Philtrum visible at centre
|Precursor||Medial nasal prominence|
The philtrum (Latin: philtrum, Greek: φίλτρον philtron, lit. "love charm"), or medial cleft, is a vertical indentation in the middle area of the upper lip, common to many mammals, extending in humans from the nasal septum to the tubercle of the upper lip. Together with a glandular rhinarium and slit-like nostrils, it is believed to constitute the primitive condition for at least therian mammals. Monotremes lack a philtrum, though this could be due to the specialised, beak-like jaws in living species.
The human philtrum, bordered by ridges, also is known as the infranasal depression, but has no apparent function. That may be because most higher primates rely more on vision than on smell. Strepsirrhine primates, such as lemurs, still retain the philtrum and the rhinarium, unlike monkeys and apes.
Society and culture
In Jewish mythology, each embryo has an angel teaching them all of the wisdom in the world while they are in utero. The Angel lightly taps an infant's upper lip before birth, to silence the infant from telling all the secrets in the universe to the humans who reside in it; the infant then somewhat forgets the Torah they have been taught. Some believers of the myth speculate that this is the cause of the philtrum, but it does not have a basis in traditional Jewish texts.
In Les Misérables by Victor Hugo, translated by Isabel F. Hapgood, Fantine's philtrum is described thus, "in the very characteristic interval which separates the base of the nose from the upper lip, she had that imperceptible and charming fold, a mysterious sign of chastity, which makes Barberousse fall in love with a Diana found in the treasures of Iconia." Book Third—In The Year 1817, Chapter III—Four And Four. 
In Key Largo (1948), Frank McCloud (Humphrey Bogart) tells a fairy tale to a child, saying that, before birth, the soul knows all the secrets of heaven, but at birth an angel presses a fingertip just above one's lip, which seals us to silence.
In the movie Mr. Nobody, unborn infants are said to have knowledge of all past and future events. As an unborn infant is about to be sent to its mother, the "Angels of Oblivion" lightly tap its upper lip, whereupon the unborn infant forgets everything it knows. The movie follows the life story of one infant, whose lip hadn't been tapped.
- hednk-032—Embryo Images at University of North Carolina
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