The
olfactory system is the
sensory system used for
olfaction, or the sense of smell. Most mammals and reptiles have two distinct parts to their olfactory system: a
main olfactory system and an
accessory olfactory system. The main olfactory system detects volatile, airborne substances, while the accessory olfactory system senses fluid-phase stimuli. Behavioral evidence indicates that most often, the stimuli detected by the accessory olfactory system are
pheromones.