An organ of smell found in many terrestrial vertebrates, but not apparent in humans. Vomeronasal organs are typically paired blind-ending sacs opening into the oral or nasal cavities. They are lined with olfactory epithelium, whose cells express receptor proteins for airborne chemicals. The vomeronasal organs tend to respond especially to pheromones used in chemical signalling between members of the same species, and their receptor proteins belong to different protein families than those expressed by the nasal epithelium. Also, the axons of vomeronasal receptors project to a different site in the brain from the nasal olfactory axons.