The structure indicated is the genioglossus muscle of the tongue.
The genioglossus muscle is one of the extrinsic muscles of the tongue. The muscles involved with the tongue consist of intrinsic muscles which lie within the tongue itself, and the extrinsic muscles which attach to the tongue and are responsible for depression, elevation, protraction and retraction of the tongue.
There are four sets of extrinsic tongue muscles:
The genioglossus is a large muscle which extends anteriorly from the chin, to the tongue. The genioglossus is involved in depression and protrusion of the tongue.
Origin: Superior mental tubercles
Insertion: Body of hyoid bone and dorsum of tongue
Action: Protrusion and depression of tongue
Innervation: Hypoglossal nerve (Cranial nerve XII)