Kaki Mudra काकी मुद्रा
Kaki काकी (kākī) is the female crow. The term also refers to the Ayurveda plant kakoli (Roscoea purpurea) and, finally, it is personified as a daughter of Kazyapa by Tamra and mother of crows and owls. This mudra is also known as the "tongue rolling gesture."
Kasyapa was the son of Marici, who was the son of Brahma. Kasyapa’s wife Tamra had many daughters like Kaki, Syeni, Bhasi, Grdhrka, Suki and Griva. From Kaki were born the crows in the world.”
Agni Purana, Chapter 19 (Google Reference)
"Shape your lips like a crow's beak" and then stick your tongue out and roll iy into a U-shape, in which the sides of the tongue are curled upwards so that it resembles a tube open in its end.
This mudra is particularly associated with Shitali (refreshing breathing) or Vatasara Dhauti.
More generally, it is associated with all practices requiring a concentration on the heart center.
The tongue consists of sixteen muscles.
The hypoglossal nucleus contains motor neurones innervating the muscles of the tongue via the hypoglossal nerve.
The caudal raphe also play a role in controlling the tongue.