Skull

The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. The neurocranium is further subdivided into the calvarium (the skullcap) and the base of the skull (the cranial fossa).

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Viscerocranium

The viscerocranium is responsible for maintaining the structure for the muscles of the face. The muscles are arranged in a complex anatomical pattern to allow for a variety of facial functions such as mastication, smiling, frowning, eye movement, and patency of the nasal passage.

Table: Bones of the viscerocranium
Viscerocranium bone Number of bones Facial features
Zygomatic Paired Cheekbone
Lacrimal Paired Anterior medial wall of the orbit
Nasal Paired Bridge of the nose
Palatine Paired Posterior quarter of the hard palate
Maxilla Paired Upper jaw and ¾ of the hard palate (the palatine process of the maxilla)
Vomer Single Posterior portion of the nasal septum
Mandible Single Lower jaw and chin
Ethmoid Single Roof of the nasal cavity, medial wall of the ocular orbit, and part of the nasal septum
Anterior view of the skull

Anterior view of the skull featuring the bones of the viscerocranium:
Zygomatic (blue), lacrimal (green), nasal (blue/midline), inferior nasal conchae (blue inside the nasal cavity), palatine (dark orange), maxillary (light orange), vomer (blue/midline), mandible (beige), and ethmoid (gray within the nasal cavity)

Image: “Anterior View of Skull” by Lumen Learning. License: CC BY 4.0
Right lateral view of the skull

Lateral view of the skull featuring the bones and sutures

Image by Lecturio.

Calvarium

The calvarium, or neurocranium, is the part of the skull most associated with the word. The calvarium consists of the unpaired frontal bone, unpaired occipital bone, bilateral parietal bones, and bilateral temporal bones. The bones are initially separated (the fontanelles at birth) and fuse over childhood. The bones are then connected by specific sutures: coronal, squamous, lambdoid, and sagittal.

Bones of the calvarium:

  • Frontal bone
  • Occipital bone
  • Parietal bones (2)
  • Temporal bones (2)
Posterior view of the skull

Posterior (slightly inferior) view of the skull featuring the bones of the calvarium:
the frontal bone (not pictured), occipital bone (blue), and parietal bone (purple)

Image: “Anterior View of Skull” by LumenLearning. License: CC BY 4.0

Main sutures are immobile joints between adjacent bones of the skull:

  • Coronal: between the frontal and parietal bones
  • Sagittal: between the parietal bones, along the midline
  • Lambdoid: between the parietal and occipital bones 
  • Squamous: between the parietal and temporal bones
Skull sutures

Lateral view of the skull featuring the coronal, lambdoid, and squamous sutures

Image: “Skull sutures” by OpenStax College. License: CC BY 3.0

Major fontanelles are areas of connective tissue within the gaps/between the bone plates, which are open during infancy:

  • Posterior, occipital, or lambdoid
  • Anterior, bregmatic, or frontal
  • Sphenoidal or anterolateral
  • Mastoid or posterolateral
Table: The major fontanelles
Fontanelle Shape Location Closure
Posterior, occipital, or lambdoid Triangle Junction of the sagittal and lambdoid sutures 3 months
Anterior, bregmatic, or frontal Diamond Junction of the sagittal and coronal sutures 18–24 months
Sphenoidal or anterolateral Irregular and bilateral Between the frontal, parietal, and sphenoid bones 6 months
Mastoid or posterolateral Irregular and bilateral Between the parietal, temporal, and occipital bones 24 months
Lateral view of a newborn's skull

Lateral view of a newborn skull featuring the posterior, anterior, sphenoidal, and mastoid fontanelles

Image: “Lateral view of a newborn’s skull” by OpenStax College. License: CC BY 3.0

Base of the Skull

The skull base is a highly complex region of the skull with many important anatomical landmarks and foramen for cranial nerves and vasculature to travel in and out of the skull. In-depth knowledge of the location of the foramen and the structures traversing the foramen are critically important.

Table: Regions of the base of the skull
Region Boundaries Bones Structures penetrating the foramina Content
Anterior cranial fossa
  • Anterior: frontal bones
  • Posterior: body and lesser wings of the sphenoid bone
Frontal (orbital plates) Foramen cecum: nasal emissary veins leading to the superior sagittal sinus
  • Frontal lobe of the cerebral cortex
  • Olfactory bulb
  • Olfactory tract
  • Orbital gyri
Ethmoid (cribriform plate and crista galli)
  • Foramina of the cribriform plate: olfactory nerve (I)
  • Anterior/posterior ethmoidal foramina: anterior/posterior ethmoidal nerves, arteries, and veins
Middle cranial fossa
  • Anterior: lesser and greater wings of the sphenoid bone
  • Posterior: superior edge of the petrous portion of the temporal bones
  • Lateral: squamous part of the temporal bone and greater wings of the sphenoid
  • Medial: lateral aspect of the sphenoid (carotid sulcus, sella turcica, and dorsum sellae)
Sphenoid (lesser and greater wings)
  • Optic canal: optic nerve (II) and ophthalmic artery
  • Superior orbital fissure: oculomotor nerve (III), trochlear nerve (IV), 3 branches of the ophthalmic nerve (V1, lacrimal, frontal, and nasociliary nerves), abducent nerve (VI), and superior ophthalmic vein
  • Temporal lobes
  • Pituitary gland
Sphenoid (greater wings)
  • Foramen rotundum: maxillary nerve (V2)
  • Foramen ovale: mandibular nerve (V3), accessory meningeal artery, and the vein connecting the cavernous sinus and pterygoid plexus
  • Foramen spinosum: meningeal branch of the mandibular nerve and the middle meningeal artery/vein
Temporal
  • Foramen lacerum: nerve and artery of the pterygoid canal and meningeal branch of ascending pharyngeal artery and vein
  • Carotid canal: internal carotid plexus and artery
  • Hiatus for greater petrosal nerve: greater petrosal nerve
  • Hiatus for lesser petrosal nerve: lesser petrosal nerve and superior tympanic artery
Posterior cranial fossa Anterior: dorsum sellae of the sphenoid/superior border of the petrous part of the temporal bones Temporal
  • Internal acoustic meatus: facial nerve (VII), vestibulocochlear nerve (VIII), and labyrinthine artery/vein
  • Jugular foramen: glossopharyngeal nerve (IX), vagus nerve (X), accessory nerve (XI), internal jugular vein, inferior petrosal sinus, occipital artery, and posterior meningeal artery
  • Cerebellum
  • Brainstem
Occipital
  • Foramen magnum: medulla oblongata, ascending fibers of accessory nerve (XI), vertebral arteries, anterior spinal artery, posterior spinal arteries, and spinal veins
  • Hypoglossal canal: hypoglossal nerve (XII)

External Fossae

Table: External fossae
Temporal fossa* Infratemporal fossa Pterygopalatine fossa
Location Above the zygomatic arch of the temporal bone
  • Medial to the zygomatic arch
  • Deep to the ramus of the mandible
  • Posterior to the maxilla
  • Medial to the infratemporal fossa
  • Below the apex of the orbit
  • Between the sphenoid bone posteriorly, maxilla anteriorly, and palatine bone medially
Boundaries
  • Superior/posterior: superior and inferior temporal lines
  • Inferior: zygomatic arch
  • Anterior: frontal process of the zygomatic bone and the zygomatic process of the frontal bone
  • Superior: greater wing of the sphenoid bone
  • Inferior: medial pterygoid muscle attachment near the mandibular angle
  • Anterior: posterior aspect of the maxilla
  • Posterior: mastoid process
  • Medial: lateral pterygoid plate of the sphenoid bone
  • Lateral: ramus of the mandible
  • Anterior: posterior surface of the maxilla
  • Posterior: root of the pterygoid process
  • Medial: perpendicular plate of the palatine bone
  • Lateral: pterygomaxillary fissure
  • Inferior: pyramidal process of the palatine bone
  • Superior: inferior orbital fissure
Content
  • Temporalis muscle
  • Deep temporal arteries
  • Auriculotemporal nerve
  • Superficial temporal artery/vein
  • Zygomaticotemporal nerve
  • Inferior alveolar, lingual, and buccal nerves
  • Otic ganglion
  • Chorda tympani
  • Maxillary artery
  • Pterygoid venous plexus
  • Medial/lateral pterygoid muscles
  • Tendon of the temporalis muscle
  • Pterygopalatine ganglion
  • Greater/lesser palatine nerves
  • Maxillary nerve
  • Maxillary artery
Foramina (and the connecting structure) Inferior orbital fissure: orbit
  • Inferior orbital fissure: orbit
  • Foramen ovale and foramen spinosum: middle cranial fossa
  • Pterygomaxillary fissure: infratemporal fossa
  • Inferior orbital fissure: orbit
  • Foramen rotundum: middle cranial fossa
  • Pterygoid canal: foramen lacerum
  • Sphenopalatine foramen: nasal cavity
  • Pterygomaxillary fissure: infratemporal fossa
  • Greater/lesser palatine foramina: oral cavity
*The temporal fossa is also called the pterion:
  • Point of convergence for the temporal, parietal, sphenoid, and frontal bones
  • One of the weakest areas of the skull (susceptible to fracture)

Clinical Relevance

  • Le Fort fractures: transverse fractures of the midface. Three types of Le Fort fractures exist and involve the separation of all or a portion of the midface from the skull base. The fractures are repaired surgically by adding stabilizing plates.
  • Skull fractures: an interruption in the continuation of the bones comprising the calvarium. The fractures usually occur due to blunt force trauma and are often associated with a traumatic brain injury. Fractures of the pterion may rupture the middle meningeal artery. Fractures of the petrous portion of the temporal bone may cause blood or cerebrospinal fluid to escape from the ear, which may result in hearing loss. 
  • Orbital fractures: fractures to the eye sockets. Orbital fractures are classified as orbital rim fractures, direct orbital floor fractures, and blowout fractures. Individuals present with bruising around the eyes accompanied by blurry, decreased, or double vision.
  • Craniosynostosis: the premature fusion of 1 or more cranial sutures. Craniosynostosis can be classified as simple or complex and typically results in an abnormally shaped head. Craniosynostosis occurs in approximately 4 per 10,000 children and can be improved with surgical intervention.
  • Acrania: a rare congenital disorder characterized by the partial or complete absence of the bones of the skull. The condition is commonly associated with anencephaly. 
  • Cranium bifidum: a neural tube defect characterized by the improper closure of the fetal skull during development. Cranium bifidum is usually associated with encephalocele or the protrusion of cerebral matter covered by meninges through the bony defect. Presentation typically includes a groove down the midline of the skull between the forehead, nose, or occipital region.

References

  1. Johnson, D., & Wilkie, A. O. (2011). Craniosynostosis. European journal of human genetics: EJHG, 19(4), 369–376. https://pubmed.ncbi.nlm.nih.gov/21248745/
  2. Okamoto, T., Nuri, T., Harada, A., Kyutoku, S., & Ueda, K. (2019). Cranial Suture Measurement by 2-point Method in Ultrasound Screening of Craniosynostosis. Plastic and reconstructive surgery. Global open, 7(5), e2225. https://pubmed.ncbi.nlm.nih.gov/31333954/
  3. Kliegman, R., Stanton, B., St. Geme, J. W., Schor, N. F., & Behrman, R. E. (2016). Nelson textbook of pediatrics (Edition 20.). Phialdelphia, PA: Elsevier.

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