A nostril (or naris, pl. nares) is one of the two channels of the nose, from the point where they bifurcate to the external opening. In birds and mammals, they contain branched bones or cartilages called turbinates, whose function is to warm air on inhalation and remove moisture on exhalation. Fish do not breathe through their noses, but they do have two small holes used for smelling which may be called nostrils.
The Procellariiformes are distinguished from other birds by having tubular extensions of their nostrils.
In humans, the nasal cycle is the normal ultradian cycle of each nostril's blood vessels becoming engorged in swelling, then shrinking.
The nostrils are separated by the septum. The septum can sometimes be deviated, causing one nostril to appear larger than the other. In extraordinary cases such as that of British television actress Danniella Westbrook, excessive use of cocaine can cause the septum to become disfigured or destroyed. In such an event, the two nostrils are no longer separated and form a single larger external opening. 
Humans have two external with two additional nostrils inside the head. These internal nostrils are called "choana" and each contain approximately 1000 strands of nasal hair. They also connect the nose to the throat aiding in respiration. Scientists believe they migrated back inside as evidenced by the discovery of "Kenichthys campbelli", a 395 million-year-old fossilized fish which shows this migration in progress. It has two nostrils between its front teeth, similar to human embryos at an early stage. If these fail to join up it causes a 'cleft palate'.
It is possible for humans to smell different olfactory inputs in the two nostrils and experience a perceptual rivalry akin to that of binocular rivalry when there are two different inputs to the two eyes.
Ala of nose
Nasal concha/meati: Superior nasal concha · Middle nasal concha · Inferior nasal concha · Superior nasal meatus · Middle nasal meatus · Inferior nasal meatus
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