https://en.wikipedia.org/wiki/Philtrum
The philtrum (Latin: philtrum from Ancient Greek φίλτρον phíltron, 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."
In most mammals, the philtrum is a narrow groove that may carry dissolved
odorants from
the rhinarium or nose pad to the vomeronasal organ via ducts inside the mouth.
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, 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.[4] Strepsirrhine primates, such as lemurs, still retain the
philtrum and the
rhinarium, unlike monkeys and apes.
https://advetresearch.com/index.php/AVR/article/view/487/432
The philtrum is a median groove in the upper lip of domestic animals
(Nickelet al.,1979). It usually found in animals that possessed a
rhinarium or a nasalplane (NP) such as carnivores and small ruminants
(Nickelet al., 1979; Evans and Christensen, 1979). The nasal plane is a
wet glabrous skin area, which covers the medial wings of the nostrils
(Nickelet al., 1979). The philtrum in such species is deep and sometimes
extends to the nostrils. On the other hand, it’s shallow or absent in
animals that lack NP, a sequine (Nickelet al., 1979). This anatomical
association is also indicating functional correlations between the
philtrum and the NP (Hillenius and Rehorek, 2005). The philtrum proposed
to drain the odoront molecules that dissolved in the fluid covering the NP
to reach the incisive papillae and then into the nasopalatine ducts
(Wöhrmann-Repenning and Bergmann, 2001). While the nasopalatine ducts or
incisive ducts are the oro-nasal passage of the vomeronasal duct system
(VNO), the philtrum thereby is considered the communication canal between
the NP and the VNO (Hillenius and Rehorek, 2005; Eshrah, 2019).