Anatomy of the Skull Note for College of Medicine

The skull is the skeleton of the head. The cranium is the skull without the mandible but sometimes the skull and the cranium are used synonymously.  

With the exception of the mandible and the bony ossicles, the bones of the skull are united by either sutures (formed by fibrous tissue) or synchrondoses (formed by cartilages).

The temporomadibular joint is a synovial joint.  Apart from the 6 bony ear ossicles there are 22 bones in the skull. These bones may be divided into two:  neurocranium and viscerocranium or splanchnocranium or facial skeleton.

Neurocranium: It is the bony covering of the brain and internal ear. It consists of a dome-like roof called the calvaria or skullcap, and a base or floor called the basicranium.

The neurocranium consists of 8 separate bones: 2 of them are paired while 4 are unpaired. The paired ones are temporal and parietal while the unpaired ones are frontal, ethmoid, sphenoid and occipital.

Viscerocranium or facial skeleton: It surrounds the orbital and nasal cavities and the upper part of the oral cavity and consists of 14 separate bones: 6 of them are paired while 2 are unpaired.

The paired ones are nasal, lacrimal, zygomatic, maxilla, palatine and inferior nasal concha while the unpaired ones are mandible and vomer.

Drawing the Proportions of the Skull

Some of the sutures of the skull:

  1. Coronal suture is the joint between the frontal bone and the two parietal bones.
  2. Sagittal suture is the joint between the left and right parietal bones.
  3. Lambdoid suture is the joint between the two parietal bones and the occipital bone. It is called lambdoid because it has the shape of the Greek Letter L (lambda) which looks like an inverted Y.
  4. Occipitomastoid suture unites the occipital bone to the mastoid part of the temporal bone.
  5. Parietomastoid suture unites the posterior part of the parietal bone to the mastoid part of the temporal bone.
  6. Parietosquamous suture unites the parietal bone to the squamous part of the temporal bone.
  7. Parietosphenoid suture unites the parietal to the greater wing of sphenoid
  8. Frontosphenoid suture unites the frontal bone to the greater wing of sphenoid
  9. Frontonasal suture unites the frontal bone to the nasal bones
  10. Internasal suture unites the left and right nasal bones
  11. Frontal suture. In the fetal skull the left and right frontal bones are separated by the frontal suture.

This suture normally begins to fuse in the 2nd year and disappears by the 8th year.

In 3-8% of adults a remnant of it persists as the metopic suture. It lies in the median plane and should be differentiated from a skull fracture

Points and Eminences of the Skull

Bregma is the point of intersection between coronal and sagittal sutures. It corresponds to the anterior fontanelle in the infant. This fontanelle closes 12-18 months after birth

Lambda is the point of intersection between sagittal and lambdoid sutures.

It corresponds to the posterior fontanelle in the infant. The fontanelle begins to close 2-3 months after birth.

Vertex is the highest point of the skull in the Frankfurt or orbitomeatal plane. It lies a few centimeters behind the bregma.

Parietal prominence or tuber is the most convex portion of the parietal bone. It is a common site of skull fracture.  

Pterion is a small circular area where four bones: the greater wing of the sphenoid, the squamous part of the temporal, the frontal and the parietal bones meet at an H-shaped junction of sutures.

It is the thinnest part of the lateral wall of the skull and overlies the middle meningeal vein and the anterior branch of the middle meningeal artery, and the stem of the lateral sulcus of the brain. It corresponds to the sphenoidal (anterolateral) fontanelle in the infant.

The centre of the pterion is about 4 cm above the midpoint of the zygomatic arch and 2.5cm behind the frontozygomatic suture.

Asterion is the point of intersection between the lambdoid, parietomastoid and occipitomastoid sutures.

It has the shape of a star and corresponds to the mastoid (posterolateral) fontanelle in the infant

Nasion is the point at the root of the nose where the frontonasal and internasal sutures meet.

Glabella is a smooth prominence on the frontal bone above the root of the nose. It is between the left and right superciliary arches.

Basion is the midpoint of the anterior margin of the foramen magnum.

Inion is the most prominent point on the external occipital protuberance.

Obelion is the point on the sagittal suture between the left and right parietal foramina.

Gnathion or mental point is the middle point of the base of the mandible.

Gonion is the lowest and most posterior point of the angle of the mandible.

Cranial Cavity

The cranial cavity is divided into anterior, middle and posterior cranial fossae by two ridges.

The sphenoidal ridge formed by the lesser wing of sphenoid separates the anterior cranial fossa from the middle cranial fossa, while the petrous ridge formed by the petrous part of the temporal bone separates the middle and posterior cranial fossae.

The anterior cranial fossa is the shallowest and smallest, while the posterior is the deepest and largest.

The anterior part of the anterior cranial fossa is formed by the frontal bone, the middle part by the ethmoid, and the posterior part by the body and lesser wings of sphenoid.  The anterior cranial fossa supports the frontal lobes of the cerebral hemispheres.

The middle cranial fossa is deeper and larger than the anterior cranial fossa, and is formed mainly by the sphenoid bone and partly by the temporal bone.

It contains the temporal lobes of the cerebral hemispheres. The posterior cranial fossa is the largest and deepest of the cranial fossae. It is formed by the sphenoid, occipital and temporal bones and contains the cerebellum, pons and medulla.

Individual Bones of the Skull

The ethmoid consists of  four parts: a horizontal or cribriform plate, a perpendicular plate which forms part of the nasal septum, and two lateral masses or labyrinths that contain the ethmoidal air sinuses. 

An upward projection from the midline of the cribriform plate is called the crista galli.  

The orbital lamina of the ethmoid bone, also known as the lamina papyracea, is the main component of the medial wall of the orbit and the lateral surface of the ethmoid air cells.

The sphenoid has the shape of a butterfly. It has a body, a pair of lesser wings, a pair of greater wings and a pair of pterygoid processes.

The body which contains the sphenoid air sinuses has superior, inferior, anterior, posterior and left and right lateral surfaces. The superior surface has the shape of a Turkish saddle and is called the sella turcica.

It consists of three parts: tuberculum sellae, hypophyseal or pituitary fossa, and dorsum sellae.

The temporal bone consists of 5 components: squamous, mastoid, petrous, styloid and tympanic. The mastoid and petrous components are morphologically one component.

The mastoid process appears in the 2nd year and mastoid air cells in the 6th year.

The occipital bone consists of 4 parts: squamous, basilar and left and right lateral or condylar parts. 

The maxilla has a body and 4 processes. The body is pyramidal in shape and contains the maxillary air sinus. It has 4 surfaces: anterior, posterior (infratemporal), orbital and nasal. The processes are: zygomatic, frontal, alveolar and palatine.

The zygomatic bone forms the prominence of the cheek and it has four processes. Its temporal process articulates with the zygomatic process of the temporal bone to form the zygomatic arch.

The palatine bone has two plates: horizontal and perpendicular plates, and three processes: pyramidal, orbital and sphenoidal.

Ossification

Bones of the calvaria and some parts of the cranial base are membranous in ossification while the floors of the middle and posterior cranial fossae are cartilaginous in ossification.

1. Bones formed in membrane: frontal, parietal, nasal, lacrimal, zygomatic, maxilla, palatine and vomer.

2. Bones formed in cartilage: ethmoid and inferior nasal concha

3. Bones formed partly in membrane and partly in cartilage: sphenoid, temporal occipital and mandible.

Sphenoid: the body, lesser wing and the base of the greater wing ossify in cartilage, while the rest of the greater wing and the pterygoid processes ossify in membrane.

Temporal: it ossifies in cartilage except the squamous and tympanic parts which ossify in membrane.

Occipital: ossifies in cartilage except the apical portion of the squamous part which ossifies in membrane.

Mandible: The mandible ossifies partly in membrane and partly in cartilage.

i) membranous ossification:

 a) the body except the symphysis menti.

 b) the ramus except the area above the mandibular foramen.

ii) cartilagenous ossification:

a) coronoid process

b) condyloid process

c) anterior portion of the body of mandible (i.e. symphysis menti)

d)  part of the ramus above mandibular foramen.

Membranous ossification of the mandible begins in the 6th week of intrauterine life. At birth the mandible consists of left and right halves united by a fibrous symphysis. The two halves begin to fuse in the 2nd year.

By the 4th year the bones of the skull have an outer compact bone called the outer table and an inner compact bone called the inner table. In between the two is the diploe.

The inner table is thinner and more brittle than the outer table, and is grooved by vessels, with the veins lying between the groove and the artery.

Fractures are more extensive on the inner table than on the outer table. Sometimes only the inner table is fractured.

Obliteration of the sutures begins between the ages of 30 and 40 years on the inner table and 10 years later on the outer table. Full obliteration may never occur in sagittal and lambdoid sutures.

Sex Differences

The male skull is larger, thicker, heavier and rougher than the female skull. It has more prominent glabella, superciliary arch and mastoid process.

Wormian or Suturual or Intrasutural Bones

They are small irregular isolated bones that are found within the sutures of the skull. They are most frequently seen in the lambdoid suture, and occasionally in the sagittal and coronal sutures.

FORAMINA OF THE SKULL AND STRUCTURES THAT PASS THROUGH THEM

A. ANTERIOR CRANIAL FOSSA

FORAMEN CAECUM: This lies between the frontal bone and the crista galli of the ethmoid bone. It usually ends blindly in adult life. When it is patent, as in early life, it transmits an emissary vein from the nasal mucosa to the superior sagittal sinus.

ANTERIOR ETHMOIDAL CANAL: This is a slit along the suture between the orbital part of the frontal bone and the cribriform plate of the ethmoid bone. It transmits:

1. Anterior ethmoidal vessels (the artery is a branch of ophthalmic artery)

2. Anterior ethmoidal nerve (a branch of nasociliary nerve)

POSTERIOR ETHMOIDAL CANAL: This opens at the posterolateral corner of the cribriform plate. It transmits:

1. Posterior ethmoidal vessels (the artery is a branch of ophthalmic artery).

2. The posterior ethmoidal nerve (a branch of nasociliary nerve).

CRIBRIFORM PLATE OF ETHMOID: The numerous small foramina transmit a bundle of nerve fibres or filaments from the nasal mucosa to the olfactory bulb. This bundle constitute the olfactory nerve

B. MIDDLE CRANIAL FOSSA

a) MEDIAN PART

OPTIC CANAL: This lies between the two roots of the lesser wing of the sphenoid, with the body of the sphenoid medially. It transmits:

1. Optic nerve

2. Ophthalmic artery

b) LATERAL PART

i) GREATER WING OF SPHENOID

SUPERIOR ORBITAL FISSURE: This is a comma-like slit bounded above by the lesser wing, below by the greater wing, and medially by the side of the body of the sphenoid. It leads into the orbit and transmits:

1. Oculomotor nerve (III) (upper and lower divisions)

2. Trochlear nerve (IV)

3. Terminal branches of the ophthalmic nerve (V)

    a) Nasociliary nerve

    b) Frontal nerve

    c) Lacrimal nerve

4. Abducent nerve (VI)

5. Sympathetic nerve fibres from the plexus around internal carotid artery

6. Orbital branch of the middle meningeal artery

7. Meningeal branch of the lacrimal artery

8. Superior and inferior ophthalmic veins

FORAMEN ROTUNDUM: This pierces the greater wing of the sphenoid immediately below the medial end of the superior orbital fissure. It leads forwards into the pterygopalatine fossa to which it transmits the maxillary nerve (V2).

FORAMEN OVALE: It passes through the greater wing of the sphenoid about 1cm posterolateral to the foramen rotundum and leads downwards into the infratemporal fossa. It transmits:

1. Mandibular nerve (V3)

2. Accessory middle meningeal artery

3. Emissary veins between the pterygoid plexus of veins and the cavernous sinus

± 4. Lesser petrosal nerve

FORAMEN SPINOSUM: This lies immediately posterolateral to the foramen ovale, and derives its name from its close relationship to the spine of the sphenoid.

It transmits:

1. Middle meningeal vessels

2. Nervous spinosus (meningeal branch of mandibular nerve)

Three inconstant foramina:

LACRIMAL FORAMEN: This lies lateral to the apex of the superior orbital fissure and allows the lacrimal artery to anastomose with the anterior branch of the middle meningeal artery.

SPHENOIDAL EMISSARY FORAMEN (OF VESALIUS): This lies between the foramen rotundum and foramen ovale. When present, it transmits an emissary vein between cavernous sinus and pterygoid plexus of veins.

CANALICULUS INNOMINATUS OR PETROSAL FORAMEN: This lies on the medial side of the foramen spinosum. When present, it transmits the lesser petrosal nerve.

ii) PETROUS TEMPORAL BONE

FORAMEN LACERUM: This lies between the apex of the petrous temporal bone, the sphenoid, and the basilar part of the occipital bone.  It is large and irregular in shape. During  life, the lower part of it is closed by a fibrocartilage. Only small structures pass through the entire length of the foramen. It transmits:

1. Meningeal branch of the ascending pharyngeal artery

2. Emissary veins between the cavernous sinus and pterygoid venous plexus

3. Lymph vessels

Structures that do not pass through the entire extent of the foramen, but are related to it include:

1. Internal carotid artery together with its accompanying sympathetic and venous plexus emerges from the carotid canal and traverses the superior end of the foramen. It does not pass through foramen lacerum.

2. Greater petrosal nerve. It enters the foramen upon emerging from the hiatus. Here it unites with the deep petrosal nerve to form the nerve of the pterygoid canal (Vidian nerve). Just above the inferior opening of the foramen lacerum, the Vidian nerve leaves the foramen and enters the pterygoid canal in the anterior wall of the foramen lacerum.

OTHERS: Two foramina open about 0.5cm and 1cm respectively to transmit:

1. Lesser petrosal nerve

2. Greater petrosal nerve

C. POSTERIOR CRANIAL FOSSA

FORAMEN MAGNUM: It is a large oval foramen in the occipital bone and is the largest foramen in the skull. It transmits:

1. Lower end of medulla oblongata

2. Spinal roots of the accessory nerve (XI)

3. Vertebral arteries

4. Sympathetic nerves associated with vertebral arteries

5. Anterior spinal artery

6. Posterior spinal arteries

7. Vertebral venous plexus

HYPOGLOSSAL/ANTERIOR CONDYLAR CANAL: This is located at the antero-lateral margin of the foramen magnum. It transmits:

1. Hypoglossal nerve (XII)

2. Meningeal branch of hypoglossal nerve

3. Meningeal branch of the ascending pharyngeal artery

4. Emissary vein between sigmoid sinus and internal jugular vein.

JUGULAR FORAMEN: This is an interosseous foramen between the occipital and the petrous temporal bones. The posterior and lower borders are smooth and regular.  The upper border is sharp and may be interrupted by a notch. The ends of the notch together with two transverse septa of fibrous dura mater, which may ossify, sometimes divide the foramen into three compartments.

It transmits:

a) Anterior compartment:

 Inferior petrosal sinus

b) Middle compartment:

1. Glossopharyngeal nerve (XI) – there may be a notch for this nerve at the upper border of the foramen.

2. Vagus nerve (X)

3. Accessory nerve (XI)

c) Posterior compartment:

1. Terminal part of sigmoid sinus/superior bulb of internal jugular vein

2. Meningeal branches of the occipital and ascending pharyngeal arteries

INTERNAL ACOUSTIC MEATUS: This lies about 1cm above the jugular foramen, and it transmits:

1. Facial nerve (VII)

2. Vestibulocochlear nerve (VIII)

3. Labyrinthine artery and accompanying vein

MASTOID CANALICULUS: This is a minute canal in the lateral wall of the jugular fossa. It transmits the auricular branch of the vagus nerve (X).

CONDYLAR/ POSTERIOR CONDYLAR CANAL: This is located in the posterolateral margin of the foramen magnum and opens behind the condyle. It is sometimes a fossa. When it is a canal, it transmits emissary veins between the sigmoid sinus and the vertebral/suboccipital venous plexus.

MASTOID FORAMEN: This is located about midway along the sigmoid sulcus. It pierces the mastoid portion of the temporal bone above the base of the mastoid process. It is near or on the occipitomastoid suture.

It transmits:

1. Emissary vein between sigmoid sinus and posterior auricular vein

2. Meningeal branch of occipital artery

II FORAMINA OF THE EXTERIOR OF THE SKULL

A. NORMA BASALIS

The inferior of the skull may be divided by two lines:

a) Anterior transverse line passing through both mandibular notches

b) Posterior transverse line passing through the anterior margins of both mastoid processes

These lines divide the inferior aspect into three areas:

i)  Anterior

ii)  Intermediate

iii)  Posterior

INCISIVE FOSSA: This is a deep fossa lying in the midline at the front of the hard palate. It receives the ends of the two lateral incisive canals from both sides which lead upwards to the corresponding side of the nasal septum and transmits:

1. Terminal part of nasopalatine nerve

2. Terminal parts of greater palatine vessels

GREATER PALATINE FORAMEN: This is the lower orifice of the canal of the same name, and opens close to the lateral border of the palate immediately behind the palatomaxillary suture. It transmits:

1. Greater palatine vessels (the artery is a branch of the descending palatine artery from the third part of maxillary artery)

2. Greater (anterior) palatine nerve (a branch of pterygopalatine ganglion)

LESSER PALATINE FORAMINA: There are usually two (sometimes one or even three) of these tiny foramina behind the greater palatine foramen. They transmit:

1. Lesser palatine vessels (the artery is a branch of the descending palatine artery from the third part of maxillary artery)

2. Lesser (posterior) palatine nerves (a branch of the maxillary nerve in the pterygopalatine fossa).

PALATOVAGINAL OR PHARYNGEAL CANAL: This is a canal between the sphenoidal process of the palatine bone and the vaginal process of sphenoid bone. It connects the nasopharynx with the pterygopalatine fossa and it transmits:

1. Pharyngeal branch of pterygopalatine ganglion (from maxillary nerve)

2. Pharyngeal branch of third part of the maxillary artery

VOMEROVAGINAL CANAL: This inconstant canal may be present on the medial side of the palatovaginal canal, and it lies between the ala of the vomer and the upper surface of the vaginal process of the medial pterygoid plate. It transmits pharyngeal branch of the sphenopalatine artery.

CAROTID CANAL: The petrous temporal bone is perforated anterior to the jugular foramen by the carotid canal. It transmits:

1. Internal carotid artery

2. Numerous sympathetic nerve fibres

TYMPANIC CANALICULUS: This small foramen lies in the ridge between the jugular fossa and the carotid canal. It transmits:

Tympanic branch of the glossopharyngeal nerve

Other small foramina on the same ridge transmit:

1. Caroticotympanic branches of the internal carotid artery

2. Sympathetic filaments from the carotid plexus

EXTERNAL AUDITORY MEATUS: This leads into the external auditory canal. 

PETROTYMPANIC FISSURE: This is the posterior part of the medial part of the squamotympanic fissure. The anterior part is the petrosquamous fissure. The division is by the edge of the tegmen tympani. It transmits:

1. Chorda tympani

2. Anterior tympanic artery (a branch of maxillary artery)

3. Tympanic veins

STYLOMASTOID FORAMEN

It is the opening between the base of the styloid and mastoid processes of the temporal bone. It transmits:

1. Facial nerve

2. Stylomastoid vessels (the artery is a branch of posterior auricular artery from the external carotid artery).

B. NORMA LATERALIS/INFRATEMPORAL FOSSA

PTERYGOMAXILLARY FISSURE: This lies between the lateral pterygoid plate and the back of the maxilla. Its upper end is continuous with the posterior end of the inferior orbital fissure. Through this fissure, the pterygopalatine fossa communicates with infratemporal fossa. It transmits:

1. Terminal part of maxillary artery

2. Posterior superior alveolar nerve (a branch of maxillary nerve)

SPHENOPALATINE FORAMEN: It is an opening in the lateral nasal wall between the inferior surface of the body of sphenoid and the superior border of the perpendicular plate of palatine bone. It connects the nasal cavity with the pterygopalatine fossa. During life, it is covered by the mucous membrane of the lateral wall of the nose, immediately behind the posterior end of the middle concha. It transmits:

1. Sphenopalatine or nasopalatine artery and vein.  The artery is also called the artery of epistaxis (nosebleeds). It is a branch of the third part of the maxillary artery, and is the main blood supply to the posterior mucosa of the nasal cavity. It is the terminal branch of the maxillary artery.

2. Nasopalatine nerve. It is also called the long sphenopalatine nerve, and it arises from the pterygopalantine ganglion in the pterygopalatine fossa. The branches of the pterygopalantine ganglion are from the maxillary division of trigeminal nerve.

3. Posterior superior lateral nasal nerves. They are also called the short sphenopalatine nerves. They are branches of pterygopalatine ganglion from the maxillary division of the trigeminal nerve and they supply the posterosuperior quadrant of the lateral nasal wall. 

INFERIOR ORBITAL FISSURE: Through this fissure, the orbit communicates anteriorly with the infratemporal, and posteriorly with the pterygopalatine fossae. Its lower lip is notched by the infraorbital groove, which passes forwards to become the infratemporal canal.

The fissure transmits:

1. Infraorbital nerve and vessels

2. Zygomatic nerve (a branch of maxillary nerve)

3. Orbital branches of pterygopalatine ganglion

4. Communicating veins between the inferior ophthalmic vein and the pterygoid plexus of veins.

C. NORMA FRONTALIS

SUPRAORBITAL FORAMEN

It lies above the orbital cavity and transmits supraorbital nerve and vessels. The nerve is a terminal branch of the ophthalmic division of trigeminal nerve. It is a direct branch of the frontal nerve. The supraorbital artery is a branch of the ophthalmic artery

INFRAORBITAL FORAMEN

It lies below the orbital cavity and transmits infraorbital nerve and vessels. The nerve is the continuation of the maxillary nerve after the latter enters the infraorbital canal. The artery is a branch of the third part of the maxillary artery.  

MENTAL FORAMEN: This is an irregular opening, just above the centre of the medial surface of the ramus of the mandible. It transmits the mental nerve and vessels. The mental nerve is branch of the posterior trunk of the inferior alveolar nerve, a branch of mandibular division of trigeminal nerve. The mental artery is a terminal branch of the inferior alveolar artery from the first part of maxillary artery.

The suprorbital, infraorbital and mental foramina tend transmit terminal branches of ophthalmic, maxillary and mandibular divisions of trigeminal nerve.  

ZYGOMATICOFACIAL FORAMEN:

It is a small foramen on the malar surface of the zygomatic bone. It transmits

1. Zygomaticofacial nerve (a branch zygomatic nerve)

2. Zygomaticofacial vessels (the artery is a branch of lacrimal artery from ophthalmic artery)

ZYGOMATICOTEMPORAL FORAMEN: This pierces the temporal surface of the zygomatic bone in the anterior wall of the temporal fossa to transmit:-

1. Zygomaticotemporal nerve (a branch of zygomatic nerve)

2. Zygomaticotemporal vessesls

D. NORMA VERTICALIS

PARIETAL FORAMEN: This lies in the posterior part of the bone, and close to the sagittal plane. It transmits:

1. Parietal emissary vein which drains into the superior sagittal sinus

2. Sometimes, a small branch of the occipital artery.    

Note by

E. N. Obikili

  

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