var chapter_list = new Array(
  ["Vol.04 Ch.019","v04/c019"],
  ["Vol.04 Ch.020","v04/c020"],
  ["Vol.04 Ch.022","v04/c022"],
  ["Vol.04 Ch.024","v04/c024"],
  ["Vol.05 Ch.025","v05/c025"],
  ["Vol.05 Ch.026","v05/c026"],
  ["Vol.05 Ch.027","v05/c027"],
  ["Vol.05 Ch.028","v05/c028"],
  ["Vol.05 Ch.029","v05/c029"]
);

var total_chapters = 9;

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;