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.