of the nasal cycle that slows down the conditioning process of the inhaled air. At the back of the nasal cavity there are two openings, called choanae (also posterior nostrils), that give entrance to the nasopharynx, and rest of the respiratory tract. On the outer wall of each cavity are three shell-like bones called conchae, arranged as superior, middle and inferior nasal conchae. Below each concha is a corresponding superior, middle, and inferior nasal meatus, or passage. Sometimes when the superior concha is narrow, a fourth supreme nasal concha is present situated above and sharing the space with the superior concha. The term concha refers to the actual bone; when covered by soft tissue and mucosa, and functioning, a concha is termed a turbinate. Excessive moisture as tears collected in the lacrimal sac travel down the nasolacrimal ducts where they drain into the inferior meatus in the nasal cavity. Most of the nasal cavity and paranasal sinuses is lined with respiratory epithelium as nasal mucosa. In the roof of each cavity is an area of specialised olfactory epithelium. This region is about 5 square cm, covering the superior concha, the cribriform plate, and the nasal septum. There is a nasal valve area that is the narrowest part of the nasal passage. An external valve exists in the larger ala part of the vestibule. An internal nasal valve typically referred to as the nasal valve, is a slit-like segment between part of the upper lateral cartilage and the septum in the middle third of the cavity. The valves regulate the airflow and resistance. Air breathed in is forced to pass through the narrow internal nasal valve, and then expands as it moves into the nasal cavity. The sudden change in the speed and pressure of the airflow creates turbulence that allows optimum contact with the respiratory epithelium for the necessary warming, moisturising, and filtering. The turbulence also allows movement of the air to pass over the olfactory epithelium and transfer odour information. The angle of the valve between the septum and the sidewall needs to be sufficient for unobstructed airflow, and this is normally between 10 and 15 degrees. The borders of each nasal cavity are a roof, floor, medial wall (the septum), and lateral wall. The middle part of the roof of the nasal cavity is composed of the horizontal, perforated cribriform plate of the ethmoid bone, through which pass