var chapter_list = new Array(
  ["Vol.01 Ch.001: File 1","v01/c001"],
  ["Vol.01 Ch.002: File 2","v01/c002"],
  ["Vol.01 Ch.003: File 3","v01/c003"],
  ["Vol.01 Ch.004: File 4 (Part One)","v01/c004"],
  ["Vol.01 Ch.005: File 4 (Part Two)","v01/c005"],
  ["Vol.01 Ch.006: File 4 (Part Three)","v01/c006"],
  ["Vol.01 Ch.007: File 5 (Part One)","v01/c007"],
  ["Vol.01 Ch.008: File 5 (Part Two)","v01/c008"],
  ["Vol.01 Ch.009: File 5 (Part Three)","v01/c009"]
);

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;