The structure indicated is the flexor digiti minimi brevis muscle of the foot.

The intrinsic muscles on the plantar aspect of the foot are divided into four layers. The flexor digiti minimi 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

Origin: Base of 5^th metatarsal

Insertion: lateral base of proximal phalanx of little toe

Action: flexes little toe at metatarsophalangeal joint

Innervation: lateral plantar nerve

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