Situated in a narrow channel high up in the nose, this thumbnail-size patch of tissue is packed with some ten million sensory neurons (4), each tipped with numerous hairlike projections, called cilia, bathed in a thin layer of mucus. So sensitive is the epithelium that it can detect 1/1,300,000,000 ounce [1/460,000,000 mg] of certain odorants in a single whiff of air.

But exactly how odors are detected is still shrouded in mystery. After all, humans can distinguish as many as 10,000 odors.

[Diagram]

(See publication)