Philtrum

The philtrum (Latin philtrum, Greek φίλτρον philtron), is a vertical groove in the middle area of the upper lip, common to many mammals, extending from the nose to the upper lip. Technically referred to as the medial cleft, together with a glandular rhinarium and slit-like nostrils, is believed to constitute the primitive condition for mammals in general. The philtrum in most mammals is a narrow groove, and may carry moisture from the mouth to the rhinarium or nose pad to keep it wet through capillary action. A wet nose pad is able to trap odor particles better than a dry one, thus it greatly enhances the olfactory system. For humans and most primates, the philtrum survives only as a vestigial medial depression between the nose and upper lip. The human philtrum, bordered by ridges, is also known as the infranasal depression, but has no apparent function. That may be because most higher primates rely more on vision than on smell and so no longer need a wet nose pad or a philtrum to keep the nose pad wet. Strepsirrhine primates, such as lemurs, still retain the philtrum and the rhinarium, unlike monkeys and apes.