Anatomy Of The Nasal Cavity and Nasopharynx

Skeleton and Framework of the External Nose

• Predominant Composition: The external nose is predominantly cartilaginous in its structure, though it contains a critical bony foundation.

• Bony Portion (Bridge of the Nose): The upper part of the nose is referred to as the "bridge" and consists of several bones:     * Nasal Bones: Two individual nasal bones are situated on either side of the midline.     * Frontal Bone: Peripheral aspects of the frontal bone contribute to the bony framework.     * Maxillary Bone: Peripheral aspects of the maxillary bones complete the bony structure.

• Cartilaginous Framework: The remainder of the external nose is formed by flexible tissues:     * Lateral Nasal Cartilages: Two large cartilages located on the sides of the nose.     * Greater Alar (Wing) Cartilages: Two cartilages that form the structure of the nostrils (alae).     * Lesser Cartilages: A variable number of smaller cartilages.     * Soft Tissue: Completes the exterior shape and flexibility of the nose.

The Choanae and Nasal Nares

• Nasal Septum: This midline structure separates the nasal cavity into distinct right and left sides. In anatomical study, removing one side (e.g., the left nasal cavity) allows for a clear view of the septum, with the opposite cavity lying deep to it.

• External Nares (Nostrils): These represent the external openings of the nasal cavity. They serve a protective function through:     * Coarse Hairs: These guard the entrance to keep out environmental debris.

• Internal Nares (Choanae): These are the posterior openings located just before the nasopharynx.

• Nasopharynx: The specific portion of the pharynx situated closest to and continuous with the nasal cavity.

Boundaries of the Nasal Cavities

Each nasal cavity is defined by the following boundaries:

• Roof: Formed by the nasal, frontal, ethmoid, and sphenoid bones.     * Anatomical Position: The anterior cranial fossa lies directly above the roof.     * Cribriform Plate: A perforated section of the ethmoid bone within the roof. It allows the passage of olfactory axons (filia olfactoria) into the olfactory bulb.

• Floor: Separates the nasal cavity from the oral cavity and corresponds to the Hard Palate.     * Anterior Portion: Formed by the palatine process of the maxilla.     * Posterior Portion: Formed by the horizontal plate of the palatine bone.

• Lateral Wall: A complex structure formed by the maxilla, palatine, and sphenoid bones.     * Superimposed Bones: The lacrimal, ethmoid, and inferior nasal concha are superimposed over the primary lateral wall bones.

• Medial Wall (Median Nasal Septum): Composed of three primary parts covered by nasal respiratory mucosa:     1. The Vomer: An unpaired bone forming the posterior-inferior part of the septum.     2. Perpendicular Plate of the Ethmoid: Forms the superior part of the septum.     3. Septal Cartilage: A cartilaginous plate forming the anterior portion of the septum.

Lateral Wall Anatomy and Nasal Conchae

• Nasal Conchae: These are three curved, shell-like bony plates (superior, middle, and inferior) that protrude into the nasal cavity.     * Structure: They are bony shelves lined with respiratory mucosa.     * Bony Origins: The upper two (superior and middle conchae) are parts of the ethmoid bone. The lowest (inferior nasal concha) is an individual, independent bone.

• Meatuses: The space located directly under each nasal concha is called a meatus. Meatuses serve as drainage sites for the paranasal air sinuses and the nasolacrimal duct.

Paranasal Air Sinuses and Specialized Drainage

There are four paired paranasal air sinuses that drain into the nasal cavity:

• Frontal Sinus: Drains into the middle meatus via the frontonasal duct.

• Maxillary Sinus: The largest sinus; drains into the middle meatus.

• Sphenoidal Sinus: Drains into the sphenoethmoidal recess, which is the space located above the superior concha.

• Ethmoidal Sinus (Ethmoidal Air Cells):     * Posterior Group: Empty into the superior meatus.     * Anterior Group: Empty into the middle meatus (specifically the hiatus semilunaris).     * Middle Group: Empty onto the surface of the ethmoidal bulla.

Detailed Meatus Anatomy

1. Superior Meatus: Receives drainage from the posterior ethmoidal air cells.

2. Middle Meatus:     * Ethmoidal Bulla: A visible protrusion of the ethmoid bone.     * Hiatus Semilunaris: A semicircular groove located under the ethmoidal bulla.     * Drainage Sites: Frontal sinus (via frontonasal duct), maxillary sinus, and anterior ethmoidal air cells.

3. Inferior Meatus:     * Nasolacrimal Duct: Located in the anterior region; it channels tears from the conjunctival sac into the nasal cavity.     * Visibility: This duct is often not visible in clinical views due to the swelling of the surrounding mucosa.

Maxillary Sinus Clinical Considerations

• Drainage Issues: The maxillary sinus is the largest sinus, but its drainage site is located at the very top of its deep cavity. This makes gravity-assisted drainage difficult.

• Infection and Pain: If secretions accumulate, infection occurs. Because the nerves supplying the upper teeth run in thin-walled tubes within the lateral wall of the maxilla, infection spreads quickly to these nerves.

• Referred Pain: Infection in the maxillary sinus often causes pain across the entire dental quadrant.

Olfactory System and CN I

• Location of Olfactory Epithelium: Found in the uppermost region of the nasal cavity, specifically on the septum, lateral wall, and roof.

• Olfactory Receptor Cells: These are bipolar neurons. Their axons leave the epithelium in groups of approximately $20$ axons, known as filia olfactoria.

• Pathway: The filia olfactoria pass upward through the cribriform plate of the ethmoid bone to synapse in the olfactory bulb.

• Terminology: The filia olfactoria are considered the "true" first cranial nerve ($CN\,I$), while the structure from the bulb inward is referred to as a "tract."

• Clinical Vulnerability: $CN\,I$ is the most easily lesioned cranial nerve. Damage often results from head trauma (e.g., hitting a steering wheel or windshield).

• Diagnostic Sign: Trauma to this area is generally accompanied by the dripping of cerebrospinal fluid (CSF), appearing as a clear fluid from the nose.

Innervation and Vascularization of the Nasal Cavity

Innervation

• Anterior/Upper Regions (Lateral and Medial Walls): Receive general sensation from the nasociliary nerve, a branch of the ophthalmic nerve ($V_1$).

• Lower/Posterior Regions (Lateral and Medial Walls): Innervated by nasal branches of the maxillary nerve ($V_2$).

• Olfactory Region: Restricted to the upper $1/3$ of the nasal cavity (location of filia olfactoria).

Blood Supply

• Primary Sources: Supply to the lateral wall and septum comes from the ophthalmic and maxillary arteries. This tracks back to both the internal and external carotid arteries ($aa.$).

• Sphenopalatine Artery: A branch of the maxillary artery (off the external carotid). It is the site of most major nosebleeds.

• Facial Artery: Off the external carotid; supplies the vestibule of each nasal cavity.

Nasopharynx and Soft Palate

• Superior Boundary: The end of the median nasal septum, specifically the vomer.

• Key Anatomical Features:     * Pharyngotympanic Tube: Also known as the auditory or eustachian tube; opens into the nasopharynx.     * Torus Tubarius: The swollen, cartilaginous end of the pharyngotympanic tube.     * Salpingopharyngeus Muscle: Descends from the torus tubarius into the pharyngeal wall. When covered with mucosa, it is known as the salpingopharyngeal fold.     * Pharyngeal Tonsil: Located within the nasopharyngeal region.     * Levator Palati Muscle: Extends out of the pharyngotympanic opening and ends in the adjacent soft palate. When covered by mucosa, it is described as being reminiscent of a "Thanksgiving cornucopia."

• Soft Palate Review: Muscles including the salpingopharyngeus and levator palati (covered by mucosa) function to manipulate the soft palate and pharyngeal wall during processes like swallowing.