Yawning, sneezing, breathing from the belly or from the upper chest affect the mucosal lining of our nasal meati (single: meatus), the labyrinths formed above the palate and oral cavity, on either side of the nasal septum, by scroll-shaped bones known as turbinates. As if this complicated spelunking stalactite cave system (connected to the sinuses and the ears) weren't complicated enough, it turns out that the mucus membranes are dynamic, alive, temperamental. They can close off one nostril or the other. They are affected by certain odors, CO2, humidity, pressure, and emotional state. By some accounts this drippy cave of stalactites and boogers acts much like a gas chromatograph, where the heavier or more reactive chemicals are sorbed (either ab- or ad-) onto the membranes first, while the lighter or less reactive ones filter deeper into the cavern. Today, Dr. Bruce Halpern of Cornell confirmed that discrimination is much easier than identification of odors. The trigeminal system includes nerves that enervate throughout the oral and nasal cavity. Interestingly, when the primary olfactory epithelium is not used, menthol and peppermint can still be sensed, presumably by nerves on the roof of the mouth.
At top right is a CT-based reconstruction of the nasal cavity from Zhao et al, 2004. Unfortunately the structure of the turbinates, septum, velum, palate (not to mention sinuses etc) are not easily visible. At lower right is an image from Heilmann and Hummel showing their procedure for discriminating between orthonasal and retronasal smelling. This is usually done under anesthesia, but for the sake of science the smiling volunteer is doing it cold-turkey.