Introduction

Bones forming the boundaries of the nasal cavity are hollowed out.  The cavities formed are the paranasal sinuses.

In the normal adult, there are eight paranasal sinuses; four on the left side and four on the right.  These are the maxillary, ethmoid, frontal and sphenoid sinuses.

Figure 1:  A sagittal section of the skull demonstrating the paranasal sinuses.
The maxillary sinus is shown in purple.  The ethmoid sinus in blue, the frontal sinus in red and the sphenoid sinus in yellow.  Look at the glossary to check the names of any structures you do not recognise.

Under normal conditions, each sinus is filled with air.

Each sinus communicates with the nasal cavitiy through a duct which opens into the nasal cavity through an ostium.  There are named ostia for each sinus and these enter the nasal cavity at different locations.

The paranasal sinuses are of great clinical importance.  They are involved in disease processes such as infection, allergy, injury and tumours.

No definitive function of the paranasal sinuses has been proven and it may be that they exist for a variety of reasons.