The structure indicated is the temporalis muscle.

This muscle is one of the muscles of mastication. The muscles of mastication include:

• Masseter
• Temporalis
• Medial pterygoid
• Lateral pterygoid

The temporalis muscle is a large fan-shaped muscle which lies in the temporal fossa above the zygomatic arch.

Origin: temporal line of parietal bone. Temporal surface of sphenoid bone

Insertion: coronoid process of mandible and ramus of mandible

Innervation: mandibular branch of trigeminal nerve (cranial nerve V)

Action: Elevation of the mandible and retraction of the mandible (via posterior horizontal fibres)

To learn more about the muscles of mastication, check out this tutorial!