(Chapter 25) Footnote 13:

In the hand, sesamoid bones embedded in tendons are found only on the palmar surface of the joints. Two (medial and lateral) are constant at the metacarpophalangeal joint of the thumb; one is frequently present at the metacarpophalangeal joint of the little finger and the index finger. Occasionally, sesamoid bones are found at the metacarpophalangeal joints of the middle and ring fingers and at the interphalangeal joints of the thumb and index fingers. Gray 1995, p. 736.