Flexor & extensor tendons

Tendines musculorum flexorum et extensorum · Šlachy ohýbačů a natahovačů

Fascia & connective tissue

Looks like

Long cords from the forearm muscles to the fingers — the flexors in sheaths on the palm side, the extensors on the back of the hand.

Bend and straighten the fingers from muscles that sit in the forearm.

Any cut on the palm or finger that leaves a finger unable to bend needs a hand surgeon within days. A finger that clicks or locks is trigger finger.