Location and Boundaries of the Choanae
The bony nasal cavity (Cavitas nasalis ossea) is trapezoidal in shape. It is divided in the midline by the bony nasal septum (Septum nasi osseum), on either side of which lie the posterior nasal apertures.
- Posterior boundary of the nasal cavity — formed directly by the choanae (choanae). By comparison, the anterior boundary of the nasal cavity is the piriform aperture (Apertura piriformis).
- Bony nasal septum (Septum nasi osseum) — divides the nasal cavity into right and left halves. The septum is formed by the perpendicular plate of the ethmoid bone, the vomer (vomer), the nasal crest of the frontal bone, and the sphenoidal crest of the body of the sphenoid bone.
Topography on the External Skull Base
When viewing the skull from the inferior (basal) norm, the external skull base (Basis cranii externa) is distinguished. It is conventionally divided into a smaller anterior part (hard palate) and a larger posterior part.
- Anterior boundary of the external skull base — anteriorly (excluding the hard palate region), the external skull base is bounded by the choanae (aperturae nasales posteriores).
- Communications of the skull base — through the choanae, the external skull base communicates freely with the nasal cavity. Posteriorly and laterally to the choanae, the base is bounded by anatomical structures of the temporal and occipital bones, while laterally it continues freely into the infratemporal fossa (Fossa infratemporalis).
Connected Structures
Posterior to the nasal cavity lies the nasopharynx (Pars nasalis pharyngis), which extends from the pharyngeal roof to the soft palate.
- Connection with the nasal cavity — the nasopharynx communicates with the nasal cavity via the choanae (choanae).
- Adjacent nasopharyngeal structures — the mucosa of the nasopharynx is lined with ciliated epithelium. On the lateral wall of the nasopharynx is the pharyngeal opening of the auditory tube (Ostium pharyngeum tubae auditivae) and the torus tubarius (Torus tubarius), which continues into the salpingopharyngeal fold (Plica salpingopharyngea). The pharyngeal tonsil (Tonsilla pharyngealis) is located in the region of the roof.
Walls and Passages of the Nasal Cavity Leading to the Choanae
The bony nasal cavity has the following walls:
- Superior wall — formed by the nasal and frontal bones, the cribriform plate of the ethmoid bone, and the body of the sphenoid bone.
- Inferior wall — corresponds to the hard palate.
- Lateral wall — includes the ethmoidal labyrinth, the nasal conchae (superior, middle, inferior), and the medial plate of the pterygoid process of the sphenoid bone.
Between the nasal conchae and the septum, the common nasal meatus (Meatus nasi communis) is formed. The nasopharyngeal meatus (Meatus nasopharyngeus) leads into the nasopharynx.
Role in Skull Communications
The external skull base, whose anterior boundary is formed by the choanae, features additional communications and foramina. Along with the choanae, which provide access to the nasal cavity, the following structures are present:
- Free communication — with the infratemporal fossa (Fossa infratemporalis), which is bounded medially by the lateral plate of the pterygoid process.
- Pterygoid canal (Canalis pterygoideus) — connects the external skull base with the pterygopalatine fossa (Fossa pterygopalatina).
- Major skull base foramina — foramen magnum (Foramen magnum), jugular foramen (Foramen jugulare), lacerum foramen (Foramen lacerum), carotid canal (Canalis caroticus), and other structures.