The philtrum is:
1) The dark pigmentation seen in the mouths of some breeds;
2) A live recording of Phil Collins at the Los Angels coliseum;
3) The line separating the left and right upper lip and nostrils of a dog;
4) The freckling occurring just on the bridge of the nose.

So what’s the answer?

The philtrum is the vertical groove between the base of the nose and the border of a dog’s upper lip, and it has a purpose. Every time a dog licks his lips, a bit of saliva stays in this groove and is drawn upwards due to capillary action. This keeps the dog’s nose moist, if not wet, which enables the dog to capture scent particles from the air. Pretty cool, eh?

Image from Wikicommons