function lx(){
    var gd = $('#pmd').css('margin-top');
    gd = parseInt(gd)-1;
    if(gd<-550){
    $('#pmd').css('margin-top','0px');
    }else{
    $('#pmd').css('margin-top',gd+'px');
    }
    }
var MyMar1=setInterval(lx,20);
$('#pmd').mouseover(function(){clearInterval(MyMar1)});
$('#pmd').mouseout(function(){MyMar1=setInterval(lx,20)});