An organ called the olfactory bulb shunts the chemical messages straight to the part of the brain that deals with stored feelings and memories, bypassing the cerebral cortex, the main part of the brain. This short-circuit is one reason smells so rapidly trigger strong emotions and memories that may have lain dormant for years. With its larger olfactory membranes, a dog's nose does amazing things. Researchers at Duke University found that a randomly selected fox terrier could after three weeks detect the scent of a fingerprint on a glass slide when compared to four clean slides. When the researchers placed the slides outside in the rain and dust, the dog was still able to pick out the slide with the fingerprint after 24 hours of weathering.
Dogs have fantastic tracking ability because humans leave a pretty good scent trail.
Because the human body sheds about 50 million cells each minute, rafts fall from the body like a shower of microscopic confetti. Dogs quickly detect these rafts, as well as other scents that may not be apparent to the producer, including breath and sweat vapor. Each person's scent trail is unique, and dogs are remarkably good at separating one person's trail from another's. In an experiment performed a century ago, G. J. Romanes lined up 11 men behind him. He started walking, with each man walking precisely in his footsteps.