var chapter_list = new Array(
  ["Vol.01 Ch.001","v01/c001"],
  ["Vol.01 Ch.002","v01/c002"],
  ["Vol.01 Ch.003.1","v01/c003.1"],
  ["Vol.01 Ch.003.2","v01/c003.2"],
  ["Vol.02 Ch.004.1","v02/c004.1"],
  ["Vol.02 Ch.004.2","v02/c004.2"],
  ["Vol.02 Ch.005.1","v02/c005.1"],
  ["Vol.02 Ch.005.2","v02/c005.2"]
);

var total_chapters = 8;

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;