As husband and father of people who claimed they wanted a dog, I spend a lot of my time walking ours. I've noticed how much time she spends smelling things. Dogs are, of course, well known for their abilities to sniff out everything from explosives to whale poop, so I wondered what and how a dog's nose knows.
Some of the world's many breeds of dog have been bred to enhance their smelling abilities. Loose skin on the face helps trap smell particles. Short legs holds the nose closer to the ground. Our mutt has droopy ears that are supposed to help collect and channel smells to the nose.
Each of a dog's nostrils can wiggle and smell independently, providing a stereo-smellic (I made that up) ability to figure out the direction of a smell. Slits at the sides of a dog's nostrils enable it to breathe out while retaining smells for further consideration. The air exits in vortices that help draw more air in.
The wetness of a dog's nose is not just for waking you up in the morning, but helps capture and dissolve odour molecules. Sometimes, a dog licks its own nose to keep it primed.
Inside the snout, a flap separates the air flow, so some air enters the lungs, and some air (12%) continues further back into a recess made of scrolled bony structures called turbinates, carpeted with a thick, spongy membrane. To keep this membrane moist, dogs go through a pint of mucus a day. Olfactory cells in the membrane detect odours dissolved in the mucus. Dogs have around 220 million olfactory cells, with as many as 300 million in bloodhounds, whereas humans have a measly 5 million.
Dogs also have another system to process chemical signals called the vomeronasal organ or Jacobson's organ, but never the Wurlitzer organ. This region under the nasal passage consists of two fluid-filled sacs with openings just behind the upper front teeth. This tissue has tiny projections called microvilli that increase surface area to enhance absorption. These enable dogs to taste social and sexual pheromones left by other dogs and other animals.
Dogs not only have more sensitive chemical sensors than we do, they also dedicate more brain power to processing that information. Nerves carry signals from olfactory cells to the olfactory lobe. In humans, this area is about the size of a postage stamp (if you remember what that is), whereas in dogs, it would cover a letter-sized sheet of paper. A dog's brain is about one-tenth that of human, but dogs are better equipped to distinguish and remember all kinds of chemical information. Meanwhile, information from vomeronasal sensors travel to the amygdala and hypothalamus, which elicit emotional and behavioural responses.
I am still far from understanding my dog, but I feel I can now at least appreciate how she makes "scents" of the world.