The paired nasal cavities lie centrally within the facial skeleton, medial to the orbits and the maxillary air sinuses (Fig. 7.47). They are separated from the oral cavity by the palate, from the anterior cranial fossa by the cribriform plates and from each other by the midline nasal septum. Anteriorly, the cavities lead into the vestibules, which are surrounded by the cartilaginous external nose and open onto the face at the nostrils.
Posteriorly, the nasal cavities are limited by the free edge of the nasal septum at the choanae (posterior nasal apertures), which open into the nasopharynx. Each cavity is partially subdivided by three shelf-like projections from the lateral wall, the superior, middle and inferior conchae (turbinates Fig. 7.48). The parts of the nasal cavity beneath each of these are called correspondingly the superior, middle and inferior meatuses, while above the superior concha is the sphenoeth-moidal recess. Into this recess and the meatuses drain the paranasal air sinuses and the nasolacrimal duct. Respiratory epithelium lines the cavity and paranasal air sinuses while the vestibule has a stratified squamous epithelium bearing nasal vibrissae (hairs).
Fig. 7.47 Coronal section showing the orbits and nasal cavities. Posterior aspect. (Compare Figs 7.84 & 7.92.)
The medial wall is the nasal septum (Fig. 7.49), common to both cavities and formed superiorly by the perpendicular plate of the ethmoid. This plate continues upwards as the crista galli, which projects into the anterior cranial fossa. The bony septum is completed posteroinferiorly by the vomer. Anteriorly, the septum is composed of hyaline cartilage which extends into the external nose.
The roof of each cavity comprises, from in front backwards, the nasal and frontal bones, the cribriform plate of the ethmoid and, finally, the body of the sphenoid bone containing the sphenoidal air sinuses. Olfactory (I) nerves from the olfactory mucosa traverse the many small foramina in the cribriform plate to reach the olfactory bulbs in the anterior cranial fossa (Fig. 7.50). These nerves are vulnerable to damage in head injuries with fracture of the cribriform plates, disrupting the sense of smell. Leakage of cerebrospinal fluid from the nose may also result from these fractures.
The floor of each nasal cavity is formed by the hard palate, consisting of the palatine process of the maxilla and the horizontal process of the palatine bone.
Numerous bones contribute to the lateral wall (Figs 7.48, 7.50 & 7.51), including the inferior concha and the maxilla, lacrimal, ethmoid, palatine and sphenoid bones. The maxilla forms the anteroinferior portion of the lateral wall and contains the maxillary air sinus. Between the maxilla and the ethmoid, part of the lacrimal bone covers the nasolacrimal canal, which opens into the inferior meatus. Each labyrinth (lateral mass) of the ethmoid is attached to the lateral part of the cribriform plate and contains numerous air cells. From the medial surface of the labyrinth project the small superior and the larger middle conchae. The ethmoidal air cells bulge into the middle meatus, forming the bulla, beneath which a curved groove, the hiatus semilunaris, separates the ethmoid from the maxilla. Forming the posterior limit of the hiatus semilunaris is the vertical plate of the palatine bone. The most posterior component of the lateral wall is the medial pterygoid plate of the sphenoid. Overlying the maxilla and palatine bones is a separate bone, the inferior concha.
Sensory nerve supply
The somatic sensory nerve supply to the walls of the nasal cavity is derived mainly from the maxillary (V2) division of the trigeminal nerve. The posterior lateral nasal nerves from the pterygopalatine ganglion (p. 354) supply most of the lateral wall, while the nasopalatine nerve supplies the septum. Lesser and greater palatine nerves supply the posterior part of the lateral wall and the floor. In addition, fibres from the ophthalmic (V1) division reach the nasal cavity via the anterior ethmoidal nerve. This nerve supplies the anterosuperior parts of the septum and the lateral wall and continues as the external nasal nerve to supply the midline part of the external nose.
Most of the blood supply to the walls of the nasal cavity is provided by branches of the maxillary artery. These vessels arise in the pterygopalatine fossa and are named according to the branches of the pterygopalatine ganglion they accompany. The anteroinferior part of the nasal septum is highly vascular (Little’s area) and commonly gives rise to nasal haemorrhage (epistaxis).
Venous blood passes to the pterygoid plexus, the facial vein and the ophthalmic veins.
Paranasal air sinuses
There are four paired groups of paranasal air sinuses (Figs 7.51– 7.54) contained within the frontal, maxillary, ethmoid and sphenoid bones. Each sinus communicates with the nasal cavity, is lined with mucous membrane and normally contains air. The frontal air sinuses are situated in the vertical and horizontal parts of the frontal bone, closely related to the frontal lobes of the brain. They are variable in size and open into the middle meatus at the infundibulum, the most anterior part of the hiatus semilunaris. The frontal air sinus is supplied by the supraorbital branch of the ophthalmic (V1) division of the trigeminal nerve.
The maxillary air sinus (antrum) occupies the body of the maxilla, lying above the oral cavity and alveolar ridge and below the orbit. Its opening at the posterior end of the hiatus semilunaris lies high on the medial wall of the antrum, permitting limited drainage for contents such as mucus or pus. Sensory innervation is from the superior alveolar nerves.
The ethmoidal air sinuses are subdivided into three groups of air cells, which communicate with the nose through many tiny foramina. The anterior cells open into the floor of the hiatus, while the middle cells open onto the bulla, both groups being supplied by the anterior ethmoidal nerve. The posterior group, innervated by the posterior ethmoidal nerve, drains into the superior meatus under the superior concha.
The sphenoidal air sinuses lie just below the sella turcica in the body of the sphenoid, through the anterior wall of which they open into the sphenoethmoidal recess. The sensory supply is from the pharyngeal branch of the pterygopalatine ganglion. The pituitary gland can be accessed surgically through the sphenoidal air sinus.
Infection of the paranasal air sinuses (sinusitis) causes thickening of the mucosal lining, which may block the openings into the nasal cavities.