The structure indicated is the flexor hallucis brevis muscle of the foot.

The intrinsic muscles on the plantar aspect of the foot are divided into four layers. The flexor hallucis brevis muscle is located in the third layer of intrinsic foot muscles. There are three muscles located in this layer:

1. Flexor hallucis brevis
2. Adductor hallucis
3. Flexor digiti minimi brevis

The flexor hallucis brevis muscle splits into two parts which insert onto the medial and lateral aspects of the base of the first phalanx of the great toe.

Origin: Plantar surface of cuboid and lateral cuneiform bone

Insertion: Lateral and medial sides of base of proximal phalanx of the great toe

Action: Flexes great toe at metatarsophalangeal joint

Innervation: medial plantar nerve

Learn more about the anatomy of the foot muscles in this tutorial.