The Upper Nose
The upper third of the nose is lined by different cells: the olfactory cells, which are responsible for our sense of smell. These are very sensitive, particularly to chemicals such as mercaptans (which are added to natural gas so we can smell if there's a leak). The sense of smell is our most primitive sense (it's how animals communicate, and why we have the perfume industry) and it mediates most of taste (which is why holding one's nose helps us to swallow unpleasant medicine). Any aroma is strongest when we first smell it, then it appears fade away - a process called accommodation. This is why we only smell garlic on other people's breath if we haven't been eating it ourselves. The olfactory cells are stimulated most directly when substances we’re smelling are driven directly at them by a blast of air, which is why we sniff at things to smell them.
The Lower Nose
The nostrils are lined by cells which produce mucous - goblet cells - and which have little hairy projections called cilia. These wave in a front-to-back direction which pushes the mucous to the back of the nose, where it is swallowed. The function of the mucous is to moisten the air, and to trap bacteria and bits of dirt that we breathe in.
The Inner Nose
The inner part of the nose is bony. At each side there are folds of bone - called turbinates - covered with the same cells and with a rich blood supply, which helps to warm the air as we breathe (our lungs don't take kindly to cold dry air, which is why asthmatics on cold days breathe better through scarves). Above and at each side of the nose are airspaces - the sinuses - which drain into the nasal cavity. The tear ducts from the eyes also drain into the bottom of the nose.
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