Traditionally humans have been thought to come equipped with five senses: sight, hearing, taste, touch, and smell. Animals possess several extra senses, including altered vision and hearing, echolocation, electric and/or magnetic field detection, and supplementary chemical detection senses. In addition to taste and smell, most vertebrates use Jacobson's organ (also termed the vomeronasal organ and vomeronasal pit) to detect trace quantities of chemicals.
While snakes and other reptiles flick substances into Jacobson's organ with their tongues, several mammals (e.g., cats) exhibit the Flehmen reaction. When 'Flehmening', an animal appears to sneer as it curls its upper lip to better expose the twin vomeronasal organs for chemical sensing. In mammals, Jacobson's organ is used not simply to identify minute quantities of chemicals, but also for subtle communication between other members of the same species, through the emission and reception of chemical signals called pheromones.
In the 1800s, Danish physician L. Jacobson detected structures in a patient's nose that became termed 'Jacobson's organ' (although the organ was actually first reported in humans by F. Ruysch in 1703). Since its discovery, comparisons of human and animal embryos led scientists to conclude that Jacobson's organ in humans corresponded to the pits in snakes and vomeronasal organs in other mammals, but the organ was thought to be vestigial (no longer functional) in humans. While humans don't display the Flehmen reaction, recent studies have demonstrated that Jacobson's organ functions as in other mammals to detect pheromones and to sample low concentrations of certain non-human chemicals in air. There are indications that Jacobson's organ may be stimulated in pregnant women, perhaps partially accounting for an improved sense of smell during pregnancy and possibly implicated in morning sickness.
Since extra-sensory perception or ESP is awareness of the world beyond the senses, it would be inappropriate to term this Sixth Sense 'extrasensory'. After all, the vomeronasal organ connects to the amygdala of the brain and relays information about the surroundings in essentially the same manner as any other sense. Like ESP, however, the sixth sense remains somewhat elusive and hard to describe.