Inside of a Dog_ What Dogs See, Smell, and Know - Alexandra Horowitz [30]
The nose is also the fastest route by which information can get to the brain. While visual or auditory data goes through an intermediate staging ground on the way to the cortex, the highest level of processing, the receptors in the nose connect directly to nerves in specialized olfactory "bulbs" (so shaped). The olfactory bulbs of the dog brain make up about an eighth of its mass: proportionally greater than the size of our central visual processing center, the occipital lobes, in our brains. But dogs' specially keen sense of smell may also be due to an additional way they perceive odors: through the vomeronasal organ.
THE VOMERONASAL NOSE
What specificity of image the name "vomeronasal" conjures up! Evoking the displeasure of getting a good sniff of fresh vomit, the "vomer" is actually a description of the part of the small bone in the nose where the sensory cells sit. Still, the name seems somehow fitting for an animal that is notorious for coprophagia (feces eating) and that may lick another dog's urine off the ground. Neither act is vomitous for dogs; it's just a way of getting even more information about other dogs or animals in the area. The vomeronasal organ, first discovered in reptiles, is a specialized sac above the mouth or in the nose covered with more receptor sites for molecules. Reptiles use it to find their way, to find food, and to find mates. The lizard who darts out its tongue to touch an unknown object is not tasting or sniffing; it is drawing chemical information toward its vomeronasal organ.
These chemicals are pheromones: hormonelike substances released by one animal and perceived by another of the same species, and usually prompting a specific reaction—such as readying oneself for sex—or even changing hormonal levels. There is some evidence that humans unconsciously perceive pheromones, perhaps even through a nasal vomeronasal organ.*
Dogs definitely have a vomeronasal organ: it sits above the roof (hard palate) of the mouth, along the floor of the nose (nasal septum). Unlike in other animals, the receptor sites are covered in cilia, tiny hairs encouraging these molecules along. Pheromones are often carried in a fluid: urine, in particular, is a great medium for one animal to send personalized information to members of the opposite sex about, say, one's eagerness to mate. To detect the pheromones in that urine some mammals touch the liquid and do a distinctive, mortifying, lip-curling grimace called flehmen. The face of a flehmening animal is notably unlovable—but it is the face of an animal who is on the hunt for a lover. The flehmen pose seems to hurtle the fluid toward the animal's vomeronasal organ, where it is pumped into the tissue, or is absorbed through capillary action. Rhinos, elephants, and other ungulates flehmen regularly; so do bats and cats, which have their own species variations. Humans may have vomeronasal organs, but we do not flehmen. Neither do dogs. But a regular observer of dogs will notice an often very intense interest in the urine of other dogs—sometimes an interest which lures them right … up … into … wait, gross! Stop licking that! Dogs may lightly lap up urine, especially urine of a female in heat. This could be their version of flehmen.
Even better than flehmen is keeping the outside of the nose nice and moist. The vomeronasal organ is probably why a dog's nose is wet. Most animals with vomeronasal organs have wet noses, too. It is difficult for an airborne odor to land squarely on the vomeronasal organ, since it is situated in a safe, dark interior recess of the face. A hearty sniff not only brings molecules into the dog's nasal cavity; little molecular bits also stick onto the moist exterior tissue of the nose. Once there, they can dissolve and travel to the vomeronasal organ through interior ducts. When your dog nuzzles against you, he is actually collecting your odor on his nose: better to confirm that you're you. In this way, dogs double their methods of smelling the world.
THE BRAVE SMELL OF A STONE
When Pump got her nose