Sneezing may be defined as a spasmodic expiration preceded by one or more spasmodic inspirations. The expiration is momentarily opposed by the closure of the nasopharynx and a more or less complete closure of the mouth. With the onset of the expiration there is a rapid rise of intrapulmonic pressure which, when a certain value is attained, suddenly forces open the nasopharyngeal partition. In consequence, a blast of air is driven into and through the nasal chambers; at the same time, however, the mouth, not infrequently, is partially opened, thus permitting a portion of the blast to be driven through the buccal orifice. The object of the sneezing spasm is the dislodgment of mucus or other fluid from the surface of the nasal mucous membrane.
The act of sneezing is, in its entirety, quite complex, and may be subdivided for convenience of presentation into two phases, a nasal and a