(Chapter 33) Footnote 23:

This is the os vesalianum tarsi, a sesamoid fibrocartilage which occasionally ossifies and develops in the portion of the tendon of m. fibularis [peroneus] longus that passes over the surface of the cuboid bone in its course towards an insertion in the base of the first metatarsal bone. For the corresponding bone in the hand, see ch. 25 above, n. 49.