var chapter_list = new Array(
  ["Vol.01 Ch.000: Chapters 0 - 7","v01/c000"],
  ["Vol.02 Ch.000: Chapters 8 - 15","v02/c000"],
  ["Vol.03 Ch.000: Chapters 16 - 23","v03/c000"]
);

var total_chapters = 3;

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;