By Dr. Marcus Judge

Next Lesson - Blood Supply of the Head and Neck

Head and Neck

Contents

  1. Introduction
  2. Why Head and Neck Anatomy Matters
  3. Regions of the Head
  4. Regions of the Neck
  5. Fascial Planes
  6. The Carotid Sheath
  7. The Cranial Nerves at a Glance
  8. Pharyngeal Arch Derivatives
  9. Lymphatic Drainage Overview
  10. Imaging the Head and Neck
  11. Summary
  12. Quiz

Abstract

  • The head and neck is one of the most anatomically dense regions of the body. Almost every clinical specialty relies heavily on its detail. These include ear, nose and throat (ENT) surgery, ophthalmology (eye medicine), neurology (medicine of the brain and nerves), neurosurgery (surgery of the brain and nerves), oncology (cancer care), dentistry, oral and maxillofacial (face and jaw) surgery, anaesthesia and emergency medicine.
  • Head and neck content is best learned in three independent ways: by region (cranial cavity, face, oral cavity, neck), by system (vascular, nervous, lymphatic and fascial, where fascia means the sheets of connective tissue that wrap the structures) and by embryology (how the body develops before birth). Embryology explains why structures in different regions share the same nerve supply, because they come from the same pharyngeal arches (ridges of tissue in the embryo that form the face, jaw and neck).
  • The neck is conventionally divided by the sternocleidomastoid (SCM, the large muscle that runs from behind the ear to the breastbone and collarbone) into anterior and posterior triangles, each subdivided further. The carotid sheath (a tube of fascia) encloses the common carotid artery (and, above its division, the internal carotid artery), the internal jugular vein, the vagus nerve and the accompanying deep cervical (neck) lymph nodes. It runs the full length of the neck within the deep cervical fascia (the layers of connective tissue in the neck).
  • The twelve cranial nerves (CN I to XII) are numbered from rostral (towards the head end) to caudal (towards the tail end). CN I and II come from the forebrain and the rest from the brainstem. Each leaves the skull through a foramen (plural foramina), a hole in the bone. A nerve can be sensory, motor or both (mixed), and some also carry autonomic (involuntary) or special sense (such as smell, sight and hearing) fibres. The four mixed nerves (V, VII, IX and X) are also the nerves of the pharyngeal arches.

Core

Introduction

The head and neck contains the brain, the special senses (sight, hearing, balance, smell and taste), 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 (a layer of connective tissue between structures). This article is a conceptual map for the rest of the head and neck curriculum. It covers the regions, the fascial framework, the great vessels (the large arteries and veins), the cranial nerves and the embryology (how the structures develop before birth) that ties it all together.

It is not a substitute for the detailed articles on each system. These are The Skull; Cranial Nerves I-VI; Cranial Nerves VII-XII; Blood Supply of the Head and Neck; and Fascial Planes, Muscles and Triangles. 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 (the large arteries and veins) supplying the brain, the cranial nerves, the thyroid and parathyroid glands (small glands behind the thyroid that help to control blood calcium), the cervical spine (the neck part of the spine) and spinal cord, the pharynx and larynx, and the lymphatic chains (lines of lymph nodes) that drain the head and neck. A 2 cm lump can compress any of these.
  • Cross-disciplinary clinical reach. Head and neck pathology presents to ear, nose and throat (ENT) surgery, ophthalmology (eye medicine), neurology (medicine of the brain and nerves), neurosurgery, dental and oral and maxillofacial (face and jaw) surgery, oncology (cancer care), 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 can all appear in objective structured clinical examinations (OSCEs). These examinations 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. It contains the brain and its coverings (the meninges), the cerebrospinal fluid, the dural venous sinuses (large veins that drain the brain, which run within the dura mater, the tough outer layer of the meninges) and the first parts of the cranial nerves. 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, which are supplied by the facial nerve (CN VII). Also contains the muscles of mastication (the chewing muscles), which are supplied by the mandibular branch of the trigeminal nerve (CN V3). The skin of the face is supplied by the sensory branches of the trigeminal nerve (CN V, the main sensory nerve of the face), through its three divisions.
  • Orbit and eye: the orbit (eye socket) is a pyramid-shaped bony cavity containing the eye, the extraocular muscles (the muscles that move the eye), 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 gums (gingivae), the openings of the salivary ducts, and the upper part of the digestive tract. See The Oral Cavity and Pharynx.
  • Ear: outer, middle and inner compartments. The outer ear has the auricle (the visible part of the ear) and the external auditory meatus (the ear canal). The middle ear has the tympanic cavity (the air-filled space behind the eardrum), the ossicles (three tiny bones) and the Eustachian tube (also called the auditory or pharyngotympanic tube, which connects the middle ear to the back of the nose and throat). The inner ear has the cochlea (for hearing) and the vestibular apparatus (for balance). See Anatomy of the Ear.
  • Pharynx and larynx: the pharynx (throat) is a muscular tube that connects the nose and mouth to the larynx and oesophagus, so both air and food pass through it. The larynx (voice box) is mainly made of cartilage. It opens at the top into the lower part of the pharynx and is continuous below with the trachea (windpipe). See The Larynx.

Regions of the Neck

The neck connects the head to the trunk and contains an exceptional concentration of vessels, nerves, viscera (internal organs) 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 large muscle that runs from behind the ear to the breastbone and collarbone).

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.

The anterior triangle is subdivided by the digastric muscle (two muscle bellies joined by a tendon that is tied to the hyoid bone, the small U-shaped bone above the larynx) and the omohyoid muscle (a thin muscle that runs from the hyoid bone down towards the shoulder blade) into four smaller triangles:

  • Submandibular (digastric) triangle: contains the submandibular gland, the facial artery and vein, the hypoglossal nerve (CN XII, the nerve to the muscles of the tongue), 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 (division into two) of the common carotid artery; also the hypoglossal nerve and ansa cervicalis (a loop of nerve fibres from the cervical plexus that supplies the strap muscles below the hyoid bone). Pulsation of the carotid is palpable here.
  • Muscular triangle: contains the strap muscles (the thin muscles below the hyoid bone: sternohyoid, sternothyroid, thyrohyoid and omohyoid) and 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.

The posterior triangle is subdivided by the inferior belly of omohyoid into the larger occipital triangle (above) and the smaller subclavian (supraclavicular) triangle (below). It 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 (injury caused by medical treatment) produces shoulder drop and weakness of shrugging.
  • The brachial plexus (the network of nerves that supplies the arm). Its trunks emerge between the anterior and middle scalene muscles (muscles on the side of the neck that run from the cervical vertebrae to the first two ribs) at the root of the neck, where the neck meets the chest, and cross the floor of the triangle.
  • 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).

The Triangles of the Neck (lateral view, right side) cranium mastoid mandible chin sternoclavicular joint acromion clavicle trapezius (ant. border) SCM digastric omohyoid (inf. belly) omohyoid (sup. belly) Anterior triangle submandibular | submental carotid | muscular Posterior triangle occipital (above omohyoid) subclavian (below) carotid bifurcation internal jugular vein CN XI (accessory) crosses posterior triangle; at risk in node biopsy brachial plexus Boundaries SCM (1° divider) digastric omohyoid Contents artery vein cranial nerve plexus roots SimpleMed Original by Dr Marcus Judge

Diagram: the triangles of the neck. Sternocleidomastoid (SCM) divides each side of the neck into anterior and posterior triangles. The anterior triangle is further subdivided by the digastric muscle and the superior belly of omohyoid (submandibular, submental, carotid and 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 (sheets of connective tissue) that divide the structures within into compartments and shape the routes by which infection spreads. Two broad layers are recognised:

Superficial fascia: subcutaneous tissue containing the platysma (a broad, thin sheet of muscle, of the same type as the muscles of facial expression, supplied by the facial nerve, 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 (salivary) 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 (the tough outer layer around the heart). This continuity is an important route by which infection from the neck can spread into the mediastinum (the central compartment of the chest, between the lungs).
  • Prevertebral layer: surrounds the cervical vertebrae (the neck bones of the spine) and the muscles in front of them (the longus colli and longus capitis, two deep muscles lying against the front of the spine). It also covers the muscles that form the floor of the posterior triangle (the scalenes, levator scapulae and splenius capitis, which are deep muscles of the neck). Its anterolateral part (the front and side) forms the fascial floor of the posterior triangle.

The retropharyngeal space lies behind the pharynx. It sits between the buccopharyngeal fascia (the fascia covering the back of the pharynx) in front and the alar layer (a thin layer of the prevertebral fascia) behind. 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 tube of thickened fascia (a fascial condensation) that extends from the base of the skull to the root of the neck, where the neck meets the chest. It contains four structures: the carotid artery (the common carotid artery and, above its division, the internal carotid artery), the internal jugular vein, the vagus nerve and the deep cervical lymph nodes. They are described below:

  • Common carotid artery (medially): bifurcating (dividing in two) at about the level of the fourth cervical vertebra (C4). This is also the level of the upper border of the thyroid cartilage (the large cartilage of the larynx that forms the Adam's apple). The two divisions are:
    • 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) is the other division, but it is not usually counted as a content of the sheath. It has eight branches (the superior thyroid, lingual, facial, occipital, posterior auricular, ascending pharyngeal, maxillary and superficial temporal), which supply 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 supplies the intrinsic laryngeal muscles (the muscles within the larynx, or voice box) except cricothyroid (a muscle that tenses the vocal cords), which is supplied by the external branch of the superior laryngeal nerve. The recurrent laryngeal nerve branches from the vagus in the chest, not in the neck, and runs back up the neck in the groove between the trachea and oesophagus (the tracheoesophageal groove), so it is not itself a content of the sheath. Injury to it during thyroidectomy (removal of the thyroid gland) is a classic cause of vocal cord palsy (weakness or paralysis of the vocal cords).
  • Deep cervical lymph nodes: running along the internal jugular vein; this chain is the final common drainage of head and neck lymph and is examined when assessing any neck lump.

The region of the carotid sheath matters clinically for two reasons. First, atherosclerosis (a build-up of fatty plaque in the artery wall) at the carotid bifurcation is a cause of stroke. Significant stenosis (narrowing) of the internal carotid artery is a major contributor to ischaemic stroke, and it can also cause transient ischaemic attacks (TIAs, brief episodes of stroke symptoms that settle completely). Second, the carotid sinus (a dilation at the start of the ICA that contains baroreceptors, which sense blood pressure) and the carotid body (a chemoreceptor lying nearby, which senses blood oxygen and carbon dioxide levels) 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 (towards the head end) to caudal as they emerge from the brain. Each leaves the skull through a foramen (plural foramina), a hole in the bone. Knowing which foramen a nerve uses helps to predict which nerves a skull-base fracture or tumour can damage. Their detailed anatomy and individual lesions (areas of damage) 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 (a sieve-like plate of bone in the roof of the nose).
  • II: Optic. Special sense of vision. Optic canal.
  • III: Oculomotor. Motor to most extraocular muscles, parasympathetic to the sphincter pupillae (the muscle that narrows the pupil). Superior orbital fissure (a slit between the roof and the outer wall of the orbit, the eye socket).
  • IV: Trochlear. Motor to superior oblique (one of the muscles that move the eye). 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 (all three are openings in the sphenoid bone at the base of the skull).
  • VI: Abducens. Motor to lateral rectus (the eye muscle that turns the eye outwards). Superior orbital fissure.
  • VII: Facial. Motor to the muscles of facial expression, parasympathetic to the lacrimal (tear), submandibular and sublingual (salivary) 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 (a muscle that lifts the pharynx), 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; each is also the nerve of one or more pharyngeal arches, as explained below).

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 (embryonic connective tissue) appearing in weeks 4 and 5 of embryonic development. Each contains a cartilage, an artery, a nerve and a muscle group, with corresponding adult derivatives:

  • Arch 1 (mandibular). Nerve: trigeminal (CN V), whose mandibular division (V3) supplies the arch muscles. Muscles: the muscles of mastication (the chewing muscles), mylohyoid, anterior belly of digastric, tensor tympani (a small middle ear muscle) and tensor veli palatini (a muscle of the soft palate). Cartilage: gives the malleus and incus (two of the middle ear bones) and Meckel's cartilage (a cartilage rod in the developing lower jaw).
  • Arch 2 (hyoid). Nerve: facial (CN VII). Muscles: the muscles of facial expression, stylohyoid, posterior belly of digastric and stapedius (a small middle ear muscle). Cartilage: gives the stapes (the third middle ear bone), the styloid process (a thin spike of bone below the ear) and the lesser horn of the hyoid (a small U-shaped bone above the larynx).
  • Arch 3. Nerve: glossopharyngeal (CN IX). Muscle: stylopharyngeus. Cartilage: gives the greater horn and lower body of the hyoid.
  • Arch 4. Nerve: superior laryngeal branch of the vagus (CN X). Muscles: cricothyroid and the pharyngeal constrictors (the muscles that squeeze food down the pharynx). Cartilage: contributes to the laryngeal cartilages.
  • Arch 6. Nerve: recurrent laryngeal branch of the vagus (CN X). Muscles: the intrinsic laryngeal muscles (except cricothyroid). Cartilage: contributes to the laryngeal cartilages. (Sources differ on which laryngeal cartilage comes from the fourth arch and which from the sixth. It is enough to learn that arches 4 and 6 together give rise to the laryngeal cartilages.)

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

Two patterns to lock in:

  • The nerve of the arch supplies the muscles derived from it (its branchial motor fibres; ‘branchial’ is an older word for pharyngeal). This is why the muscles of mastication, the mylohyoid and the tensor tympani are all supplied by the mandibular branch of the trigeminal nerve (CN V3): they share an embryological home in the first arch. (CN V also carries general sensation from the face through all three divisions, V1, V2 and V3. Only V3 carries the branchial motor fibres to the first-arch muscles.)
  • 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 (a fetal vessel that links the pulmonary artery to the aorta; after birth it closes and becomes the ligamentum arteriosum, a fibrous cord). The ligamentum arteriosum that remains tethers 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 (enlarged lymph nodes in the neck) in an adult is clinically important because cancer is part of the differential diagnosis (the list of possible causes). The cancers to consider include head and neck cancers, lymphomas (cancers of the lymph nodes) and metastases (spread of cancer) from the chest, breast or abdomen.
  • The levels of cervical nodes (I-VI) are a standard surgical and radiological framework for describing nodal disease and planning neck dissection (surgery to remove lymph nodes from the neck). The levels are introduced in Lymph Nodes and Neck Lumps.

The deep cervical chain ultimately drains into the jugular trunk (a collecting lymph vessel on each side of the neck). On the right, the trunk joins the right lymphatic duct. On the left, it joins the thoracic duct (the large lymph vessel that carries lymph from most of the body). Both ducts usually empty 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 (X-rays): for a suspected neck (cervical spine) injury, NICE guideline NG41 sets out which scan to use. In adults it recommends CT when the Canadian C-spine rule (a clinical decision rule that identifies who needs neck imaging) indicates that imaging is needed. In children (under 16) it recommends MRI when a cervical spinal cord injury is strongly suspected, and says to consider plain X-rays only if the criteria for MRI are not met but suspicion remains after repeated clinical assessment. X-rays are also used to check the teeth, and dental X-rays are usually done in a dental practice.
  • Ultrasound: the initial test recommended by NICE (guideline NG230) when thyroid nodules are being investigated for cancer. It can also be used to look at salivary glands, neck lumps and lymph nodes. It is safe, cheap, and good at distinguishing solid from cystic (fluid-filled) structures.
  • Computed tomography (CT): the workhorse for trauma, sinus disease, intracranial pathology (disease inside the skull) requiring fast assessment, and skull-base bony anatomy. CT is widely used in cancer staging, often with contrast (a dye injected into a vein that makes blood vessels and tumours stand out). For example, NICE recommends conventional imaging such as chest CT when a person with head and neck cancer needs staging for spread elsewhere in the body and PET-CT (described below) is not indicated.
  • Magnetic resonance imaging (MRI): particularly useful for detailed images of soft tissue in the brain, cranial nerves, oropharynx (the part of the throat behind the mouth) and tongue base, and for showing how far a tumour extends. MRI is not the first-line investigation for every head and neck problem. The choice of scan depends on the question being asked, for example whether bone detail (CT) or soft-tissue detail (MRI) matters more.
  • PET-CT (positron emission tomography combined with CT, using a radioactive sugar tracer to show active tissue such as tumour). In head and neck oncology (cancer care), NICE recommends it in selected patients: to find the primary tumour when cancer has spread to neck lymph nodes from an unknown site, to stage some advanced cancers, and to assess response 3 to 6 months after chemoradiotherapy (radiotherapy given with chemotherapy).

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

Summary

  • The head and neck is one of the most anatomically dense regions of the body, and many clinical specialties rely on its detail. 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 the full length of the neck, passing through the carotid triangle of the anterior triangle, and contains the common and internal carotid arteries, internal jugular vein, vagus nerve and deep cervical lymph nodes.
  • The deep cervical fascia has three layers (investing, pretracheal, prevertebral). The retropharyngeal space behind the pharynx is a clinically important route between the neck and the posterior mediastinum (the back part of the central compartment of the chest).
  • 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 and X) are also the nerves of the pharyngeal arches.
  • 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

Quiz

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