Philtrum

Overview
The philtrum (Greek philtron, from philein, "to love; to kiss"), also known as the infranasal depression, is the vertical groove in the upper lip, formed where the nasomedial and maxillary processes meet during embryonic development.

The philtrum allows humans to express a much larger range of lip motions than would otherwise be possible, which enhances vocal and non-verbal communication.

Pathology
When these processes fail to fuse fully, a cleft lip (sometimes called a "hare lip") can result.

A flattened or smooth philtrum can be a symptom of Fetal alcohol syndrome.

Etymology
The ancient Greeks used to believe that the philtrum was one of the most erogenous spots on the human body, hence the etymology.

Folklore
According to the Jewish Talmud (Niddah 30b), God sends an angel to each womb and teaches a baby all the wisdom that can be obtained. Just before the unborn baby comes out, the angel touches it between the upper lip and the nose and all that it has taught the baby is forgotten.

Commentries on this particular story can be found in "What the Angel Taught you" by Rabbi Noah Weinberg and Yaakov Salomon (ISBN 1-57819-134-3).

Similarly, in other folksayings, it is said that an angel "shushes" the baby in the womb, to stop it from talking about heaven, or to forget. Other stories say that it is an indent left by the finger of God. Still more say that it is the spot where the angel put his finger to "shush" the child after having told it a secret. (This was memorably referenced in the film The Prophecy by the arch-angel Gabriel (Christopher Walken).)