The structure indicated is the abductor hallucis muscle of the foot.
The intrinsic muscles on the plantar aspect of the foot are divided into four layers. The abductor hallucis muscle is found in the first layer of muscles. There are three muscles in the first layer:
- Abductor hallucis
- Flexor digitorum brevis
- Abductor digiti minimi
The abductor hallucis muscle is located in the medial border of the foot and is responsible for the palpable bulk of muscle that you can feel on the inside of the foot. It inserts via a common tendon, together with the flexor hallucis brevis, onto the base of the proximal phalanx of the great toe.
Origin: Medial process of calcaneal tuberosity
Insertion: Medial plantar portion of proximal phalanx of great toe
Action: Abducts great toe at MTP joint and flexes great toe at MTP joint.
Innervation: Medial plantar nerve
Learn more about the muscles of the foot in this tutorial.