(25) The Pharynx and Face Anatomy

Anatomy and Structure of the Pharynx

The pharynx is described as a funnel-shaped fibromuscular tube. It serves as a shared conduit for both the respiratory and digestive systems, allowing the passage of air into the respiratory system and food into the digestive system.

Dimensions and Extent: It extends approximately $12\,cm$ from the base of the skull down to the lower border of the cricoid cartilage.
Wall Connectivity: * Posterior and Lateral Walls: These walls are continuous. * Anterior Wall: This wall is open, communicating with the nasal cavity, oral cavity, and larynx.
Inferior Boundary: At its lower end, the pharynx becomes continuous with the esophagus.

Regional Divisions of the Pharynx

The pharynx is anatomically divided into three distinct regions: the nasopharynx, the oropharynx, and the laryngopharynx.

1. The Nasopharynx

This region is located most superiorly, extending from the base of the skull to the soft palate. It communicates inferiorly with the oropharynx.

Anterior Communication: It opens into the nasal cavity via a pair of openings known as choanae (singular: choana).
Lymphoid Tissue: The submucosa contains the pharyngeal tonsil. When these tonsils are enlarged, the clinical condition is referred to as adenoids.
Lateral Wall Features: It contains the opening of the pharyngotympanic (auditory) tube.
Salpingopharyngeus Muscle: * Attachment: Attaches to the lower margin of the pharyngotympanic tube. * Structure: Produces a vertical mucosal elevation called the salpingopharyngeal fold. * Function: Elevates the pharynx during swallowing and opens the auditory tube to equalize pressure on both sides of the eardrum.

2. The Oropharynx

This middle region extends from the soft palate to the upper border of the epiglottis. It opens anteriorly into the oral cavity.

Floor: Formed by the posterior third of the tongue and the space between the tongue and the epiglottis.
Anterolateral Walls: Formed by mucosal elevations called palatine arches (tonsillar pillars): * Palatoglossal Arch: Located anteriorly; formed by the palatoglossus muscle. * Palatopharyngeal Arch: Located posteriorly; formed by the palatopharyngeus muscle.
Palatine Tonsil: Located within the tonsillar fossa, which is the recess between the palatoglossal and palatopharyngeal arches.

3. The Laryngopharynx (Hypopharynx)

This inferior region extends from the epiglottis to the lower border of the cricoid cartilage. It continues inferiorly as the esophagus.

Anterior Communication: It opens into the laryngeal inlet and the posterior aspects of the arytenoid and cricoid cartilages.
Piriform Recesses: Recesses located on either side of the laryngeal inlet. They serve to guide food into the esophagus.
Clinical Vulnerability: The mucosa of the piriform recesses covers the superior and inferior laryngeal vessels, as well as the internal and recurrent laryngeal nerves. These structures can be easily damaged due to their superficial location.

Pharyngeal Wall and Muscular Layers

The pharyngeal walls are covered by a mucous membrane that is continuous with the linings of the nasal and oral cavities, larynx, auditory tubes, middle ear, and esophagus. The muscular layer consists of six voluntary muscles categorized into two groups.

1. Pharyngeal Constrictors (Outer Circular Layer)

These three muscles form the outer layer and are fan-shaped. They overlap such that the inferior constrictor overlaps the middle, and the middle overlaps the superior.

Superior Pharyngeal Constrictor: Originates from the fibrous pterygomandibular raphe, the mandible, and the lateral pterygoid plate.
Middle Pharyngeal Constrictor: Originates from the hyoid bone.
Inferior Pharyngeal Constrictor: Originates from the thyroid and cricoid cartilages.
Median Pharyngeal Raphe: This is the posterior midline structure where the right and left constrictors merge.

2. Pharyngeal Elevators (Inner Longitudinal Layer)

These three muscles interdigitate with the constrictors to form an incomplete inner layer.

Palatopharyngeus Muscle: * Origin: Hard palate. * Insertion: Side of the pharynx and the thyroid cartilage. * Function: Elevates the pharynx and larynx during swallowing and speaking. * Innervation: Vagus nerve (CN X).
Salpingopharyngeus Muscle: * Origin: Pharyngotympanic tube. * Insertion: Blends with the palatopharyngeus muscle. * Function: Similar to the palatopharyngeus. * Innervation: CN X.
Stylopharyngeus Muscle: * Origin: Styloid process of the temporal bone. * Insertion: Pharynx and thyroid cartilage. * Function: Shortens and widens the pharynx during swallowing and speaking. * Innervation: Glossopharyngeal nerve (CN IX). It is the only muscle innervated by the motor component of CN IX.

Anatomical Gaps and Transversing Structures

Specific structures enter the pharynx by passing through gaps between or around the constrictor muscles:

Location of Gap	Structures Entering Gap
Above Superior Constrictor	Pharyngotympanic tube, Levator veli palatini muscle
Above Middle Constrictor	Stylopharyngeus muscle, Glossopharyngeal nerve (CN IX)
Above Inferior Constrictor	Internal laryngeal nerve
Below Inferior Constrictor	Recurrent laryngeal nerve

Innervation of the Pharynx

Specific nerves provide motor and sensory functions, which mingle in the pharyngeal plexus.

Motor Innervation: Primarily provided by CN X (Vagus nerve). The exception is the stylopharyngeus muscle, which is innervated by CN IX (Glossopharyngeal nerve).
Sensory Innervation: * Nasopharynx: Provided by $CN\,V_2$ (Maxillary division of the Trigeminal nerve). * Oropharynx and Hypopharynx: Provided by CN IX (Glossopharyngeal nerve).

Functional Mechanics of Swallowing

Swallowing is a complex process requiring neuromuscular integration, split into two phases:

1. Voluntary Stage

Food is chewed and mixed with saliva to form a bolus on the dorsum (top) of the tongue.
The bolus is pushed upward and backward against the hard palate.
The palatoglossus muscles contract, squeezing the bolus backward through the oropharyngeal isthmus into the oropharynx.

2. Involuntary Stage

Nasopharynx Sealing: The nasopharynx is separated from the oropharynx to prevent food from entering the nasal cavity. This is achieved by: * Tensing the soft palate (tensor veli palatini muscle). * Raising the soft palate (levator veli palatini muscle). * Pulling the posterior pharyngeal wall forward (upper fibers of the superior constrictor).
Airway Protection: The palatopharyngeal arches are pulled together. The larynx and laryngopharynx are elevated toward the epiglottis, and the laryngeal opening is narrowed.
Bolus Movement: The bolus moves around the epiglottis and laryngeal opening. It is propelled through the lower pharynx by gravity and successive contractions of the superior, middle, and inferior pharyngeal constrictor muscles.
Esophageal Entry: The cricopharyngeal sphincter relaxes, allowing the bolus to enter the esophagus, where peristaltic contractions move it toward the stomach.

Anatomical Composition of the Face

The face comprises several key components:

Bony Framework: Frontal, nasal, zygomatic bones, maxilla, and mandible.
Muscles of Facial Expression: Superficial skeletal muscles.
Innervation: Both motor and sensory.
Vascular Supply: Detailed blood vessels.

Muscles of Facial Expression

These are paired superficial skeletal muscles that insert into the skin rather than bone.

Orbicularis Oculi: * Orbital Part: Encircles the orbit; used for forceful eye closure. * Palpebral Part: Located within the eyelids; used for gentle closure (blinking) to protect the cornea and spread tears.
Orbicularis Oris: A sphincter muscle encircling the mouth within the lips. It controls lip movements (eating, drinking, whistling, kissing), oral competence, and speech.
Zygomaticus Major: Extends from the zygomatic bone to the orbicularis oris; raises the corners of the mouth for smiling.
Levator Anguli Oris: Assists the zygomaticus major in smiling, speech, and chewing.
Depressor Anguli Oris: Functions as the frowning muscle.
Occipitofrontalis: Consists of two muscle bellies connected by an aponeurosis.
Buccinator: * Origin: Alveolar processes of the maxilla and mandible, and the pterygomandibular raphe. * Insertion: Fuses with the orbicularis oris. * Function: Maintains cheek tightness and presses cheeks against teeth during chewing.

Innervation and Clinical Conditions of the Face

Motor Innervation

All muscles of facial expression are innervated by the terminal branches of the Facial nerve (CN VII).

Clinical Condition: Bell’s Palsy: * Cause: Damage to CN VII within the facial canal of the temporal bone. * Effect: Results in weakness or paralysis of facial expression muscles on the ipsilateral (same) side of the damage.

Sensory Innervation

Provided by all three divisions of the Trigeminal nerve (CN V):

$CN\,V_1$ (Ophthalmic): Supraorbital and supratrochlear nerves.
$CN\,V_2$ (Maxillary): Infraorbital nerve.
$CN\,V_3$ (Mandibular): Mental nerve.

The Parotid Gland

Description: A large salivary gland located anterior to the ear and superficial to the ramus of the mandible.
Secretion: Produces a serous fluid containing $\alpha$ -amylase, lysozyme, IgA, and other biologically active substances.
Pathology: Enlargement occurs during inflammation, such as in mumps.
Relationship to Nerves: Branches of the facial nerve emerge from this gland.
Parotid Duct: Crosses the masseter muscle, pierces the buccinator muscle, and empties into the oral cavity.

Muscles of Mastication

While discussed with the face, these are distinct from muscles of facial expression.

1. Masseter Muscle

Origin: Zygomatic arch.
Insertion: Lateral surface of the angle and lower ramus of the mandible.
Innervation: $CN\,V_3$ (Mandibular division of the Trigeminal nerve).
Action: Elevates the mandible for forceful mouth closure.

2. Temporalis Muscle

Origin: Lateral surface of the temporal bone.
Insertion: Coronoid process of the mandible.
Innervation: $CN\,V_3$ .
Action: Elevates and retracts the mandible.

Blood Supply of the Face

The primary blood supply stems from branches of the external carotid artery:

Facial Artery branches.
Superficial Temporal Artery branches.