Serpent jaws enclosing a human head are known collectively as a Vision Serpent, an important actor in the vision quest bloodletting ritual performed by the new ruler before being publically enthroned.
The serpent is a conduit linking the underworld with the human realm, a road for the divine ancestor to come back to the world and accept/bless the new ruler as his successor.
There are several such sculptures of serpent jaws with human heads in the Puuc Region. They are found in corners and entablatures of public buildings, for example in the sub-structure of the Great Pyramid of Uxmal.
The composition in the southwest corner of Room 18 of the Palace in Labna is not a sculpture, but a highly elaborated mosaic mask which includes a crocodilian monster with fins, stars, vegetation and feathers which represent a combination of celestial, terrestrial, and aquatic worlds.
As an interesting detail, there is a human face carved at the basal corner molding below the vision serpent.
Most probably the carved face symbolizes an important ancestor of the ruler who promoted the construction of this building, consecrated on September 19, 862 AD. Similar ancestor heads were found in the basal molding of the older throne anteroom, Room 23.