Nose

The nose is a vertebrate organ that handles both breathing and sensing smells. It has two main parts: an internal nasal cavity and an external nose on the face. The external part contains the nostrils, also called nares, which are paired tubes that let air flow in for respiration.

Inside the nasal cavity, these tubes widen into areas known as nasal fossae, which hold turbinates and olfactory mucosa. The nasal cavity also links to the paranasal sinuses—dead-end air pockets that help buffer pressure and add moisture. From the nasal cavity, the nostrils lead into the pharynx, a switching valve that connects the respiratory and digestive systems.

In humans, the nose sits centrally on the face and provides an alternate breathing route, especially useful for infants during suckling. A protruding nose that is completely separate from the mouth is a trait found only in therian mammals. Scientists think this unique mammalian nose evolved from the front part of the upper jaw in reptilian-like ancestors called synapsids.

Air treatment

As the first contact point between the outside world and an animal’s delicate lungs, the nose conditions incoming air. It regulates temperature, filters particles, and enables the sense of smell. Hair inside the nostrils filters out dust, smoke, and other obstructions, acting as a first defense against airborne illness.

Mucus produced in the nose also helps maintain temperature and adds moisture to the respiratory system. Capillary structures inside the nose warm and humidify air as it enters; later, this moisture retention allows alveoli in the lungs to properly exchange oxygen for carbon dioxide during respiration. On exhalation, these capillaries recover some moisture, mainly as part of thermal regulation.

Sense of direction

A dog’s wet nose helps it sense direction. Cold receptors in the skin detect where the nose is cooled the most, pointing to the source of a particular smell the animal just picked up.

Structure in air-breathing forms

In amphibians and lungfish, nostrils open into small sacs that then open into the forward roof of the mouth through the choanae. These sacs contain a little olfactory epithelium; in caecilians, this tissue also lines several nearby tentacles. Despite structural similarities to amphibians, lungfish nostrils are not used for breathing—these animals breathe through their mouths. Amphibians also have a vomeronasal organ lined by olfactory epithelium, but unlike in amniotes, it is usually a simple sac with little connection to the rest of the nasal system, except in salamanders.

In reptiles, the nasal chamber is generally larger, with choanae located much farther back in the roof of the mouth. Crocodilians have an exceptionally long chamber, allowing them to breathe while partly submerged. The reptilian nasal chamber has three parts: an anterior vestibule, the main olfactory chamber, and a posterior nasopharynx.

The olfactory chamber is lined with olfactory epithelium on its upper surface and has several turbinates to increase sensory area. The vomeronasal organ is well-developed in lizards and snakes, where it no longer connects to the nasal cavity but opens directly into the roof of the mouth. It is smaller in turtles, keeping its original nasal connection, and is absent in adult crocodilians.

Birds have a nose similar to other reptiles, with nostrils at the upper rear part of the beak. Because birds generally have a poor sense of smell, the olfactory chamber is small, though it contains three turbinates that can be complex, like those in mammals. In many birds, such as doves and fowls, the nostrils are covered by a horny protective shield. The vomeronasal organ in birds is either underdeveloped or absent, depending on the species.

In mammals, the nasal cavities are fused into one. In most species, they are exceptionally large, often taking up half the skull’s length. However, in some groups—including primates, bats, and cetaceans—the nose has been secondarily reduced, giving these animals a relatively poor sense of smell.

The mammalian nasal cavity enlarged partly because a palate developed, cutting off the upper surface of the original oral cavity, which then became part of the nose, leaving the palate as the new roof of the mouth. This enlarged cavity contains complex, coiled turbinates that warm air before it reaches the lungs. The cavity also extends into neighboring skull bones, forming additional air cavities called paranasal sinuses.

In cetaceans, the nose has been reduced to one or two blowholes—nostrils that migrated to the top of the head. This adaptation gave cetaceans a more streamlined body and the ability to breathe while mostly submerged.

Conversely, the elephant’s nose has evolved into a long, muscular, manipulative organ called the trunk. The mammalian vomeronasal organ is generally similar to that of reptiles. In most species, it sits in the floor of the nasal cavity and opens into the mouth via two nasopalatine ducts running through the palate.