var chapter_list = new Array(
  ["Vol.01 Ch.001: Angel's Pocket","v01/c001"],
  ["Vol.01 Ch.002: To You, the Avante-gardes","v01/c002"],
  ["Vol.01 Ch.003: I Have Love","v01/c003"],
  ["Vol.01 Ch.004: 9 o'clock Bomb","v01/c004"]
);

var total_chapters = 4;

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;