Nose

Lore & Background

In humans, the nose is located centrally on the face and serves as an alternative respiratory passage, especially during suckling for infants. The protruding nose that is completely separate from the mouth is a characteristic found in therian mammals and monotremes, theorized to have evolved from the anterior part of the upper jaw of reptilian-like ancestors (synapsids). Across vertebrates, the nose varies greatly. In amphibians and lungfish, nostrils open into small sacs that open into the forward roof of the mouth. Reptiles have a larger nasal chamber, with crocodilians possessing an exceptionally long chamber for breathing while partially submerged. Birds have a similar nose to other reptiles, with nostrils at the upper rear part of the beak, though their olfactory chamber is small. In mammals, the nasal cavities are divided by the nasal septum into two separate chambers and are exceptionally large, occupying up to half the length of the skull in most species, though reduced in primates, bats, and cetaceans. The nose also plays a role in direction sensing: the wet nose of dogs is useful for perception of direction, as cold receptors detect where the nose is cooled most, indicating the direction of a particular smell. In fish, the nose has no connection with the mouth nor any role in respiration, generally consisting of paired pouches lined by olfactory epithelium.

Reader's Guide

The nose is significant as a multifunctional organ central to respiration, olfaction, and air conditioning across vertebrate species. Its structure and function vary widely, reflecting adaptations to different environments and lifestyles. In therian mammals, the evolution of a protruding nose separate from the mouth represents a key anatomical innovation. The nose's role in filtering, warming, and humidifying incoming air is critical for protecting delicate lung tissues and enabling efficient gas exchange. The presence of turbinates and paranasal sinuses in mammals enhances these functions. The sense of smell, mediated by olfactory epithelium within the nasal cavity, is vital for many animals, though reduced in some groups like primates, bats, and cetaceans. In cetaceans, the nose has been reduced to blowholes for streamlined breathing, while in elephants it has elaborated into a trunk. The vomeronasal organ, present in many mammals and reptiles, further contributes to chemosensation. Understanding the nose's diversity and function provides insight into vertebrate evolution and adaptation.

Did You Know?

• The protruding nose separate from the mouth is found in therian mammals and monotremes.
• The wet nose of dogs detects the direction of a smell by sensing where the nose is cooled most.
• In cetaceans, the nose has been reduced to one or two blowholes located on top of the head.
• In some early vertebrates (e.g., cyclostomes), the nasohypophyseal opening is a single median nostril leading to a blind sac associated with the pituitary.