Paul McPharlin provides an extensive history of manicules starting on page 47 of his 1942 book, Roman Numerals, Typographic Leaves and Pointing Hands [1].

"Why did so many readers, until relatively recently, take the trouble to stop and draw a whole hand when they simply wanted to mark something as important? And why did so many authors and printers use the increasingly heavy-handed image of the manicule to direct the attention of their readers?" William H. Sherman explains in his 2005 paper, Toward a History of the Manicule [2].

[1] http://hdl.handle.net/2027/uc1.b4200870

[2] http://www.livesandletters.ac.uk/papers/FOR_2005_04_001.pdf