var checkflag = "false";
function check(field)
 {
  if (checkflag == "false") 
   {
    for (i = 0; i < field.length; i++) 
     {
      field[i].checked = true;
     }
    checkflag = "true";
    return "Desmarcar Todas"; 
   }
  else 
   {
    for (i = 0; i < field.length; i++)
     {
      field[i].checked = false;
     }
    checkflag = "false";
    return "Marcar Todas"; 
   } 
 }