The structure indicated is the hyoglossus muscle of the tongue.
The hyoglossus 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:
Origin: Greater horn and body of hyoid bone
Insertion: Lateral aspect of tongue
Action: Depression and retraction of tongue
Innervation: Hypoglossal nerve (cranial nerve XII)