Illustration of a frontal section through the nasal cavity, which reaches from the nostrils to the end of the nasopharynx. The nasal septum divides the nasal cavity into two halves that blend together at their back end via the choanae. The hard palate builds the bottom of the nasal cavity; the lateral walls have three horizontal outgrowth, the 'conchae', that break, warm and moisturise the inhaled air and direct the airflow to the olfactory epithelium. Either side are the nasal sinuses, air-filled spaces surrounding the nasal cavity. The largest is the maxillary sinus under the eyes. Between the eyes are the ethmoidal sinuses and the frontal sinuses are above the eyes. All sinuses are connected with the nasal cavity. Every cavity is lined with respiratory or olfactory epithelium.