Triangles and Compartments of the Neck

• The neck is divided into three compartments: posterior, anterior/median visceral, and posterior triangles.

Posterior Compartment

• Specialized for support.
• Contains the vertebral column and musculature for flexion/extension of the neck.

Anterior (Median Visceral) Compartment

• Divided into two anterior triangles superficially
• Contains the larynx/trachea, pharynx, and a vertical neurovascular bundle.

Posterior Triangles

• Two areas continuous with the axilla via the cervicoaxillary canal.
• The brachial plexus and subclavian artery/vein are the main components.

Triangles of the Neck

• The sternocleidomastoid muscle (SCM) is the key landmark in the neck.
• The SCM divides the neck into anterior and posterior triangles on either side of the midline.
• The SCM is innervated by CN XI (spinal accessory).
• The SCM attaches superiorly to the mastoid process.
• Inferiorly attaches to the sternum (sternal head - round) and the clavicle (clavicular head - flat).
• Bilateral contraction tilts the head toward the shoulder of the same side, rotating the face to the opposite side, and protracts the head.

Cervical Fasciae

• Four varying layers of deep cervical fascia exist in the neck: investing, pretracheal, prevertebral, and carotid sheath.
• The investing layer surrounds the entire neck and roofs the posterior and anterior triangles.
• The pretracheal layer forms a sheath around the thyroid gland.
• The prevertebral layer surrounds the support compartment of the neck (vertebra and musculature) and forms the fascial carpet over the floor of the posterior triangle.
• The carotid sheath condenses all three layers to enclose the vertical neurovascular bundle (common carotid artery, internal jugular vein, and vagus nerve).

Extent of Fasciae Inferiorly

• The "danger space" is between the pretracheal fascia (buccopharyngeal fascia in the neck region) and the prevertebral fascia and is composed of the retropharyngeal and prevertebral spaces.
• The retropharyngeal space extends from the base of the skull to C7-T2.
• The prevertebral space extends from the base of the skull to the diaphragm.
• These spaces communicate with the posterior mediastinum of the thorax, providing a potential route for infection spread.

Superficial Venous Drainage

• Includes the superficial temporal, maxillary, retromandibular, posterior auricular, communicating, facial, external jugular, anterior jugular, jugular arch, and internal jugular veins.

Cervical Plexus

• The cervical plexus (C1-4) provides sensory innervation to the neck and upper thorax via the lesser occipital, transverse cutaneous nerve of the neck, great auricular, and supraclavicular nerves
• A cervical block can be administered by injecting anesthetic subcutaneously at the nerve point of the neck, approximately halfway up the posterior border of the sternocleidomastoid.

Cutaneous (Sensory) Innervation

• Includes the posterior (dorsal) ramus of C2 (greater occipital), posterior (dorsal) rami C3-5, lesser occipital (C2), great auricular (C2/3), transverse cervical (C2/3), and supraclavicular nerves (C3/4).

Posterior Triangle (Floor)

• The floor is formed by the semispinalis capitis, splenius capitis, levator scapulae (with CN XI crossing anteriorly), scalenus medius, and scalenus anterior (with the phrenic nerve crossing anteriorly).
• The brachial plexus is located in the floor between the scalenus anterior and medius, entering the axilla via the cervicoaxillary canal.
• The scalene muscles arise from the transverse processes of the cervical vertebrae and insert on the R1 (anterior and medius) and R2 (posterior), elevating them on contraction.
• The scalenus posterior is behind the scalenus medius, or may be absent or fused with it.

Anterior Triangle and Subdivisions

• Boundaries: inferior border of the mandible, anterior border of the sternocleidomastoid, and midline of the neck.

Digastric Triangle

• Submandibular
• Contains the submandibular gland, facial artery, lymph nodes, and mylohyoid muscle (floor).

Submental Triangle

• Contains submental lymph nodes and mylohyoid (floor).

Carotid Triangle

• Contains the common carotid artery, internal jugular vein, vagus nerve (CN X), and hypoglossal nerve (CN XII).

Muscular Triangle

• The infrahyoids form the floor
• Contains the thyroid, larynx, trachea, and esophagus beneath the floor.

Suprahyoid and Infrahyoid Muscles

• Four pairs of muscles attach or relate to the superior aspect of the hyoid bone

Suprahyoid Muscles

• Digastric muscle (anterior belly by nerve to mylohyoid - V3, posterior belly by CN VII - facial nerve)
• Stylohyoid (CN VII)
• Mylohyoid (with a midline raphe, by nerve to mylohyoid - V3)
• Geniohyoid (deep to the mylohyoid in the floor of the mouth - C1 via CN XII)

Infrahyoid Muscles

• Superficial are sternohyoid (ansa) and omohyoid (ansa)
• Deep are sternothyroid (ansa) and thyrohyoid (C1 via XII)
• Infrahyoid muscles depress and stabilize the hyoid bone.
• Suprahyoid muscles raise the hyoid bone, pulling the larynx up.

Cervical Plexus - Motor and Sensory Derivations

• The anterior (ventral) rami of C1-4 in this plexus provide sensory innervation to the skin of the neck and motor innervation to all infrahyoid muscles, as well as one suprahyoid muscle.

Carotid Triangle & the Vertical Neurovascular Bundle

• Components of the vertical neurovascular bundle: common carotid artery, internal jugular vein, and vagus nerve (CN X) are all ensheathed in the carotid sheath.
• The common carotid artery terminates by bifurcating into the internal and external carotid arteries.
• The internal carotid artery has an initial swelling, the carotid sinus (a baroreceptor). The internal carotid has no branches until emerging in the cranial cavity.
• The carotid body is a chemoreceptor responding to CO² levels, innervated by the pharyngeal branch of the vagus nerve (CN X).

Branches of the External Carotid Artery

• Includes: superior thyroid artery, ascending pharyngeal artery, lingual artery, facial artery, occipital artery, and posterior auricular artery.
• The external carotid ends in the parotid bed, bifurcating into the superficial temporal and maxillary arteries.
• The internal jugular vein drains the brain, face, and neck.
• The vagus nerve (CN X) is motor and sensory and provides parasympathetic supply. Major branches off the vagus in the neck include the superior laryngeal, and right recurrent laryngeal nerves concerned with larynx innervation.
• Other major nerves are the spinal accessory nerve (CN XI), hypoglossal nerve (CN XII), and the sympathetic trunk.
• The pharyngeal plexus innervates the pharynx, formed by branches of the vagus nerve, glossopharyngeal nerve (CN IX), and postganglionic sympathetic fibers. Motor fibers are from the vagus nerve (CN X), and sensory fibers from the glossopharyngeal nerve (CN IX).

Carotid Triangle Clinical Notes

• Used in surgical approaches to the carotid arterial system and internal jugular vein, CN X, XII, and the cervical sympathetic trunk.
• The carotid sinus responds to blood pressure increases, signaling the heart to slow down via the vagus nerve.
• Pressure on the sinus can cause fainting.
• A carotid pulse can be detected in the external carotid artery.
• The carotid body (chemoreceptor) responds to changes in blood composition, supplied by CN IX, X, and sympathetic fibers. The carotid body responds to an increase in CO2 or a decrease in O₂.

Muscular Triangle

• Used in surgical approaches to the thyroid and parathyroid glands, as well as the esophagus and trachea. All lymphatic vessels from the head and neck drain into the deep cervical lymph nodes that radical resections require their removal and external and internal jugular vv when necessary.