilla, palatine, nasal, and vomer bones.
  • Paranasal sinuses include the frontal, maxillary, sphenoidal, and ethmoidal sinuses.[8] These air-filled cavities lighten the skull and enhance voice resonance.

Foramina and Canals

The skull contains numerous foramina and canals that allow the passage of nerves and blood vessels:

Hyoid Bone

  • The hyoid bone, although not part of the skull proper, is often associated with it.
  • Located in the neck, it provides attachment for muscles of the tongue and larynx.

Notable Features

  • Pneumatized Bones
    • Some cranial and facial bones (e.g., frontal, maxilla, ethmoid) are hollow and contain sinuses to reduce weight and improve vocal resonance.[1]
  • Protective Design
    • The skull’s robust structure and rounded shape provide excellent protection for the brain and sensory organs.
  • Sexual Dimorphism
    • Male and female skulls differ in size and shape, with males typically having more pronounced brow ridges and a larger mastoid process.

Function

  • Protection of the Brain
    The primary function of the skull is to encase and protect the brain from mechanical injury. The robust structure of the cranium acts as a protective barrier against impacts and trauma.
  • Housing and Protection of Sensory Organs
    • The skull houses and safeguards sensory organs such as the eyes, ears, nose, and tongue, which are crucial for vision, hearing, smell, and taste.[4]
    • The orbits, nasal cavity, and internal ear structures provide specific enclosures for these organs.
  • Support for Facial Structures
    The facial skeleton provides a framework for the face, supporting soft tissues and creating the shape and structure of the face.
  • Facilitation of Respiration and Digestion
    • The nasal cavity facilitates breathing, filtering, and humidifying air.
    • The oral cavity, supported by the maxilla and mandible, assists in chewing and speaking.
  • Attachment for Muscles
    The skull provides attachment points for muscles involved in chewing, facial expression, head movement, and speech.
  • Formation of Joints
    • The temporomandibular joint (between the mandible and temporal bone) allows for jaw movement essential for speaking and chewing.
    • Sutures between skull bones provide immovable connections that contribute to its rigidity and stability.
  • Communication and Aesthetic Role
    • The skull gives shape to the face, influencing identity and expression.
    • It supports the voice by housing resonating structures like the paranasal sinuses.

Clinical Significance

The skull is critical for protecting vital structures, but its complex anatomy makes it susceptible to various clinical conditions:

  • Fractures:
    • Skull Fractures: Commonly result from trauma, with types including linear, depressed, and basilar fractures. Basilar fractures can lead to cerebrospinal fluid leakage or cranial nerve injuries.
  • Traumatic Brain Injury (TBI):
    • Skull fractures or impacts can lead to brain injuries such as concussions, contusions, or hematomas (e.g., epidural or subdural hematomas).
  • Congenital Anomalies:
    • Craniosynostosis: Premature fusion of cranial sutures, leading to abnormal head shapes and potential intracranial pressure issues.
    • Cleft Palate: A defect in the fusion of maxillary and palatine bones.
  • Infections:
    • Sinusitis: Inflammation of the paranasal sinuses can spread to nearby structures like the orbit or brain.
    • Osteomyelitis: Infection of the skull bones, often following trauma or surgery.
  • Tumors:
    • Skull base tumors or bone malignancies like osteosarcoma can affect surrounding structures, including the brain and cranial nerves.[6]
  • Neurological Implications:
    • Foramen Compression: Narrowing of foramina can compress cranial nerves, causing symptoms like vision loss or facial weakness.

References

  1. Lang, J. (1989). Clinical Anatomy of the Skull. Springer. ISBN 9783540186021.
  2. McMinn, R. M. H., & Hutchings, R. T. (1990). A Colour Atlas of Human Anatomy (3rd ed.). Wolfe Publishing. ISBN 9780723416265.
  3. Mahadevan, V. (2013). Anatomy of the Head, Neck, and Brain (3rd ed.). Hodder Arnold. ISBN 9781444122235.
  4. Kierszenbaum, A. L., & Tres, L. L. (2020). Histology and Cell Biology: An Introduction to Pathology (5th ed.). Elsevier. ISBN 9780323672726.
  5. Romanes, G. J. (1981). Cunningham’s Manual of Practical Anatomy, Volume 3: Head and Neck and Brain (15th ed.). Oxford University Press. ISBN 9780192631400.
  6. Frahm, H. D., & Zilles, K. (2013). The Skull: Principles of Structure and Function. Springer. ISBN 9783642734762.
  7. Warwick, R., & Williams, P. L. (1973). Gray’s Anatomy (36th ed.). Longman. ISBN 9780443023163.
  8. Fitzgerald, M. J. T. (2007). Human Embryology and Developmental Biology (4th ed.). Saunders. ISBN 9781416037064.