3.2 The Face & The Ear
Key Takeaways
- The scalp has five layers (Skin, Connective tissue, Aponeurosis, Loose connective tissue, Pericranium — SCALP), with the loose layer allowing dangerous infections to spread and the aponeurosis (galea aponeurotica) holding tension to limit bleeding.
- Muscles of facial expression are innervated by the facial nerve (CN VII), while the trigeminal nerve (CN V) provides sensory supply in three divisions (V1 ophthalmic, V2 maxillary, V3 mandibular).
- The external ear is supplied by the auriculotemporal (V3), great auricular (C2–C3), and auricular branch of the vagus (CN X) nerves; the tympanic membrane receives its outer surface from V3 and CN X and its inner surface from CN VII.
- The middle ear houses the three ossicles — malleus, incus, stapes — and connects to the nasopharynx via the pharyngotympanic (auditory) tube, which equalizes pressure and is shorter and more horizontal in children, predisposing them to otitis media.
- The inner ear contains the bony labyrinth (cochlea, vestibule, three semicircular canals) housing the membranous labyrinth and the organ of Corti (hearing) and vestibular receptors (balance).
The Scalp and Face
Quick Answer: The scalp is a five-layered structure (remember SCALP: Skin, Connective tissue, Aponeurosis, Loose connective tissue, Pericranium) whose deep loose layer allows infection and hemorrhage to spread widely. Muscles of facial expression are all innervated by CN VII (facial nerve), while CN V (trigeminal) provides the sensory supply in three divisions.
Scalp Layers
| Layer | Structure | Clinical Note |
|---|---|---|
| S — Skin | Thick, hair-bearing | Lacerations bleed briskly |
| C — Connective tissue | Dense, highly vascular | Hemorrhage limited by density |
| A — Aponeurosis (galea aponeurotica) | Links frontalis and occipitalis | Tension limits gapping |
| L — Loose connective tissue | "Dangerous layer" | Allows spread of infection/effusion |
| P — Pericranium (periosteum) | Adherent to bone | Subperiosteal fluid stops at suture lines |
The first three layers move as a unit because the dense connective tissue and galea are surgically inseparable. The loose subaponeurotic layer is the surgical "danger zone" — it permits fluid to accumulate and infection to pass into the skull via emissary veins.
Muscles of Facial Expression
The muscles of facial expression are derived from the second pharyngeal arch and innervated by CN VII (facial nerve). The principal groups:
- Orbital group — orbicularis oculi (closes eyelids; palpebral and orbital parts); corrugator supercilii.
- Nasal group — nasalis, procerus, depressor septi.
- Oral group — orbicularis oris (closes lips); buccinator (compresses cheek against teeth, keeps food out of vestibule); zygomaticus major/minor, levator labii superioris, depressor anguli oris, mentalis, risorius.
- Auricular group — auricularis anterior/superior/posterior (often vestigial).
- Scalp group — frontalis and occipitalis bellies linked by the galea aponeurotica.
Sensory Innervation of the Face
The trigeminal nerve (CN V) provides nearly all facial sensation through three divisions:
| Division | Foramen | Territory |
|---|---|---|
| V1 Ophthalmic | Superior orbital fissure | Forehead, upper eyelid, nose tip, cornea |
| V2 Maxillary | Foramen rotundum | Lower eyelid, cheek, lateral nose, upper lip, upper teeth |
| V3 Mandibular | Foramen ovale | Lower face, lower lip, chin, anterior tongue (general sensation), lower teeth, auricle anterior part |
A small area over the angle of the mandible is supplied by the great auricular nerve (C2–C3) from the cervical plexus, not by CN V.
The Ear: External, Middle, and Inner
The ear is divided into three parts, each with distinct embryologic and anatomic features.
External Ear
- Auricle (pinna) — elastic cartilage covered by skin; supplied by the auriculotemporal nerve (V3), great auricular nerve (C2–C3), lesser occipital nerve, and the auricular branch of the vagus (CN X). The external acoustic meatus has an outer cartilaginous portion and an inner bony portion (temporal bone) with an S-shape; the tympanic membrane closes its medial end.
- Tympanic membrane — obliquely oriented, ~1 cm diameter; the cone of light points anteroinferiorly. Outer surface supplied by V3 (auriculotemporal) and CN X (auricular branch); inner surface by CN VII (via the tympanic plexus and chorda tympani).
Middle Ear (Tympanic Cavity)
The middle ear lies within the temporal bone and contains the ossicular chain:
| Ossicle | Attachment | Function |
|---|---|---|
| Malleus ("hammer") | Tympanic membrane → incus | Vibrates with the eardrum |
| Incus ("anvil") | Malleus → stapes | Middle articulation |
| Stapes ("stirrup") | Footplate on oval window | Smallest bone; transmits to perilymph |
The pharyngotympanic (auditory) tube connects the middle ear to the nasopharynx, equalizing pressure and draining mucus. It is shorter, wider, and more horizontal in children, which explains the high incidence of otitis media in early life — nasopharyngeal pathogens ascend the tube easily.
The middle ear also houses the facial nerve (CN VII) in its bony canal (the facial canal) and the chorda tympani branch, which crosses the tympanic cavity to carry taste from the anterior two-thirds of the tongue.
Inner Ear
The bony labyrinth lies within the petrous part of the temporal bone and contains the membranous labyrinth suspended in perilymph. Its three parts:
- Cochlea — snail-shaped; contains the organ of Corti within the cochlear duct (scala media), seated between the scala vestibuli and scala tympani. The cochlear nerve (CN VIII cochlear division) transmits hearing.
- Vestibule — houses the utricle and saccule, which detect linear acceleration and gravity via maculae.
- Semicircular canals (anterior, posterior, lateral) — detect rotational acceleration via cristae ampullares; the vestibular nerve (CN VIII vestibular division) carries balance signals.
The inner ear is supplied by the labyrinthine artery, a branch of the anteroinferior cerebellar artery (AICA) — a clinically important relationship because AICA stroke can cause sudden unilateral hearing loss and vertigo.
Temporal Bone Landmarks
The temporal bone has four parts:
- Squamous — thin lateral plate forming part of the cranial vault; the zygomatic process contributes to the zygomatic arch.
- Tympanic — forms the walls of the external acoustic meatus.
- Mastoid — contains mastoid air cells that communicate with the middle ear (mastoiditis is a complication of otitis media).
- Petrous — dense bone housing the inner ear, facial canal, carotid canal, and the internal acoustic meatus.
Which nerve provides motor innervation to the buccinator muscle, and which nerve provides general sensation to the cheek skin overlying it?
Why are children more prone than adults to acute otitis media?