Introduction to Head & Neck Anatomy

Core

Introduction

The head and neck contains the brain, the special senses, the airway, the upper digestive tract, the major arteries to the brain, and a dense lymphatic drainage system. Almost every structure is in contact with several others, frequently sharing a nerve, an artery or a fascial plane. This article gives the conceptual map for the rest of the head and neck curriculum: the regions, the fascial framework, the great vessels, the cranial nerves, and the embryological origins that tie it all together.

It is not a substitute for the system-specific articles: The Skull, Cranial Nerves I-VI and Cranial Nerves VII-XII, Blood Supply of the Head and Neck, Fascial Planes, Muscles and Triangles, but the orientation it provides should make those articles much easier to follow.

Why Head and Neck Anatomy Matters

Three reasons explain the disproportionate weight head and neck anatomy carries in pre-clinical teaching:

• Density of structure. The neck contains the airway, the great vessels supplying the brain, the cranial nerves, the thyroid and parathyroid glands, the cervical spine and spinal cord, the pharynx and larynx, and the lymphatic chains that drain the entire upper body. A 2 cm lump can compress any of these.
• Cross-disciplinary clinical reach. Head and neck pathology presents to ENT, ophthalmology, neurology, neurosurgery, dental and oral and maxillofacial surgery, oncology, vascular surgery, plastic surgery, paediatrics and emergency medicine. Few other regions are touched by so many specialties.
• Examination-friendliness. Cranial nerve, neck and oral examinations feature in nearly every objective structured clinical examination (OSCE). They draw heavily on anatomy and rely on simple bedside equipment such as a torch, a tongue depressor, a tuning fork and a stethoscope.

Regions of the Head

The head is conventionally divided into:

• Cranial cavity: bounded by the skull, contains the brain and its meningeal coverings, the cerebrospinal fluid, and the proximal segments of the cranial nerves and dural venous sinuses. Detailed in The Skull and The Meninges.
• Face: the anterior visible region of the head bounded superiorly by the hairline and inferiorly by the chin. Contains the muscles of facial expression (innervated by the facial nerve, CN VII), the muscles of mastication (innervated by the mandibular branch of the trigeminal nerve, CN V₃), and the sensory territories of all three branches of the trigeminal nerve.
• Orbit and eye: cone-shaped bony cavity containing the eye, the extraocular muscles, the optic nerve and the orbital fat. See The Eye.
• Nose and paranasal sinuses: air-conducting passages with mucosa-lined cavities that lighten the skull and resonate the voice. See The Nose.
• Oral cavity: bounded by the lips, cheeks, hard and soft palates and the tongue. Contains the teeth and gingivae, the openings of the salivary ducts, and the upper part of the digestive tract. See The Oral Cavity and Pharynx.
• Ear: outer (auricle, external auditory meatus), middle (tympanic cavity, ossicles, Eustachian tube) and inner (cochlea, vestibular apparatus) compartments. See Anatomy of the Ear.
• Pharynx and larynx: the vertically-oriented muscular tubes through which air and food traverse from the head to the thorax. See The Larynx.

Regions of the Neck

The neck connects the head to the trunk and contains an exceptional concentration of vessels, nerves, viscera and lymphatic structures. The single most useful starting framework is the division of each side of the neck into anterior and posterior triangles by the sternocleidomastoid muscle.

The Anterior Triangle

The anterior triangle is bounded by:

• Anteriorly: the midline of the neck.
• Posteriorly: the anterior border of sternocleidomastoid.
• Superiorly: the inferior border of the mandible.

It is subdivided by the digastric muscle and the omohyoid into four smaller triangles:

• Submandibular (digastric) triangle: contains the submandibular gland, the facial artery and vein, the hypoglossal nerve, and the cervical branch of the facial nerve.
• Submental triangle: the midline triangle below the chin, containing submental lymph nodes; a common site of nodal enlargement from oral cavity infection or malignancy.
• Carotid triangle: contains the carotid sheath structures and the bifurcation of the common carotid artery; also the hypoglossal nerve and ansa cervicalis. Pulsation of the carotid is palpable here.
• Muscular triangle: contains the strap muscles (sternohyoid, sternothyroid, thyrohyoid, omohyoid), the thyroid and parathyroid glands.

The Posterior Triangle

The posterior triangle is bounded by:

• Anteriorly: the posterior border of sternocleidomastoid.
• Posteriorly: the anterior border of trapezius.
• Inferiorly: the middle third of the clavicle.

It is subdivided by the inferior belly of omohyoid into the larger occipital triangle (above) and the smaller subclavian (supraclavicular) triangle (below). The posterior triangle contains:

• The spinal accessory nerve (CN XI), running across the triangle to supply trapezius. It is superficial and at risk during lymph node biopsy; iatrogenic injury produces shoulder drop and weakness of shrugging.
• The brachial plexus, emerging between the scalene muscles: the source of the regional anatomy at the root of the neck.
• The third part of the subclavian artery.
• The cervical plexus branches (lesser occipital, great auricular, transverse cervical, supraclavicular nerves) emerging at the midpoint of the posterior border of sternocleidomastoid (Erb's point).

Diagram: the triangles of the neck. Sternocleidomastoid divides each side of the neck into anterior and posterior triangles. The anterior triangle is further subdivided by the digastric muscle (submandibular, submental, carotid, muscular sub-triangles); the posterior triangle is subdivided by the inferior belly of omohyoid (occipital and subclavian sub-triangles).

Fascial Planes

The neck is wrapped in concentric layers of fascia that compartmentalise the structures within and shape the routes by which infection spreads. Two broad layers are recognised:

Superficial fascia: subcutaneous tissue containing the platysma (a broad sheet of muscle of facial-expression type, innervated by CN VII), small cutaneous nerves and superficial veins (external jugular, anterior jugular).

Deep cervical fascia: in three layers, plus the carotid sheath:

• Investing layer: the most superficial layer, splitting to enclose the trapezius and sternocleidomastoid muscles and the submandibular and parotid glands. It forms a sleeve that wraps the entire neck.
• Pretracheal layer: encloses the thyroid and parathyroid glands, trachea and oesophagus anteriorly. It is continuous inferiorly with the fibrous pericardium; an important route by which infection from the neck can spread into the mediastinum.
• Prevertebral layer: envelops the cervical vertebrae and the muscles in front of them (the longus colli and longus capitis), and the floor of the posterior triangle muscles (the scalenes, levator scapulae, splenius capitis). This is the fascial floor of the posterior triangle.

Between the buccopharyngeal fascia covering the pharynx (anteriorly) and the alar layer of the prevertebral fascia (posteriorly) is the retropharyngeal space. It extends from the base of the skull down to approximately the upper thoracic vertebrae. Behind it, between the alar and prevertebral fasciae, is the ‘danger space’, which runs all the way to the diaphragm. Together these spaces provide a route by which deep neck infection can spread into the mediastinum. Detailed treatment is in Fascial Planes, Muscles and Triangles.

The Carotid Sheath

The carotid sheath is a tubular fascial condensation extending from the base of the skull to the root of the neck. It contains four structures:

• Common carotid artery (medially): bifurcating at approximately the level of the upper border of the thyroid cartilage (C4) into:
  • The internal carotid artery (ICA), which has no branches in the neck and ascends to enter the skull through the carotid canal.
  • The external carotid artery (ECA), which has eight branches (including the superior thyroid, lingual, facial, occipital, posterior auricular, ascending pharyngeal, maxillary and superficial temporal) supplying the soft tissues of the head and neck.
• Internal jugular vein (laterally): the major venous drainage of the head and neck.
• Vagus nerve (CN X): in the angle behind and between the artery and vein. Its recurrent laryngeal branch gives rise to all but one of the intrinsic laryngeal muscles; iatrogenic injury during thyroidectomy is the classic preventable cause of vocal-cord palsy.
• Deep cervical lymph nodes: running along the IJV; this chain is the final common drainage of head and neck lymph and is examined when assessing any neck lump.

The carotid sheath is also the focus of one of the most consequential vascular territories in medicine: atherosclerosis at the carotid bifurcation is a major cause of stroke, and the carotid sinus (a baroreceptor-rich dilation at the proximal ICA) and carotid body (a chemoreceptor lying nearby) are essential to cardiovascular and respiratory reflex control.

The Cranial Nerves at a Glance

There are twelve cranial nerves, conventionally numbered I-XII from rostral to caudal as they emerge from the brain. Their detailed anatomy and individual lesions are covered in Cranial Nerves I-VI and Cranial Nerves VII-XII; the orientation here is the function and exit foramen of each:

• I: Olfactory. Special sense of smell. Cribriform plate of the ethmoid bone.
• II: Optic. Special sense of vision. Optic canal.
• III: Oculomotor. Motor to most extraocular muscles, parasympathetic to the sphincter pupillae. Superior orbital fissure.
• IV: Trochlear. Motor to superior oblique. Superior orbital fissure.
• V: Trigeminal. Sensory to the face (in three divisions: V₁ ophthalmic, V₂ maxillary, V₃ mandibular) and motor to the muscles of mastication. V₁ exits through the superior orbital fissure, V₂ through the foramen rotundum, V₃ through the foramen ovale.
• VI: Abducens. Motor to lateral rectus. Superior orbital fissure.
• VII: Facial. Motor to the muscles of facial expression, parasympathetic to the lacrimal, submandibular and sublingual glands, and special sensory taste from the anterior two-thirds of the tongue. Internal acoustic meatus, then stylomastoid foramen.
• VIII: Vestibulocochlear. Special senses of hearing and balance. Internal acoustic meatus.
• IX: Glossopharyngeal. Sensory and taste from the posterior third of the tongue, motor to stylopharyngeus, parasympathetic to the parotid gland, sensory from the carotid sinus and body. Jugular foramen.
• X: Vagus. Motor and sensory to the pharynx and larynx, parasympathetic to thoracic and most abdominal viscera, sensory from the aortic arch baroreceptors and chemoreceptors. Jugular foramen.
• XI: Accessory. Motor to sternocleidomastoid and trapezius. Jugular foramen.
• XII: Hypoglossal. Motor to the muscles of the tongue (except palatoglossus). Hypoglossal canal.

A standard mnemonic for the modality of each nerve (Sensory, Motor or Both): "Some Say Marry Money But My Brother Says Big Brains Matter Most":

• I, II, VIII: Sensory.
• III, IV, VI, XI, XII: Motor.
• V, VII, IX, X: Both (the four mixed-modality nerves; all are pharyngeal-arch derivatives).

Pharyngeal Arch Derivatives

Many of the apparently disparate facts about cranial nerves and the structures they supply collapse into a much simpler pattern when read against the embryological pharyngeal arches. The arches are paired ridges of mesenchyme appearing in week 4 of embryonic development. Each contains a cartilage, an artery, a nerve and a muscle group, with corresponding adult derivatives:

• Arch 1 (mandibular): nerve: trigeminal V₃; muscles of mastication, mylohyoid, anterior belly of digastric, tensor tympani, tensor veli palatini; cartilage gives malleus and incus and Meckel's cartilage.
• Arch 2 (hyoid): nerve: facial VII; muscles of facial expression, stylohyoid, posterior belly of digastric, stapedius; cartilage gives stapes, styloid process, lesser horn of hyoid.
• Arch 3: nerve: glossopharyngeal IX; stylopharyngeus; cartilage gives the greater horn and lower body of the hyoid.
• Arch 4: nerve: superior laryngeal branch of vagus X; cricothyroid and the pharyngeal constrictors; contributes to the laryngeal cartilages.
• Arch 6: nerve: recurrent laryngeal branch of vagus X; the intrinsic laryngeal muscles (except cricothyroid); contributes to the laryngeal cartilages. (The exact assignment of individual laryngeal cartilages to the fourth versus sixth arch varies between sources and is best learned at the level of ‘arches 4 and 6 together give rise to the laryngeal cartilages other than the epiglottis’.)

(Arch 5 regresses in humans without significant adult derivatives.)

Two patterns to lock in:

• The nerve of the arch supplies the branchial-motor muscles derived from it. This is why the muscles of mastication, the mylohyoid and the tensor tympani are all innervated by the mandibular branch of the trigeminal nerve (CN V₃): they share an embryological home in the first arch. (Note that CN V also carries general sensation from the face through V₁ and V₂; only its branchial motor component is arch-derived.)
• The recurrent laryngeal nerve takes asymmetrical courses because the sixth-arch arteries have different fates. On the left, the distal sixth arch persists as the ductus arteriosus (the future ligamentum arteriosum), tethering the left recurrent laryngeal nerve under the arch of the aorta; the aorta itself is a fourth-arch derivative. On the right, the distal sixth arch regresses, allowing the right recurrent laryngeal nerve to migrate up and loop under the right subclavian artery.

Pharyngeal arch development is covered in detail in Development of the Head and Neck.

Lymphatic Drainage Overview

Almost every structure in the head and neck drains, sooner or later, into the deep cervical chain running along the internal jugular vein within the carotid sheath. Two points are worth knowing now:

• Persistent cervical lymphadenopathy in an adult is clinically important because malignancy; head and neck cancers, lymphomas and metastases from the chest, breast or abdomen; is part of the differential diagnosis.
• The levels of cervical nodes (I-VI) are a standard surgical and radiological framework for describing nodal disease and planning neck dissection. They are introduced in Lymph Nodes and Neck Lumps.

The deep cervical chain ultimately drains into the jugular trunk, then either the right lymphatic duct (right side) or the thoracic duct (left side), and into the venous system at the junction of the internal jugular and subclavian veins.

Imaging the Head and Neck

Modern head and neck assessment leans heavily on imaging because of the inaccessibility of so many of the structures. The pre-clinical principles are:

• Plain radiographs: mostly of historical interest, occasionally still used for cervical-spine trauma and dental imaging.
• Ultrasound: first-line for thyroid nodules, salivary glands, neck lumps and lymph nodes; safe, cheap, and good at distinguishing solid from cystic structures.
• Computed tomography (CT): the workhorse for trauma, sinus disease, intracranial pathology requiring fast assessment, and skull-base bony anatomy. Contrast-enhanced CT is the standard for cancer staging.
• Magnetic resonance imaging (MRI): the gold standard for soft tissues (brain, cranial nerves, oropharynx, tongue base) and for staging head and neck cancers where soft-tissue detail is critical.
• PET-CT: used in head and neck oncology to detect occult metastatic disease and to monitor treatment response.

The principles are detailed in Imaging of the Head and Neck.

Summary

• The head and neck is the most anatomically dense region of the body and the substrate for at least eight clinical specialties. Approach it along three axes: region, system and embryology.
• The neck is divided by sternocleidomastoid into anterior and posterior triangles; each is subdivided. The carotid sheath runs through the anterior triangle and contains the common/internal carotid artery, internal jugular vein, vagus nerve and deep cervical nodes.
• The deep cervical fascia has three layers (investing, pretracheal, prevertebral), with the retropharyngeal space a clinically important continuation between neck and posterior mediastinum.
• The twelve cranial nerves are best learned with their function (sensory, motor, both) and their exit foramen. The four mixed-modality nerves. V, VII, IX, X; are pharyngeal-arch derivatives.
• The pharyngeal arches explain why apparently unrelated muscles share a nerve supply, and why the recurrent laryngeal nerves take asymmetrical routes.
• Almost everything drains to the deep cervical lymph chain; a persistent neck lump in an adult requires investigation.

Reviewed by: Dr. Marcus Judge

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