var chapter_list = new Array(
  ["Vol.01 Ch.001: Op. 1","v01/c001"],
  ["Vol.01 Ch.002: Op. 2","v01/c002"],
  ["Vol.01 Ch.003: Op. 3","v01/c003"],
  ["Vol.01 Ch.004: Op. 4","v01/c004"],
  ["Vol.01 Ch.005: Op. 5","v01/c005"],
  ["Vol.02 Ch.006: Op. 6","v02/c006"],
  ["Vol.02 Ch.007: Op. 7","v02/c007"],
  ["Vol.02 Ch.008: Op. 8","v02/c008"],
  ["Vol.02 Ch.009: Op. 9","v02/c009"],
  ["Vol.02 Ch.010: Op. 10","v02/c010"],
  ["Vol.02 Ch.011: Op. 11","v02/c011"],
  ["Vol.03 Ch.012: Op. 12","v03/c012"]
);

var total_chapters = 12;

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;