var chapter_list = new Array(
  ["Vol.01 Ch.001","v01/c001"],
  ["Vol.01 Ch.002","v01/c002"],
  ["Vol.01 Ch.002.1: Zanouka part 1","v01/c002.1"],
  ["Vol.01 Ch.002.2: Zanouka part 2","v01/c002.2"],
  ["Vol.01 Ch.002.5","v01/c002.5"],
  ["Vol.02 Ch.003","v02/c003"]
);

var total_chapters = 6;

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;