(Chapter 26) Footnote 17:

“The part between carpos and fingers is called the metacarpos. It is synarthrosed to the carpos but diarthrosed to the first phalanges, a name given to the finger-bones. Only the thumb has its first phalanx diarthrosed to the metacarpos at the side. Of the fingers, each is formed of three bones, the end of each phalanx fitting into the cavity placed at the beginning of the next. It seems reasonable to say that the thumb too is made up of three bones and not to add a first metacarpal to it, for it has a diarthrosis at both ends, as with the phalanges but not as with the metacarpals. Thus some would reckon the metacarpals as four, and the bones of the five fingers as fifteen. Alternatively some reckon fourteen phalanges and five metacarpals.” De ossibus 19 (2.771.7-772.7), tr. Singer 1952, p. 774.