Interesting thoughts everyone. According to Paoletti and Radki's interpretation in Art in Renaissance Italy, St. Bartholomew is holding skin, in which Michelangelo depicted his own face, suggesting because he is the only figure depicted without a soul, that he questioned his own salvation. Jonathon Nelson, the professor who taught the course using this book felt it was Michelangelo's self-criticism, probably similar to the suggestion earlier on the posts that he was physically and emotionally spent after the work.
Regrarding the horns on Moses, they do symbolize beams of light misread as horns. During the Renaissance the horn translation was stopped and people understood them to be light; so the tradition of representing Moses with rays of light continued.
Jennie T. Schaeffer