Dear Susan,
I don't think that the anatomy book referred to in _Tom Sawyer_ can be
Gray's, unless Twain was committing an anachronism.  This year marks the
150th
anniversary of its publication in Britain, but I don't believe it was
published in the U.S. until twenty years later--1878 or even later.   But
there
were a fairly large number of anatomy books available in the first half of
the 19th C, I believe, including some that had been around and in use since
the mid-18th C.  

K. Patrick Ober's excellent _Mark Twain and Medicine_ includes discussion of
some of the books and practitioners who influenced Twain.  Dr. Ober might
 actually know offhand which anatomy is most likely to be the one referred
to in _TS_.  I don't know whether he is still on the list, but you might
email him.

Huck mentions several books that are not "textbooks," per se, but were
common in people's homes, such as _Gunn's Domestic Medicine_, but that may
go far afield from what you're looking for.

A fascinating topic.

Best,
Sharon McCoy