It has been estimated that if you took all the sensory mucous membrane in a horse’s nose and laid it flat it would cover the entire animal.
Horses not only smell more things and from greater distances but they can also analyse them, using the ‘Organ of Jacobsen’ (vomeronasal organ). In horses this organ is found in each nasal cavity with ducts into the back of the hard palate (roof of the mouth were it meets the nasal septum). This organ connects to the olfactory part of the brain and analyses smells for the presence of pheromones (chemicals that are released in sweat, urine, faeces and other secretions). The vomeronasal organ is like a mini-laboratory and the information that is passed to the brain is retained and remembered over the lifetime of the horse.
Many animals have this ability, including cats, however it is believed that this organ and its function has been eliminated from humans.
Because the vomeronasal ducts open at the back of the mouth, a new smell may cause a horse to have what is called a ‘Flehmen’ response: the horse will put its head in the air, stop breathing and take the scent in through its mouth, passing the duct openings.