Chapter 5: Situs Faciales et Colli: Face & Neck

This chapter covers the soft-tissue anatomy of the face and neck: the scalp, the muscles of facial expression and mastication, the nerves and vessels of the face, the fascial compartments and triangles of the neck, the neck viscera, and lymphatic drainage.

The scalp

The scalp has five layers. Their initials spell SCALP:

The first three layers are firmly bound together and move as one. A cut that gapes has gone through the galea.

The face

The face (regio facialis) extends from the arcus superciliaris above to the lower border of the mandible below, and back to the ears on each side. From outside in, its layers are:

• skin;
• dense subcutaneous tissue;
• the muscles of facial expression (musculi faciales);
• deep fascia;
• the muscles of mastication.

Muscles of facial expression

The facial muscles lie in the subcutaneous tissue and insert into the skin. When they contract, they move the skin, creating expressions. They develop from the second pharyngeal arch, and all are supplied by the facial nerve (CN VII). They are grouped by the opening they surround:

Many of the oral muscles meet at the modiolus, a fibromuscular knot just lateral to the corner of the mouth.

Muscles of mastication

The muscles of mastication move the mandible at the temporomandibular joint. They develop from the first pharyngeal arch and are supplied by the mandibular nerve (V3).

Three muscles close the jaw (temporalis, masseter, medial pterygoid). Only one, the lateral pterygoid, actively opens it, helped by gravity and the suprahyoid muscles.

Nerves and vessels of the face

Arteries

The face is supplied mainly by the external carotid artery. A small area around the eye and forehead is supplied by the internal carotid artery, through the ophthalmic artery (supraorbital and supratrochlear branches).

Branches of the external carotid artery, from below upward:

1. A. thyroidea superior
2. A. pharyngea ascendens
3. A. lingualis
4. A. facialis, which curves over the lower border of the mandible just in front of masseter (where its pulse can be felt), then runs tortuously toward the medial angle of the eye. It gives off the aa. labiales superior and inferior and ends as the a. angularis.
5. A. occipitalis
6. A. auricularis posterior
7. Its two terminal branches, inside the parotid gland:
   • a. temporalis superficialis, whose pulse can be felt in front of the ear. It gives off the a. transversa faciei.
   • a. maxillaris, which supplies the deep face, jaws and teeth, and gives off the a. meningea media.

A mnemonic for the eight branches: "Some Anatomists Like Freaking Out Poor Medical Students" (superior thyroid, ascending pharyngeal, lingual, facial, occipital, posterior auricular, maxillary, superficial temporal).

Veins

The v. facialis begins at the medial angle of the eye as the v. angularis and runs down and back behind the facial artery. It joins the v. retromandibularis and drains into the v. jugularis interna, which drains most of the face and head.

The facial vein has no effective valves. Through the ophthalmic veins it connects with the cavernous sinus, and through the plexus pterygoideus it connects with deep veins. Infection of the skin around the nose and upper lip can therefore spread to the cavernous sinus. This region is called the danger triangle of the face.

Nerves

The face has two separate nerve supplies:

• Sensory: trigeminal nerve (CN V), in three divisions:

  • V1, n. ophthalmicus: the forehead, upper eyelid and bridge of the nose (it reaches the skin through the supraorbital foramen and supratrochlear branches);
  • V2, n. maxillaris: the lower eyelid, cheek, side of the nose and upper lip (infraorbital foramen);
  • V3, n. mandibularis: the lower lip, chin, lower jaw and the skin over the temple (mental foramen). V3 is also motor to the muscles of mastication.

  The angle of the mandible is supplied not by V but by the great auricular nerve (from the cervical plexus, C2–C3). The back of the scalp is supplied by dorsal rami of cervical nerves. This matters clinically: loss of sensation over the angle of the jaw points away from a trigeminal lesion.

• Motor: facial nerve (CN VII). It leaves the skull through the foramen stylomastoideum and enters the parotid gland, where it divides into five terminal branches that fan out to the facial muscles: Temporal, Zygomatic, Buccal, Marginal mandibular, Cervical. (Mnemonic: "To Zanzibar By Motor Car.") It passes through the parotid gland but does not supply it.

Lymphatic drainage of the face

Lymph from the face drains to a ring of superficial nodes at the junction of head and neck (from front to back: submental, submandibular, parotid and preauricular, retroauricular, occipital), and from there to the deep cervical nodes.

The neck (regio colli)

Layers and fascia

Deep to the skin, the neck has two layers of fascia:

• Fascia cervicalis superficialis: subcutaneous tissue containing the platysma, cutaneous nerves and superficial veins.
• Fascia cervicalis profunda: the deep cervical fascia, in several layers that divide the neck into compartments:
  • investing layer: surrounds the whole neck, enclosing sternocleidomastoid and trapezius;
  • pretracheal layer: encloses the thyroid, trachea and oesophagus;
  • prevertebral layer: surrounds the vertebral column and its muscles;
  • carotid sheath: a tube containing the common and internal carotid arteries, the internal jugular vein and the vagus nerve.

Compartments of the neck

The fascial layers create four compartments:

Os hyoideum

The hyoid bone is a U-shaped bone in the anterior midline, between the mandible and the thyroid cartilage, at the level of C3.

• It has a corpus (body), two cornua majora (greater horns) and two cornua minora (lesser horns). The lig. stylohyoideum runs from the lesser horn to the styloid process.
• It is the only bone in the body that does not articulate with another bone. It is held in place by muscles and ligaments.
• It anchors the floor of the mouth in front, the pharynx behind and the larynx below. Muscles attached above it are suprahyoid and muscles below it are infrahyoid.

Cervical triangles

M. sternocleidomastoideus (SCM) runs obliquely from the sternum and clavicle to the mastoid process. It divides each side of the neck into an anterior and a posterior triangle.

Trigonum cervicale anterius

The anterior triangle is bounded by the midline of the neck, the anterior border of SCM and the lower border of the mandible. It is subdivided into four smaller triangles:

The suprahyoid muscles form the floor of the mouth and raise the hyoid in swallowing:

• m. digastricus: its anterior belly (supplied by the nerve to mylohyoid, from V3) and posterior belly (supplied by CN VII) are joined by a tendon slung to the hyoid;
• m. stylohyoideus (CN VII);
• m. mylohyoideus (nerve to mylohyoid, from V3), which forms the muscular floor of the mouth;
• m. geniohyoideus (C1, carried by CN XII).

The infrahyoid (strap) muscles lower the hyoid and larynx:

• m. sternohyoideus;
• m. omohyoideus (superior and inferior bellies joined by a tendon);
• m. sternothyroideus;
• m. thyrohyoideus.

All four are supplied by the ansa cervicalis (C1–C3), except thyrohyoid (C1 via CN XII).

Trigonum cervicale posterius

The posterior triangle is bounded by the posterior border of SCM, the anterior border of trapezius and the middle third of the clavicle. Its floor is formed by splenius capitis, levator scapulae and the scalene muscles. The inferior belly of omohyoid divides it into:

• the trigonum occipitale (above), which contains the accessory nerve (CN XI) crossing to trapezius and the cutaneous branches of the cervical plexus;
• the trigonum omoclaviculare (subclavian triangle, below), which contains the subclavian artery, the trunks of the brachial plexus and the external jugular vein.

Nerves and vessels of the neck

• Arteries: the anterior triangle contains the common carotid artery and the early branches of the external carotid. The posterior triangle contains the subclavian artery and its branches.
• Veins:
  • the v. jugularis externa crosses SCM superficially and drains into the subclavian vein; the v. jugularis anterior runs near the midline. Together they are the main superficial veins of the neck;
  • the v. jugularis interna runs deep in the carotid sheath.
• Nerves: the facial nerve (VII; cervical branch to platysma), the mandibular nerve (V3; to mylohyoid and anterior digastric), the accessory nerve (XI), the hypoglossal nerve (XII), the vagus (X), the cervical plexus (ventral rami of C1–C4, including the ansa cervicalis, the phrenic nerve and cutaneous branches) and the roots and trunks of the brachial plexus (C5–T1).

Viscera of the neck

Glandula thyroidea

• Structure: two lobes joined by an isthmus, which lies over the 2nd to 4th tracheal rings. The gland extends from about C5 to T1.
• Function: produces thyroid hormones (triiodothyronine, T3, and thyroxine, T4), which set the basal metabolic rate and are essential for growth and brain development. Its C cells produce calcitonin, which lowers blood calcium.
• Arteries:
  • a. thyroidea superior, the first branch of the external carotid;
  • a. thyroidea inferior, from the thyrocervical trunk of the subclavian;
  • sometimes a small a. thyroidea ima from the brachiocephalic trunk or aortic arch.
• Veins: superior, middle and inferior thyroid veins.
• Nerves: sympathetic fibres from the cervical ganglia. The recurrent laryngeal nerve runs in the groove between trachea and oesophagus, right behind the gland, close to the inferior thyroid artery. It supplies the laryngeal muscles, not the gland itself, and it is the structure most at risk in thyroid surgery: injury causes hoarseness.

Glandulae parathyroideae

• Location: usually four small glands (two superior, two inferior) on the posterior surface of the thyroid lobes.
• Function: secrete parathyroid hormone (PTH), which raises blood calcium. It does this by releasing calcium from bone, increasing calcium reabsorption in the kidney, and (through vitamin D) increasing calcium absorption from the gut.
• Arteries: mainly the inferior thyroid artery, also the superior thyroid artery.
• Nerves: vasomotor sympathetic fibres from the superior and middle cervical ganglia. Secretion is controlled by blood calcium, not by nerves.

Thymus

• Location: the superior and anterior mediastinum, sometimes extending up into the neck. It is large in children and is gradually replaced by fat after puberty.
• Function: the site where T lymphocytes mature and learn self-tolerance. It is central to cellular immunity rather than to antibody production.
• Blood supply: small branches of nearby vessels (the internal thoracic and inferior thyroid arteries).

Lymphatic drainage of the head and neck

All lymph from the head and neck ultimately reaches the deep cervical lymph nodes (nodi lymphoidei cervicales profundi). These form a chain along the internal jugular vein, mostly under cover of SCM.

• Superficial nodes drain into the deep nodes. They are:
  • a ring at the junction of head and neck: occipital, retroauricular (posterior auricular), preauricular, parotid, submandibular and submental;
  • superficial cervical nodes along the external jugular vein (posterior triangle) and the anterior jugular vein (anterior triangle).
• Other named groups: tonsillar (jugulodigastric), posterior cervical and supraclavicular.
• The deep nodes drain into the jugular trunk, which empties into the thoracic duct on the left and the right lymphatic duct on the right.

Clinical relevance. An enlarged left supraclavicular node (Virchow's node) can be the first sign of a cancer in the abdomen, because the thoracic duct drains past it.

Clinical correlations

• Bell's palsy: an idiopathic paralysis of the facial nerve (CN VII), usually at or near the stylomastoid foramen. All the facial muscles on the same side are weak:
  • the patient cannot wrinkle the forehead, close the eye or smile on that side;
  • the corner of the mouth droops.

Because the whole side, forehead included, is affected, it is a lower motor neuron lesion. An upper motor neuron lesion (such as a stroke) spares the forehead.

• Paralysis of platysma: damage to the cervical branch of the facial nerve (for example in neck surgery). The skin of the neck loses its tone and falls into slack folds.
• Myasthenia gravis: an autoimmune disease in which antibodies block the acetylcholine receptors at the neuromuscular junction. It causes weakness that worsens with use, typically beginning in the eyelid and eye muscles (drooping lids and double vision), then the muscles of the face, chewing and swallowing. Because it affects the junction and not the nerve, sensation is normal.

Key teaching point

Two nerves run the face: V feels, VII moves, and V3 also works the muscles of mastication. In the neck, SCM is the key landmark:

• the carotid sheath lies under it;
• the posterior triangle behind it holds the accessory nerve and the brachial plexus;
• the deep cervical nodes along it drain the whole head and neck.