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

The intrinsic muscles on the plantar aspect of the foot are divided into four layers. The flexor digitorum brevis muscles is found in the first layer of muscles. There are three muscles in the first layer:

1. Abductor hallucis
2. Flexor digitorum brevis
3. Abductor digiti minimi

The flexor digitorum brevis muscle lies above the  central part of the plantar aponeurosis and just below the tendons of flexor digitorum longus.

Origin: medial process of calcaneal tuberosity and plantar aponeurosis.

Insertion: Plantar sides of middle phalanges of lateral four toes

Action: flexes lateral four toes at proximal interphalangeal joint

Innervation: medial plantar nerve from tibial nerve.

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