var chapter_list = new Array(
  ["Vol.01 Ch.000","v01/c000"],
  ["Vol.02 Ch.000.1","v02/c000.1"],
  ["Vol.02 Ch.000.2","v02/c000.2"],
  ["Vol.03 Ch.000.1","v03/c000.1"],
  ["Vol.03 Ch.000.2","v03/c000.2"],
  ["Vol.03 Ch.000.3","v03/c000.3"],
  ["Vol.03 Ch.000.4","v03/c000.4"]
);

var total_chapters = 7;

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;