The structure indicated is the palmaris longus muscle of the forearm.

The palmaris longus is one of four muscles in the superficial layer of muscles in the anterior compartment of the forearm. The anterior compartment of the forearm consists of three layers of muscles:

1. Superficial layer
2. Intermediate layer
3. Deep layer

Four muscles are located in the superficial layer (from lateral to medial):

1. Flexor carpi ulnaris
2. Palmaris longus
3. Flexor carpi radialis
4. Pronator teres

All four muscles of the superficial layer have a common origin on the medial epicondyle of the humerus.

Origin:  medial epicondyle of humerus

Insertion: distal half of flexor retinaculum and palmar aponeurosis

Action: flexion of wrist. Tightens palmar aponeurosis

Innervation: Median nerve

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