17.1 Head I: Skull & Cranial Foramina, Scalp, Face & Parotid, Orbit, Temporal & Infratemporal Fossae, TMJ, Oral Region, Nose & Ear

Why this area matters

The TOS head list includes:

• skull (walls of the cranial cavity);
• face (muscles, nerves, vasculature) and the parotid gland;
• scalp;
• orbit;
• temporal region, infratemporal fossa and TMJ;
• oral region (lips, cheeks, gingivae, teeth, palate, tongue, salivary glands);
• pterygopalatine fossa;
• nose and paranasal sinuses;
• ear.

Clinical topics include scalp lacerations, facial palsy, parotid tumors, orbital fractures, epistaxis, sinusitis, dental infections and otitis media.

Cranial Foramina & Skull Base Neurovascular Conduits

The skull base is organized into three descending cranial fossae perforated by specific foramina that transmit cranial nerves, arterial conduits, and venous sinuses:

Anterior Cranial Fossa

• Cribriform Plate of Ethmoid: Transmits the unmyelinated primary olfactory nerve filia (CN I) from the nasal olfactory neuroepithelium to the olfactory bulbs. Fractures through the cribriform plate produce anosmia and CSF rhinorrhea.

Middle Cranial Fossa

• Optic Canal: Located within the lesser wing of the sphenoid; transmits the optic nerve (CN II) encased in meningeal sheaths and the ophthalmic artery.
• Superior Orbital Fissure (SOF): Triangular gap between the greater and lesser wings of the sphenoid; transmits the oculomotor nerve (CN III), trochlear nerve (CN IV), ophthalmic division of the trigeminal nerve ($CN\ V_1$: lacrimal, frontal, and nasociliary branches), abducens nerve (CN VI), sympathetic pupillodilator fibers, and the superior ophthalmic vein. Trauma or retro-orbital tumors cause Superior Orbital Fissure Syndrome (complete ophthalmoplegia, ptosis, proptosis, and sensory loss over the forehead and cornea).
• Foramen Rotundum: Pierces the greater wing of the sphenoid; transmits the maxillary nerve ($CN\ V_2$) horizontally into the pterygopalatine fossa.
• Foramen Ovale: Transmits the mandibular nerve ($CN\ V_3$), the accessory meningeal artery, the lesser petrosal nerve (parasympathetic secretomotor to parotid via otic ganglion), and emissary veins connecting to the pterygoid venous plexus (mnemonic: MALE).
• Foramen Spinosum: Transmits the middle meningeal artery, middle meningeal vein, and the meningeal branch of $CN\ V_3$ (nervus spinosus). Traumatic pterion fractures tear the underlying middle meningeal artery, producing an acute epidural hematoma (lens-shaped, biconvex hyperdensity on CT that does not cross cranial suture lines).
• Carotid Canal: Transmits the internal carotid artery (ICA) and the surrounding sympathetic perivascular carotid plexus.
• Foramen Lacerum: Occluded in vivo by fibrocartilage; the internal carotid artery passes horizontally across its superior roof rather than vertically traversing it.

Posterior Cranial Fossa

• Internal Acoustic Meatus (IAM): Located within the petrous temporal bone; transmits the facial nerve (CN VII), vestibulocochlear nerve (CN VIII), and the labyrinthine artery (branch of AICA). Acoustic neuromas (vestibular schwannomas) at the cerebellopontine angle compress structures in the IAM, presenting with sensorineural hearing loss, tinnitus, vertigo, and facial weakness.
• Jugular Foramen: Located between the petrous temporal bone and occipital bone; transmits the inferior petrosal sinus, the sigmoid sinus (forming the internal jugular vein), glossopharyngeal nerve (CN IX), vagus nerve (CN X), accessory nerve (CN XI), and the posterior meningeal artery. Vernet syndrome (Jugular Foramen Syndrome) causes dysphagia, loss of gag reflex, hoarseness, and shoulder shrug weakness.
• Hypoglossal Canal: Pierces the occipital condyle; transmits the hypoglossal nerve (CN XII) and an emissary vein.
• Foramen Magnum: The largest skull base opening; transmits the medulla oblongata / upper spinal cord, vertebral arteries, spinal roots of CN XI, anterior and posterior spinal arteries, and the tectorial membrane / alar ligaments.

Cranial Foramina & Transmitting Structures

Scalp

The five layers spell SCALP:

1. Skin.
2. Connective tissue: dense and richly vascular. Vessels are held open by the fibrous tissue, so lacerations bleed profusely.
3. Aponeurosis (galea aponeurotica) of the occipitofrontalis. A wound that splits it gapes widely.
4. Loose areolar tissue: the "danger area". Blood and pus spread freely here, and emissary veins connect it to the intracranial dural sinuses.
5. Pericranium: the periosteum of the skull. A cephalohematoma in a newborn lies beneath it and is limited by the sutures.

Blood supply comes from branches of the external carotid artery (superficial temporal, occipital, posterior auricular) and the internal carotid artery (supraorbital, supratrochlear).

Face and parotid gland

• Muscles of facial expression develop from the second pharyngeal arch and are supplied by CN VII. Sensation comes from CN V (V1, V2, V3).
• The facial artery crosses the mandible at the anterior border of the masseter, where its pulse can be felt.
• The facial vein connects to the cavernous sinus through the ophthalmic veins and the pterygoid plexus. Infection of the "danger triangle" (the upper lip and the side of the nose) can therefore cause cavernous sinus thrombosis.
• Parotid gland:
  • lies in front of the ear over the masseter, enclosed by a tough fascial capsule, so mumps parotitis is painful;
  • structures within it, from superficial to deep: facial nerve branches, the retromandibular vein and the external carotid artery;
  • the five facial nerve branches ("To Zanzibar By Motor Car") are temporal, zygomatic, buccal, marginal mandibular and cervical;
  • the parotid (Stensen) duct crosses the masseter, pierces the buccinator and opens opposite the upper second molar;
  • secretomotor supply: glossopharyngeal nerve, then the lesser petrosal nerve, then the otic ganglion, then the auriculotemporal nerve;
  • Frey syndrome (gustatory sweating) after parotid surgery is caused by misdirected regrowth of these fibers.

The orbit

• Walls: the floor (maxilla) and medial wall (ethmoid, lamina papyracea) are thinnest.
  • A blowout fracture of the floor can trap the inferior rectus, limiting upward gaze, and numb the cheek through the infraorbital nerve.
  • Ethmoid sinusitis can spread through the medial wall and cause orbital cellulitis.
• Extraocular muscles: LR6 SO4, the rest 3.
  • The lateral rectus is supplied by CN VI and the superior oblique by CN IV.
  • The medial, superior and inferior recti, the inferior oblique and the levator palpebrae superioris are supplied by CN III.
  • To test the superior oblique, ask the patient to look down while the eye is adducted. To test the inferior oblique, ask the patient to look up while it is adducted.
• Eyelids and lacrimal apparatus:
  • Ptosis follows paralysis of the levator (CN III, complete ptosis) or of the superior tarsal muscle (sympathetic, partial ptosis in Horner syndrome).
  • Tears from the lacrimal gland (parasympathetic fibers via CN VII) drain through the puncta, canaliculi and lacrimal sac into the nasolacrimal duct, which opens into the inferior meatus.
• Vessels: the ophthalmic artery (from the internal carotid) gives the central retinal artery, whose occlusion causes sudden painless monocular blindness with a cherry-red spot.

Temporal and infratemporal fossae and the TMJ

• The infratemporal fossa contains:
  • the lateral and medial pterygoid muscles;
  • the maxillary artery (its middle meningeal branch passes through the foramen spinosum);
  • the pterygoid venous plexus;
  • the mandibular nerve with its branches: the inferior alveolar nerve (target of the dental block), the lingual nerve (joined by the chorda tympani), the buccal nerve and the auriculotemporal nerve.
• Muscles of mastication (all supplied by V3): masseter, temporalis and medial pterygoid close the jaw; the lateral pterygoid opens it and protrudes it.
• The temporomandibular joint is a synovial joint with an articular disc. Opening the mouth combines hinge and gliding movements. The joint usually dislocates anteriorly, with the condyle in front of the articular eminence, after yawning or a blow. Reduction is by pressing down and back on the lower molars.

Oral region and salivary glands

• Tongue:
  • Motor: CN XII, except palatoglossus (CN X).
  • General sensation: anterior two-thirds by the lingual nerve (V3); posterior third by CN IX.
  • Taste: anterior two-thirds by the chorda tympani (CN VII); posterior third by CN IX; epiglottic region by CN X.
• Palate: the soft palate muscles are supplied by CN X, except tensor veli palatini (V3). With a CN X lesion, the uvula deviates away from the lesion.
• Submandibular gland: its duct (Wharton) opens at the sublingual caruncle. It is the most common site of salivary stones because its secretion is mucous and alkaline and the duct runs uphill. Secretomotor fibers come from CN VII through the chorda tympani and the submandibular ganglion.
• Sublingual gland: many small ducts open on the sublingual fold.
• Teeth and dental infections: infections of the lower molars can spread into the submandibular and sublingual spaces, producing Ludwig angina, which threatens the airway.

Pterygopalatine fossa

The pterygopalatine fossa is a small space behind the maxilla. It contains:

• the maxillary nerve (V2);
• the pterygopalatine ganglion, which receives parasympathetic fibers from CN VII through the greater petrosal nerve and supplies the lacrimal, nasal and palatal glands;
• the third part of the maxillary artery, whose sphenopalatine branch is the main supply to the nasal cavity and a source of posterior epistaxis.

Nose and paranasal sinuses

• Nasal cavity: superior, middle and inferior conchae and meatuses.
• Sinus drainage:
  • the frontal, anterior ethmoidal and maxillary sinuses drain into the middle meatus (hiatus semilunaris);
  • the posterior ethmoidal sinuses into the superior meatus;
  • the sphenoid sinus into the sphenoethmoidal recess;
  • the nasolacrimal duct into the inferior meatus.
• The maxillary sinus opening lies high on its medial wall, so it drains poorly and is the most commonly infected sinus. Its floor is close to the roots of the upper molars, so dental infection can cause sinusitis.
• Kiesselbach's plexus (Little's area) on the anterior septum is where the anterior ethmoidal, sphenopalatine, greater palatine and superior labial (facial) arteries anastomose. It is the source of about 90% of nosebleeds, which are controlled by firm anterior pressure.

The ear

• External ear: the auricle (elastic cartilage) and the external acoustic meatus. Sensory supply comes from V3, CN VII, CN IX, CN X and C2 and C3. A cough on ear cleaning is due to the auricular branch of the vagus.
• Middle ear:
  • the tympanic membrane; the ossicles (malleus, incus, stapes); the auditory (Eustachian) tube to the nasopharynx;
  • the stapedius (CN VII) and tensor tympani (V3) muscles;
  • the mastoid antrum and air cells, through which mastoiditis spreads;
  • the chorda tympani crosses it.
  • In children the Eustachian tube is shorter, wider and more horizontal, so otitis media is common.
  • Middle-ear infection can spread to the meninges, the sigmoid sinus or the temporal lobe.
• Inner ear: the cochlea (hearing) and vestibular apparatus (balance), supplied by CN VIII and lying in the petrous temporal bone.