Noses (comparative anatomy)

In vertebrates, the back of the throat is connected to the outside air through a passageway called the nose. The outside opening of the nose is referred to as the external naris, whereas the opening from the nose to the back of the throat is called the internal naris. In reptiles and amphibians, air drawn into the lungs passes through the external naris and into a tubelike structure which is the simplest form of a nose. In animals like the salamander, the nasal air passageway is a straight tube. Air exits this tube via the internal naris, where it proceeds into the back of the throat. In some amphibians, such as the bullfrog, the channel between the external and internal naris has a bony bump on the floor of the nasal passageway. This bump, called the ementia, apparently functions like a baffle plate so that the incoming air stream is deflected. As a result of this deflection, air moving through the nose is turbulent. This turbulence likely increases the ability to detect smells and improves the efficiency of the other nasal functions.

In mammals, the nose is divided into two halves along the midline by a bony structure called the nasal septum. The internal anatomy of the nose is defined by bony structures called turbinates. These turbinates produce multiple and convoluted air flow paths for the inspired air. In some mammals, like rats, the septum is incomplete, and so there is some mixing in the nose of air that comes into the left and right nostrils.

In some animals, noses are located on a protruding or regionalized section of their faces, like muzzles, snouts, rostrums, or proboscis. In many furry animals, the area near the nostrils, called the rhinarium, is hairless and often wet.

Air entering the nose travels in the airspace between the turbinates and the nasal septum. The surface of these nasal structures has a very rich blood supply, and so blood flow to these areas can be quickly changed. By changing the amount of blood flow to the nasal structure, the diameter of the nasal airspace itself can be quickly changed. Because of the speed and amount of diameter change that can occur in these areas, they are called nasal “swell spaces.”

Heat, Humidity, and Purification

When cold air enters the nose, it causes the blood flow to the swell spaces to increase dramatically. This causes a swelling of the nasal tissue, reducing the airspace through which the incoming air must travel. Because the air passageways are now more narrow, more heat can be transferred from the bloodstream to the incoming air. Thus, the cold air is effectively warmed before it enters the lungs. When air that is warmer than the body temperature enters the nose, the reverse happens, and the nasal air passageways are made wider.

The material covering the turbinates and nasal septum is called mucus. This mucus layer is mostly composed of water and serves to humidify the incoming air. When the air is dry, water is evaporated from the mucus into the inspired air. When the air is dry, a considerable quantity of water can be lost through the nose. For animals living in dry desert conditions, the nasal humidification process is critical because it is necessary to save every drop of water while still humidifying the incoming air. When these animals take air into the lungs, the inspired air passes through the extensive turbinate structures of the nose, where it is humidified by the evaporation of water from the nasal mucus. As evaporation takes place, the surface of the mucus is cooled. When it is time to discharge the air from the lungs, the expired air passes over the cooled surface of the mucus. As a result, water in the inspired air is picked up by the cooled surface so that it is not lost in the expired air. This type of water conservation method has been seen in kangaroo rats living in the deserts of Australia and in certain birds, such as the cactus wren, which inhabit warm, dry areas.

In addition to supplying water for humidification purposes, the nasal mucus serves as a trap for particulate matter like smoke, dust, and airborne bacteria. Beneath the nasal mucus is a layer of cells that contain hairlike protrusions called cilia. As the cilia of these cells beat, they create a wavelike action in the mucus. The mucus is, thus, moved through the nose and to the back of the throat. Once in the throat, the mucus is swallowed, and the stomach must rid the body of the particles and bacteria. The movement of nasal mucus is an ongoing process, and trapped particles are continually removed from the nose. There are also white blood cells and enzymes in the mucus that destroy bacteria.

The detection of potentially harmful chemicals occurs both through the smell receptors and pain receptors found in the nose. When particles (and sometimes even air) touch parts of the nose, a sneeze occurs—breathing stops, and the air is forcibly expelled from the lungs at a high flow rate. The sneeze is an attempt to expel the particles from inside the nose. Breathing may also temporarily stop when an odorant is very foul or stings the nose. This protects the lungs from potentially damaging chemicals.

Some animals have noses designed to accommodate hunting or living in their environment, and others have noses that perform specialized functions. The shape of the hammerhead shark's head places its nostrils, called nares, far apart. They can smell their prey long before they can see it and know which direction to swim based on which nostril sensed the prey, which could be hundreds of meters away. Their nostrils are keyhole-shaped, they have a minor nasal groove that parallels their nostrils, and their nasal bridge is designed to control the water flow into their nose to protect their sophisticated scent organ. The endangered proboscis monkey's nose can grow up to four inches long—the largest nose of any primate. It is supported by specialized cartilage structures in the skull. Males use their noses to attract mates and communicate. Females prefer males with larger noses and loud calls, and since their nose acts as an echo chamber, the larger the monkey's nose is, the louder their call will be. The blind star-nosed mole's nose is the most sensitive touch organ known to exist in any mammal species. Over 100,000 nerve endings called Eimer's organs cover twenty-two tentacles that form a star shape on the mole's face. Underwater, the mole can smell by blowing bubbles and then sucking them back into their nose. Though they can not see, they are among the fastest foragers in the animal kingdom.

Principal Terms

Mucus: The watery material covering the internal nasal structures that aids in humidification, warming, and particle filtration

Olfaction: The sense of smell

Septum: The bony structure that divides the nose into two sections

Turbinates: The bony structures that define the internal nasal anatomy

Bibliography

Getchell, T. V., et al. Smell and Taste in Health and Disease. New York: Raven Press, 1991.

Gibbons, Byron. “The Intimate Sense of Smell.” National Geographic, vol. 170, no. 3, 1986, pp. 321-61.

Miriello, Lisa. "The Nose That 'Sees.'" Carnegie Museum of Natural History, 31 Aug. 2023, carnegiemnh.org/the-nose-that-sees. Accessed 15 Sept. 2024.

"Resources for the Public." Association for Chemoreception Sciences, achems.org/web/resources-public.php. Accessed 15 Sept. 2024.

Vroon, Piet. Smell: The Secret Seducer. New York: Farrar, Straus and Giroux, 1997.

Whittle, Patrick. “Scientists: Mussels, without Noses, Use Smell to Find Homes.” Business Insider, 10 May 2016, www.businessinsider.com/ap-scientists-mussels-without-noses-use-smell-to-find-homes-2016-5. Accessed 4 July 2023.