The structure indicated is the pisiform bone of the hand.
There are three groups of bones in the hand:
- Carpal bones (8 in total)
- Metacarpal bones
The carpal bones are separated into two rows:
- Proximal row
- Distal row
The pisiform bone is one of four bones in the proximal row of carpals (scaphoid, lunate, triquetral, pisiform). The pisiform bone is a tiny sesamoid bone located in the tendon of the flexor carpi ulnaris muscle. It articulates with the palmar surface of the triquetral bone.
Learn more about the anatomy of the bones of the hand in this tutorial.