var chapter_list = new Array(
  ["Ch.001.1","c001.1"],
  ["Ch.001.2","c001.2"],
  ["Ch.001.3","c001.3"],
  ["Ch.002","c002"],
  ["Ch.003.1","c003.1"],
  ["Ch.003.2","c003.2"],
  ["Ch.004","c004"],
  ["Ch.005","c005"],
  ["Ch.006","c006"],
  ["Ch.007","c007"],
  ["Ch.008","c008"],
  ["Ch.009","c009"],
  ["Ch.010","c010"]
);

var total_chapters = 13;

for (var i = 0; i < chapter_list.length; i++) {
  document.getElementById('top_chapter_list').options[i] = new Option(chapter_list[i][0], chapter_list[i][1]);
  document.getElementById('bottom_chapter_list').options[i] = new Option(chapter_list[i][0], chapter_list[i][1]);
}
document.getElementById('top_chapter_list').value=current_chapter;
document.getElementById('bottom_chapter_list').value=current_chapter;
var current_chapter_index = document.getElementById('top_chapter_list').selectedIndex;