The structure indicated is the lunate 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 lunate bone is one of four bones in the proximal row of carpals (scaphoid, lunate, triquetrum, pisiform). The lunate bone is located centrally in the wrist between the scaphoid and triquetral bones. It gets its name from its crescent shape (lunar = relating to the moon). It has the following articulations:
- Proximal: Radius
- Distal: capitate and hamate
- Lateral: scaphoid
- Medial: triquetral
Learn more about the anatomy of the bones of the hand in this tutorial.