THE NASAL CAVITIES.

General structure

     The right and left nasal cavities are separated by the vertical nasal septum. Each nasal cavity is tall, narrow, and elongated, resembling the nave of a gothic cathedral. Its roof is the floor of the anterior cranial fossa, its floor is the roof of the oral cavity. Laterally, it is flanked by the orbit (above) and the maxillary sinus (below).

     The nasal vestibule, just inside the nostrils, is lined with stratified squamous epithelium, bearing coarse hairs. The nasal cavities themselves are lined with typical pseudostratified columnar ciliated respiratory epithelium. This is modified over the upper septum, roof, and lateral wall, where the olfactory epithelium contains the receptors for the sense of smell.

Bones:

Ethmoid. The unpaired ethmoid bone is the key element in the structure of the anterior two-thirds of the nasal cavity. It consists of three major parts.

The vertical plate of the ethmoid lies on or slightly to one side of the midline. Above the floor of the anterior cranial fossa, it forms the crista galli. It continues downward as the upper, anterior part of the bony nasal septum.

The paired, horizontal cribriform (perforated, sieve-like) plates of the ethmoid constitute the roof of the nasal cavities, separating them from the overlying anterior cranial fossa.

The paired lateral masses of the ethmoid form the upper, lateral walls of the nasal cavities, separating them from the orbits. Although the lateral masses are relatively thick, they are lightly built, being hollowed out by a series of paranasal ethmoid air cells. The air cells vary considerably in number from skull-to-skull and are grouped loosely into anterior, middle and posterior sets. All communicate with the nasal cavity.

The superior and middle conchae (turbinates = Latin turbo, turbin- spinning top) are scroll-like inward projections of the lateral masses of the ethmoid. They increase the mucosal surface of the nasal cavities and so improve warming and humidification of inspired air. In many mammals these can become quite elaborate as in this picture of a sheep nasal cavity from the anterior nasal aperture.

Maxillae. The paired maxillae form the lower lateral nasal walls, which serve as partitions between the nasal cavities and the paranasal maxillary air sinuses.

The horizontal or palatine shelf of the Maxila of the maxillae form the floor of the nasal cavities and separate them from the underlying oral cavity.

Inferior conchae. Although the paired inferior conchae (turbinates) are more heavily constructed than those belonging to the ethmoid, they serve the same purpose, namely to increase mucosal surface area to facilitate warming and moistening of inspired air.

Vomer. The midline, unpaired vomer extends downward and forward from the body of the sphenoid to the upper surface of the hard palate. It completes the bony nasal septum.

Palatine bones. The paired palatine bones are the key structural elements in the posterior third of the nasal cavities. Each is essentially L-shaped, consisting of vertical and horizontal plates meeting at a right angle.

The vertical plate of the palatine forms the posterior third of the lateral wall of the nasal cavity. It serves as a partition between the nasal cavity and the adjoining pterygopalatine fossa.

The horizontal plate of the palatine forms the posterior third of the hard palate, the floor of the nasal cavity.

A pyramidal process projects downward, laterally, and posteriorly from the intersection of the two plates to intervene between the maxilla and the medial and lateral pterygoid plates.

Sphenoid bone. The sphenoidal crest on the anterior surface of the body of the sphenoid forms the roof of the nasal cavity behind the cribriform plate of the ethmoid. It also extends downward, forming a bony posterior wall for the upper portion of the nasal cavity.

The medial pterygoid plate projects downward from the body of the sphenoid. It fuses with the rear edge of the vertical plate of the palatine bone and extends the lateral wall of the nasal cavity beyond the posterior margin of the hard palate.

Parieties

Floor: The hard palate forms the floor of the bony nasal cavity. Its anterior two thirds consists of the palatine process of the maxilla. The posterior third is provided by the horizontal plate of the palatine bone, and the interpalatine suture intervenes between the two.

Roof: The roof of the nasal cavity is formed largely by the cribriform plate of the ethmoid. Behind the cribriform plate, the roof is completed by the body of the sphenoid,
which extends downward to anchor the vomer. This creates a sphenoethmoidal recess behind the superior concha. The sphenoid paranasal air sinus opens into this recess.

Medial wall: The medial wall of the nasal cavity is the nasal septum. It consists of the perpendicular plate of the ethmoid (anterosuperiorly) and the vomer (inferoposteriorly). In life, the septum is completed by the septal cartilage, which is continuous with the alar cartilages that frame the nostrils.

The upper, septal surface of the ethmoid is furrowed by numerous vertical grooves for small bundles of olfactory nerve fibers.

The surface of the vomer is marked by the long delicate groove for the nasopalatine nerve and vessels, which run downward and forward to reach the upper surface of the hard palate.

Lateral wall: The basic structure of the lateral nasal wall is provided, from anterior to posterior, by the nasal bone, the maxilla, the lateral mass of the ethmoid, the vertical plate of the palatine, and the medial pterygoid plate of the sphenoid.

The plate-like nasal bone curves toward the septum and so forms an anterior wall for the uppermost part of the nasal cavity.

Three thin plates of bone, the nasal turbinates or conchae. scroll inward and downward from the lateral wall. The small superior concha and the larger middle concha are extensions from the lateral mass of the ethmoid. The stouter inferior concha is a separate bone joined to the maxilla (anteriorly) and the palatine (posteriorly).

The superior, middle, and inferior nasal meati are narrow anteroposterior channels. Each lies below the corresponding concha and extends upward between the concha and the lateral nasal wall.

The sphenoidal air sinus enters the sphenoethmoidal recess. All other paranasal air sinuses and the nasolacrimal duct open into the nasal cavity under cover of the conchae.

The nasolacrimal duct drains into the inferior meatus.
The posterior ethmoid air cells open into the superior meatus.

All remaining paranasal sinuses (maxillary, frontal, anterior ethmoidal, posterior ethmoidal) reach the middle meatus.

Neurovascular exit portals

Roof:

Perforations in the cribriform plate transmit bundles of olfactory nerve (Cranial Nerve I) fibers from the olfactory epithelium lining the roof of the nasal cavity. These fibers pass upward to end in the olfactory bulb, which lies on the upper surface of the cribriform plate. The bulb is an extension of the forebrain and belongs to the central nervous system.

One pore at the anterior extremity of the plate transmits the anterior ethmoidal nerve and vessels. The nerve is a branch of the nasociliary (ophthalmic division of Cranial Nerve V), the artery a branch of the ophthalmic artery, the vein a tributary to the superior ophthalmic vein. Nerve and vessels escape through the medial wall of the orbit onto the upper surface of the cribriform plate and turn downward to supply anterior portions of the lateral and medial nasal walls. The external nasal nerve and vessels continue onward, emerge between the nasal bone and nasal cartilage, and supply the side and tip of the nose.

Lateral wall:

The sphenopalatine foramen is situated atop the posterior portion of the lateral nasal wall, between the vertical plate of the ethmoid and the under surface of the sphenoid. It transmits the posterior superior nasal nerves and the sphenopalatine vessels into the nasal cavity from the pterygopalatine fossa. The nerves are branches of the maxillary division of Cranial Nerve V. The artery is the terminal continuation of part three of the maxillary artery.

 Lateral posterior superior nasal nerves and vessels turn forward (to supply the mucous membrane) over the and the superior meatus.

Lateral posterior inferior nasal nerves and vessels run forward [to supply the mucous membrane] over the middle meatus, inferior concha, and inferior meatus. They enter the nasal cavity from the greater palatine canal, through small foramina in the vertical plate of the palatine bone. The nerves arise from the greater palatine branch of the maxillary division of Cranial Nerve V, the arteries from the descending palatine branch of the maxillary artery.

Medial posterior superior nasal nerves and vessels cross the roof of the nasal cavity to reach the septum. The nasopalatine nerve and posterior septal artery run downward and forward on the septum in a groove on the vomer.

Floor:

The incisive canal is located anteriorly, at the base of the septum and at the end of the groove for the nasopalatine nerve. It leads obliquely downward and forward through the hard palate to open into the incisive fossa in the roof of the oral cavity, on the midline behind the central incisors. The canal transmits the nasopalatine nerve, passing downward, and the ascending branch of the greater palatine artery, passing upward. The nerve is one of the posterior superior medial nasal branches of the maxillary division of Cranial Nerve V. The artery arises from the descending palatine branch of the maxillary.