The Skull

The Skull

Overview of the Skull

  • Definition: Part of the axial skeleton, consists of 22 bones (excluding the ossicles of the middle ear and the hyoid bone).

  • Bones Breakdown:

    • Cranial Bones (8): Protect the brain.

      • Unpaired (4): Frontal, Occipital, Sphenoid, Ethmoid.

      • Paired (4): Parietal (2), Temporal (2).

    • Facial Bones (14): Form the structure of the face.

      • Unpaired (2): Vomer, Mandible.

      • Paired (12): Maxilla (2), Zygomatic (2), Nasal (2), Lacrimal (2), Palatine (2), Inferior Nasal Conchae (2).

Major Sutures of the Cranium

  • Coronal Suture: Located between the frontal bone and the two parietal bones.

  • Sagittal Suture: Located between the two parietal bones along the midline of the skull.

  • Lambdoid Suture: Located between the two parietal bones and the occipital bone posteriorly.

  • Squamosal Suture: Located on the lateral aspect of the skull, between each parietal bone and its corresponding temporal bone.

Views of the Skull

  • Views: Anterior, Posterior, Lateral, Superior, Inferior.

Detailed Bone Features

Frontal Bone

  • Features: Supra-orbital notch (foramen), glabella.

  • Function: Forms forehead, part of the anterior cranial fossa, and upper eye sockets.

Temporal Bones

  • Features: Zygomatic process (forms part of zygomatic arch), external acoustic meatus, mastoid process, styloid process, carotid canal, mandibular fossa.

Occipital Bone

  • Features: Foramen magnum (spinal cord exit), Occipital condyles (articulate with the first cervical vertebra, atlas).

Sphenoid Bone

  • Features: Medial and lateral pterygoid plates, sella turcica (houses pituitary gland).

Ethmoid Bone

  • Internal Surface: Crista galli, cribriform foramina (for olfactory nerve, CN ICN \, I).

  • Anterior View: Perpendicular plate, nasal conchae (superior and middle).

Maxillary Bones

  • Features: Infra-orbital foramen, alveolar processes (for teeth), palatine processes (form anterior 2/32/3 of hard palate).

Zygomatic Bone

  • Features: Temporal process (forms part of zygomatic arch).

Zygomatic Arch

  • Components: Formed by the zygomatic process of the temporal bone and the temporal process of the zygomatic bone.

Hard Palate

  • Components: Formed by the palatine processes of the maxillary bones (anterior 2/32/3) and the horizontal plates of the palatine bones (posterior 1/31/3).

The Hyoid Bone

  • Structure: Greater and lesser horns.

  • Function: Involved in swallowing and speech; it is the only bone in the body not directly articulated with another bone.

Paranasal Sinuses

  1. Sinuses: Frontal, Sphenoid, Ethmoid, Maxillary.

  2. Function: Lightens skull, improves voice resonance, warms and humidifies inhaled air, produces mucus, hinders infection spread.

Nasal Septum Structure

  • Components: Perpendicular plate of ethmoid bone (superiorly), vomer bone (inferiorly and posteriorly), septal cartilage (anteriorly).

  • Deviated Septum: Displaced nasal septum causing congestion, nosebleeds, breathing issues.

Fetal/Infant Skull and Fontanels

  • Fontanels: Soft spots in the skull of an infant where cranial sutures have not yet fused. They are composed of fibrous connective tissue.

  • Major Fontanels:

    • Anterior Fontanel (Bregma): Largest, diamond-shaped, located at the junction of the sagittal and coronal sutures. Closes around 18-24 months.

    • Posterior Fontanel (Lambda): Smaller, triangular, located at the junction of the sagittal and lambdoid sutures. Closes around 2-3 months.

    • Sphenoid (Anterolateral) Fontanels: Located at the junction of the frontal, parietal, temporal, and sphenoid bones. Close around 6 months.

    • Mastoid (Posterolateral) Fontanels: Located at the junction of the parietal, occipital, and temporal bones. Close around 6-18 months.

  • Purpose: Allow for rapid brain growth during infancy (the brain doubles in size in the first year) and enable the skull to compress slightly during passage through the birth canal.

Summary of Cranial Nerve Passageways and Other Foramina

  • Cranial Nerve I (Olfactory Nerve): Passes through the cribriform foramina of the ethmoid bone.

  • Cranial Nerve II (Optic Nerve): Passes through the optic canal of the sphenoid bone.

  • Cranial Nerve III (Oculomotor Nerve): Exits the cranial cavity via the superior orbital fissure of the sphenoid bone.

  • Cranial Nerve IV (Trochlear Nerve): Exits the cranial cavity via the superior orbital fissure of the sphenoid bone.

  • Cranial Nerve V (Trigeminal Nerve):

    • V1V_1 (Ophthalmic division): Exits the cranial cavity via the superior orbital fissure of the sphenoid bone.

    • V2V_2 (Maxillary division): Passes through the foramen rotundum of the sphenoid bone.

    • V3V_3 (Mandibular division): Passes through the foramen ovale of the sphenoid bone.

  • Cranial Nerve VI (Abducens Nerve): Exits the cranial cavity via the superior orbital fissure of the sphenoid bone.

  • Cranial Nerve VII (Facial Nerve): Enters the temporal bone via the internal acoustic meatus and exits via the stylomastoid foramen.

  • Cranial Nerve VIII (Vestibulocochlear Nerve): Enters the temporal bone via the internal acoustic meatus.

  • Cranial Nerve IX (Glossopharyngeal Nerve): Exits the cranial cavity via the jugular foramen (between temporal and occipital bones).

  • Cranial Nerve X (Vagus Nerve): Exits the cranial cavity via the jugular foramen (between temporal and occipital bones).

  • Cranial Nerve XI (Accessory Nerve): Exits the cranial cavity via the jugular foramen (between temporal and occipital bones).

  • Cranial Nerve XII (Hypoglossal Nerve): Exits the cranial cavity via the hypoglossal canal of the occipital bone.

  • Other Important Foramina:

    • Foramen magnum: Spinal cord, vertebral arteries, ascending portion of the accessory nerve (CN XICN \, XI).

    • Carotid canal: Internal carotid artery.

    • Jugular foramen: Internal jugular vein.

    • Superior orbital fissure: Ophthalmic vein.