In follow-up of Sharon's comments...
The 2007 Norton Critical Edition of _The Adventures of Tom Sawyer_ (ed.
Beverly Lyon Clark) sheds some light on the question of the anatomy
In a discussion of the background and context of _Tom Sawyer_, page 208
depicts an anatomic drawing of a genderless human being with chest
cavity and abdominal cavity opened (although I would note that several
important organs are lacking). The picture is labeled speculatively as
"The Page That Becky Tore?" The drawing was used as the frontispiece in
Calvin Cutter's 1848 book, _Anatomy and Physiology_ (a book that Twain
may have owned, according to Alan Gribben).
Along similar lines, as noted by Sharon, Huck Finn referred to _Gunn's
Domestic Medicine, or Poor Man's Friend, in the Hours of Affliction,
Pain and Sickness_. First published in 1830, it was the leading domestic
medical reference in the West. (According to Huck, "Dr. Gunn's Family
Medicine...told you all about what to do if a body was sick or dead.")
The book was a medical do-it-yourself book, written so that the average
citizen could do his own doctoring without resorting to a physician.
Even surgical procedures such as amputations of arms and legs could be
done by anyone who would follow the simple instructions in the book,
"unless he be an idiot or an absolute fool," according to Gunn. To
perform an amputation, for example, nothing was required beyond some
common household items (including a carving knife, penknife, and
shoemaker's awl), and no particular skills were needed except "firmness
and common dexterity."
Nowadays, few of my patients seem to be reading Dr. Gunn's book, but
rely mostly on the Internet for such thoughtful medical guidance.