The horse’s olfactory receptors are located in the mucus membranes within the upper portions of the nasal cavities. When odor molecules enter the nasal passage, they contact the protein and lipid surfaces of the mucous membranes to stimulate the microscopic tufts of hair projecting from the receptor cells. By sniffing, even more hairs are stimulated. The olfactory cells have two branches, one that covers the surface of the olfactory mucosa and another that leads directly to the brain. The twin olfactory bulbs (scent–dedicated areas of the brain) are situated at the front of the cerebrum (one on each lobe) and are directly connected to the receptors in the nasal passages through the main olfactory nerves. Curiously, the olfactory bulbs in the horse are one of the only structures in the brain that don’t overlap; the receptors of the right nostril are directly connected with the right olfactory bulb and the left one is directly connected to the left olfactory bulb.
What's more, at the back and lateral sides of the nasal cavity exist four delicate bones called the turbinates—a pair of dorsal (upper) turbinates and a pair of ventral (lower) turbinates. Thin honey–combed bones, scroll–like in shape and covered in thick mucus membranes, they help to increase the surface area to which the tissues are exposed to air, providing a broad surface area for discriminating scent. The first two are the dorsal and ventral turbinates, and the fifth, in the back, the ethmoturbinate, is rich in olfactory nerves and transmits the scent stimuli to the brain. All five are rich in nerve and blood supplies, and are thickly covered with mucus and fluid–producing glands to also warm or cool, moisten the air, and filter out particles. These three chonchæ further divide the nasal passage into three airway channels, the dorsal meatus, middle meatus, and ventral meatus which channel airflow directly to the olfactory nerves. The ventral meatus is the largest of the three and a direct pathway from the nostrils to the pharynx.
But the horse really has two olfactory systems! Specifically, he has a specialized smelling organs called the Organs of Jacobsen (or vomeronasal organ) (VNOs). In fact, nearly all animals have VNOs, and only people and cetaceans (such as whales and dolphins) are among the few species without them. In horses, the VNOs are about 5” long (12cm), tubular, and cartilaginous. They’re lined with mucous membranes and contain sensory fibers of the olfactory nerve. The VNOs expand and contract from stimulation from strong odors, and have their own pathways to the brain, acting almost like independent sensory organs. Functionally, the VNOs are thought to detect and analyze pheromones, the chemical signals produced by other horses, especially to identify another horse’s sexual status. In this way, they can be considered a sexual organ, mostly to help stallions identify a mare in season.
The horse is constantly using scent to identify threats, evaluate his habitat, pick suitable food and water—and refuse medicated feed no matter how carefully it's been doctored with molasses! He also uses smell to identify and interact with herd mates…even with us. In fact, it’s thought a horse can identify a specific person from 100 paces! And if we coat a foal's nostrils with menthol cream, he typically cannot find his mother! And all this olfactory sensitivity is good since scent is extremely helpful by providing information about something that’s hidden from view, travels great distances, and remains for a long time. This makes it a highly effective means of communication, threat assessment, and identification.