A nostril (or naris , plural ''nares'' ) is either of the two
orifices of the
nose
A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for respiration alongside the mouth. Behind the nose are the olfactory mucosa and the sinuses. Behind the nasal cavity, air next pass ...
. They enable the entry and exit of air and other gasses through the
nasal cavities. In
bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s and
mammals, they contain branched bones or cartilages called
turbinates, whose function is to warm air on inhalation and remove moisture on exhalation.
Fish
Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% ...
do not breathe through noses, but they do have two (but
cyclostomes have merged into one) small holes used for
smelling, which can also be referred to as nostrils.
In
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
s, 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. With extreme damage to the septum and columella, the two nostrils are no longer separated and form a single larger external opening.
Like other
tetrapod
Tetrapods (; ) are four-limb (anatomy), limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant taxon, extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (p ...
s, humans have two external nostrils (anterior nares) and two additional nostrils at the back of the nasal cavity, inside the head (posterior nares, posterior nasal apertures or
choanae). They also connect the nose to the throat (the nasopharynx), aiding in respiration. Though all four nostrils were on the outside the head of the aquatic ancestors of modern tetrapods, the nostrils for outgoing water (excurrent nostrils) migrated to the inside of the mouth, 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
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...
s at an early stage. If these fail to join up, the result is a
cleft palate.
Each external nostril contains approximately 1,000 strands of
nasal hair, which function to filter foreign particles such as pollen and dust.
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.
The
Procellariiformes are distinguished from other birds by having tubular extensions of their nostrils.
Widely-spaced nostrils, like those of the
hammerhead shark, may be useful in determining the direction of an odour's source.
See also
*
Dilator naris muscle
*
Nasal cycle
References
External links
*
{{Authority control
Nose
Facial features