(Chapter 25) Footnote 51:

Straus and Temkin 1943 p. 630 describe this os Vesalianum carpi as “an extremely rare anomaly, in fact one of the most exotic of all skeletal aberrations,” and they cite research documenting its presence in only two of 1,456 European hands studied. They characterize Vesalius’ account here as “dominated by teleology,” noting that Vesalius downplays its rarity and exaggerates its importance as a support for the fifth metacarpal bone. Cf. Chapt. 33 n. 22 below (p. 148 of the 1543 Fabrica) for the corresponding anomalous sesamoid bone in the foot, the os Vesalianum tarsi.