This is the name applied to two irregular cavities which are continuous with the nasal cavities; they rest against each other on the median line, and are bounded below by the palatine arch, and above by the cribriform plate (cribrum, a sieve, being perforated with numerous holes) of the ethmoid bone. They open posteriorly just above the throat A partition formed by the perpendicular plate of the ethmoid, the vomer (a ploughshare), and a cartilage separates the nasal fossa; on the median line; the prolongation of this cartilage separates the nasal cavity into two parts, as we have already seen. On the external walls of the nasal fossa; there are bony folds, which are called upper, middle, and lower spongy bones, and are separated from each other by corresponding passages. The nasal fossa: communicate with numerous sinuses in the substance of the bones of the face and skull.

The whole internal surface of the olfactory apparatus is lined with a mucous membrane called the pituitary membrane, this is the immediate organ of smell. This membrane dips into the numerous inequalities of the spongy bones and the passages, thus presenting a larger surface to olfactory impressions. The olfactory nerve is ramified in the pituitary membrane. It penetrates the nasal fossæ through the cribriform plate of the ethmoid, but is distributed over the upper portion only. In the lower portion of the fossæ the pituitary membrane receives only nervous filaments from the fifth pair, a circumstance to be noted in reference to the mechanism and seat of smell.