In reply to Nick Huffman, it depends whether he is asking about the "chapter
headings" or the introduction to the chapters -- the "Chapter I" and onward
were drawn by E. W. Kemble and sometimes incorporated into the initial
letter.  Some of the "chapter headings, on the other hand,  were written by
Twain as were some of the captions for the illustrations.  For the
introductions to the chapters, when the illustrations were deleted, the
format changed for the various editions.  As for the "chapter headings," some
of them are very interesting and add a bit of humor to the chapter.  Check
some of them in the reproduced first edition done by Oxford University Press.
 I hope this answers some of the questions.