var chapter_list = new Array(
  ["Vol.01 Ch.001.1: Water Breath","v01/c001.1"],
  ["Vol.01 Ch.001.2: [continuation]","v01/c001.2"],
  ["Vol.01 Ch.002: Pension","v01/c002"],
  ["Vol.01 Ch.003: Dance","v01/c003"],
  ["Vol.01 Ch.004: Longing","v01/c004"]
);

var total_chapters = 5;

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;