$(function($){
    $("#thumb-next").mousedown(function(){
        scrolling = true;
        startScrollingNext($("#thumb-list"), "-=20px");
    }).mouseup(function(){
        scrolling = false;
    });
});

function startScrollingNext(obj, param) {

    var thumbLeft = $('#thumb-list').css('left');
    var thumbLeft = parseInt(thumbLeft);

    if (thumbLeft >= -526) {
        if ( !scrolling ) {
            obj.stop();
        } else {
            obj.transition({'left': param}, 10, function(){
                if ( scrolling ) {
                    startScrollingNext(obj, param);
                }
            });
        }
    }
}