Chapter 4: Cranium

The cranium (skull) protects the brain and the special sense organs, forms the skeleton of the face, and gives passage to the cranial nerves and the great vessels of the head. For practicum you need to name every bone, find its key features, and recognise the skull from five external views and from inside.

Development of the skull

The skull develops in two parts:

• Neurocranium, the case around the brain.
  • The flat bones of the vault form directly in membrane (intramembranous ossification).
  • Most of the base forms first in cartilage, the chondrocranium (endochondral ossification).
• Viscerocranium, the skeleton of the face and jaws. It develops from the pharyngeal arches and is built largely from neural crest cells (chapter 2).

The face

The face forms around the primitive mouth (stomodeum) from five prominences that appear in week 4:

• one frontonasal prominence, which gives rise to the forehead, the bridge of the nose and the medial and lateral nasal prominences around each nasal placode;
• paired maxillary prominences, forming the upper cheeks and most of the upper lip;
• paired mandibular prominences, forming the lower jaw and lower lip.

The two medial nasal prominences fuse to form the middle of the upper lip (the philtrum) and the primary palate. A cleft lip results when a maxillary prominence fails to fuse with the medial nasal prominence on its side.

The neonatal skull and fontanelles

At birth the vault bones are separated by fibrous sutures that widen at their corners into fontanelles (fonticuli). These let the bones overlap as the head passes through the birth canal and leave room for the brain to grow.

The anterior fontanelle is felt in every infant examination:

• a bulging fontanelle suggests raised intracranial pressure;
• a sunken fontanelle suggests dehydration;
• premature fusion of a suture (craniosynostosis) distorts the shape of the head.

Bones of the skull

The adult skull has 22 bones (plus the hyoid and six ear ossicles):

Neurocranium

Os frontale

The frontal bone forms the forehead and the roofs of the orbits. It meets the frontal process of the maxilla and the nasal bones medially, and the zygomatic bone laterally. Key features:

• Arcus superciliaris: the brow ridges.
• Glabella: the smooth area between them.
• Margo supraorbitalis: the upper rim of the orbital opening (aditus orbitae). It carries the foramen (or incisura) supraorbitale, which transmits the supraorbital nerve and vessels.
• Processus zygomaticus, meeting the zygomatic bone.
• Sinus frontalis, an air sinus within the bone.

Os ethmoidale

A light, cube-shaped bone between the orbits:

• Lamina cribrosa (cribriform plate): perforated by the foramina cribrosa, which carry the olfactory nerve fibres (CN I). It forms part of the floor of the anterior cranial fossa. The crista galli rises from it in the midline.
• Lamina perpendicularis: forms the upper part of the bony nasal septum.
• Labyrinthus ethmoidalis: contains the ethmoidal air cells. Its medial surfaces carry the superior and middle nasal conchae, and its thin lateral surface forms part of the medial wall of the orbit.

Os sphenoidale

A bat-shaped bone at the centre of the cranial base:

• Corpus: contains the sphenoidal sinus. Its upper surface is the sella turcica, which holds the pituitary gland.
• Ala minor (lesser wing): carries the optic canal at its root.
• Ala major (greater wing): forms part of the middle cranial fossa and the side of the skull at the pterion. It contains the foramina rotundum, ovale and spinosum.
• Processus pterygoideus (medial and lateral plates): projects downward. The canalis pterygoideus runs through its base.

Os occipitale

Forms the back and much of the base of the skull:

• Squama occipitalis, carrying:
  • the protuberantia occipitalis externa (its most prominent point is the inion);
  • the lineae nuchales superior and inferior, where neck muscles attach.
• Foramen magnum, flanked by the condyli occipitales, which articulate with the atlas.
• Pars basilaris: runs forward to the sphenoid as the clivus.
• Sutura lambdoidea: its junction with the parietal bones.

Os temporale

The temporal bone has four parts:

1. Pars squamosa: the thin, flat part on the side of the skull. It carries the processus zygomaticus and the fossa mandibularis (for the temporomandibular joint).
2. Pars tympanica: surrounds the meatus acusticus externus (external ear canal).
3. Pars petrosa: a dense wedge in the cranial base that houses the inner ear. It contains the meatus acusticus internus and the canalis caroticus.
4. Pars mastoidea: behind the ear. It carries the processus mastoideus, which contains mastoid air cells.

The processus styloideus projects down from its underside, with the foramen stylomastoideum (where CN VII leaves the skull) just behind it.

Os parietale

The paired parietal bones form most of the roof and sides of the vault:

• Tuber parietale: the parietal eminence, the widest point of the head.
• Lineae temporales superior and inferior: attachments of the temporal fascia and temporalis muscle.
• Foramen parietale: a small emissary foramen near the sagittal suture.
• Suturae: coronal (anterior border), sagittal (between the two parietals), lambdoid (posterior border) and squamous (lower border).

Viscerocranium

Mandibula

The only movable skull bone, with a corpus (body) and two rami.

• Angulus mandibulae: where the body meets the ramus.
• Processus condylaris: head and neck, articulating at the temporomandibular joint.
• Processus coronoideus: attachment of the temporalis muscle.
• The incisura mandibulae (mandibular notch) lies between the condylar and coronoid processes.
• Foramen mandibulae: on the inner surface of the ramus, where the inferior alveolar nerve and vessels enter.
• Foramen mentale: on the outer surface of the body, below the premolars, where the mental nerve emerges.
• Arcus alveolaris: holds the lower teeth.
• Symphysis mandibulae: the midline union of the two halves.
• Spina mentalis: tubercles on the inner surface of the symphysis, where genioglossus and geniohyoid attach.
• Tuberositas pterygoidea: a rough area on the inner surface of the angle, where medial pterygoid inserts. (Lateral pterygoid inserts into the fovea pterygoidea on the neck of the condyle.)

Maxilla

Forms the upper jaw, most of the hard palate, the floor of the orbit and the lateral wall of the nasal cavity. Its body contains the sinus maxillaris, the largest paranasal sinus.

• Foramen infraorbitale: below the orbit, for the infraorbital nerve (from V2).
• Processus alveolaris and arcus alveolaris: carry the upper teeth.
• Processus palatinus: forms the front two-thirds of the hard palate.
• Processus frontalis and processus zygomaticus.

Os palatinum and the hard palate

The palatum durum (hard palate) is formed by the palatine processes of the maxillae in front and the horizontal plates of the palatine bones behind. Seen from below, it has three openings:

• the foramen incisivum, behind the central incisors, for the nasopalatine nerve;
• the foramen palatinum majus, near the third molar, for the greater palatine nerve and vessels;
• the foramina palatina minora, just behind it.

A cleft palate results when the palatal shelves fail to fuse in the midline.

Other facial bones

• Vomer: forms the lower and back part of the bony nasal septum, below the perpendicular plate of the ethmoid.
• Os nasale: the paired bones of the nasal bridge. Each meets four bones:
  • the frontal bone (sutura frontonasalis);
  • the ethmoid (sutura ethmoidonasalis);
  • its partner (sutura internasalis);
  • the maxilla (sutura nasomaxillaris).
• Os lacrimale: the smallest facial bone, in the medial wall of the orbit. Its groove helps form the fossa for the lacrimal sac.
• Concha nasalis inferior: a separate curled bone on the lateral wall of the nasal cavity, below the ethmoidal conchae.
• Os zygomaticum (cheek bone):
  • it meets the frontal bone at the sutura frontozygomatica, the maxilla at the sutura zygomaticomaxillaris, and the temporal bone at the sutura temporozygomatica;
  • with the zygomatic process of the temporal bone, it forms the arcus zygomaticus;
  • the foramen zygomaticofaciale transmits a small cutaneous nerve.

The skull from outside (facies externa)

Anatomists describe the external skull from five views (normae).

Norma frontalis (facialis)

• Glabella and nasion (where the frontonasal suture meets the midline).
• The orbits (cavitas orbitae) with the foramen supraorbitale above and foramen infraorbitale below.
• The apertura piriformis (the pear-shaped nasal opening) and the septum nasi.
• Sutures: frontozygomatica, frontonasalis, internasalis, nasomaxillaris, intermaxillaris.
• The foramen mentale on the mandible.

The supraorbital, infraorbital and mental foramina lie roughly on one vertical line. They are where the three divisions of the trigeminal nerve (V1, V2, V3) reach the face.

Norma lateralis

The landmarks to find on the side of the skull:

• Pterion: the H-shaped junction of the frontal, parietal, sphenoid (greater wing) and temporal bones, about 3–4 cm above the midpoint of the zygomatic arch.
• Asterion: the junction of the parietal, occipital and temporal (mastoid) bones.
• Stephanion: where the coronal suture crosses the superior temporal line.
• Fossa temporalis: the shallow area above the zygomatic arch, bounded by the temporal lines, where temporalis lies.
• Meatus acusticus externus, with the mastoid and styloid processes behind and below it.

Clinical relevance of pterion. The bone at pterion is thin, and the anterior branch of the middle meningeal artery runs in a groove just deep to it. A blow to the side of the head can fracture pterion and tear the artery, causing an epidural (extradural) haematoma: arterial bleeding between the skull and the dura. The patient may have a lucid interval before rapidly deteriorating, which makes it a neurosurgical emergency.

Norma verticalis

Seen from above:

• Sutura coronalis (between frontal and parietals).
• Sutura sagittalis (between the two parietals).
• Sutura lambdoidea (between parietals and occipital).
• Bregma: where the coronal and sagittal sutures meet (the site of the anterior fontanelle).
• Lambda: where the sagittal and lambdoid sutures meet (the site of the posterior fontanelle).
• Vertex: the highest point of the skull.
• Obelion: the point on the sagittal suture between the two parietal foramina.
• Tuber parietale and the lineae temporales superior and inferior.

Norma occipitalis

From behind:

• Sutura lambdoidea.
• Protuberantia occipitalis externa and inion.
• Lineae nuchales superior and inferior.
• Processus mastoidei.

Norma basalis

From below, from front to back:

• Hard palate: the incisive, greater palatine and lesser palatine foramina; the spina nasalis posterior.
• Processus pterygoidei: medial and lateral plates, with the hamulus pterygoideus at the tip of the medial plate.
• Foramen ovale and foramen spinosum in the greater wing, and the spina ossis sphenoidalis beside them.
• Foramen lacerum, at the apex of the petrous temporal bone.
• The canalis caroticus (external opening), the foramen jugulare and the fossa jugularis.
• Processus styloideus and foramen stylomastoideum; processus mastoideus.
• Foramen magnum, flanked by the condyli occipitales, with the canalis nervi hypoglossi just above each condyle and the canalis condylaris behind it.
• Lineae nuchales.

The skull from inside (facies interna)

Calvaria

The inner surface of the skull cap shows:

• Sulcus sinus sagittalis superioris: a midline groove for the superior sagittal sinus, ending in front at the crista frontalis.
• Sulci arteriosi et venosi: branching grooves for the middle meningeal artery and veins.
• Foveolae granulares: small pits beside the sagittal groove, made by the arachnoid (Pacchionian) granulations, through which CSF drains into the venous sinus.
• In section, a layer of spongy bone (diploë) between the inner and outer tables (lamina interna and lamina externa).

Basis cranii interna

The floor of the cranial cavity steps down, from front to back, in three cranial fossae.

The lesser wings of the sphenoid separate the anterior and middle fossae. The petrous ridges and dorsum sellae separate the middle and posterior fossae.

Where the cranial nerves leave the skull

The deck's summary slide highlights the canalis condylaris next to "CN XII". The hypoglossal nerve actually exits through the canalis nervi hypoglossi. The condylar canal, which is not always present, only carries an emissary vein.

A mnemonic for the three divisions of the trigeminal nerve, through consecutive openings in the sphenoid: "Standing Room Only": superior orbital fissure (V1), rotundum (V2), ovale (V3).

Key teaching point

Learn the skull in layers:

1. The 8 bones of the neurocranium and the 14 of the viscerocranium.
2. The sutures and their meeting points (bregma, lambda, pterion, asterion).
3. The five external views.
4. The three cranial fossae and the table of foramina.

Most clinical questions about the skull come back to that table, or to pterion.