You might have noticed your dog has the ability to move each nostril independently of the other, but that doesn't explain why he needs slits on the outside of each nostril. These slits aren't an accident. They help your dog smell, especially when he's trying to determine where a scent is coming from.
Sense of Smell
A dog's sense of smell is one his most important adaptations. It helps him find food, although that's probably not difficult in your home when you put out the dog dish. His nose helps your pooch distinguish between smells, such as whether the pheromones he detects at a neighbor's tree are from a male or a female that's ready to mate, for example. His sense of smell is much more sensitive than yours; you might smell dinner cooking on the stove, but he's sniffing each ingredient separately.
What Slits Do
The side slits on your dog's nose allow him to exhale without disturbing the scent particles in front of his nose -- the ones that need to come inside to tell him what's going on. The front nostrils are designed to take in air, but the side slits allow air to escape as part of normal respiration. Some air escapes through the front nostrils as well, but the slits release most of it, creating a kind of vortex in front of his nose.
Why They're Important
A dog can tell which nostril is breathing in a specific scent, helping him determine the direction of the smell. Breathing out the side slits helps release air while holding scent particles in the nostrils. It also creates a sort of circular air motion in front of his nose, helping draw more scent particles in with the next breath. This means a dog isn't as likely to lose a scent once he has it.
Harnessing the Smelling Power
Although your dog's nose developed to help him survive, people use the power of dog's sniffers to assist in many emergency situations. Commonly used for search and rescue, tracking dogs help find lost children and adults, and some sniff for drugs or bombs. Others can detect illnesses such as cancer in people. Hunters also use dogs to help find and flush prey.