Philtrim and alar folds……..it occurs to us that in our discussion about a dog’s nose, we’ve not yet mentioned the external nares. The whole sniffing process begins with those nares (or nostrils) which, of course, which leads to the nasal passages. The outer nares are mobile and allow for expansion on inspiration and contraction (breathing in and out) to prevent the entry of unwanted objects, say, an umbrella (just checking to see if you’re paying attention). When a dog sniffs, he inhales the delectable scented chemicals into his nasal cavities where they’re trapped in mucus and processed by the dog’s sensory cells. You’ve read this part before, but remember that expiration forces air out the side of the dog’s nares so that the exit doesn’t interfere with odors from the air or ground ready to be inhaled. Put another way, if our noses were built the same way, it would help the smell of rotten eggs exit our nose without interfering with the smell of vanilla extract that we’re about to smell.