var pageArray = new Array ("index", "1p", "11", "12", "13", "14", "15", "16", "17", "18", "1a", "19", "110", "111", "112");

var thisIndex = -1;

for (i=0; i<pageArray.length; i++) {
 if (pageArray[i] == pageId) {
  thisIndex = i;
 }
}

if (thisIndex > -1) {
 if (thisIndex != 0) {
  document.write('<a href="page' + pageArray[thisIndex-1] + '.shtml">');
  document.write('<img src="images/button-back.gif" id="buttonprev" /></a>');

 }
 if (thisIndex != pageArray.length-1) {
  document.write('<a href="page' + pageArray[thisIndex+1] + '.shtml">');
  document.write('<img src="images/button-next.gif" id="buttonnext" /></a>');

 }
}