如何在引导模式对话框中只为模态体设置滚动条

I have following element

我有以下元素

<div class="modal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true"> 
    <div class="modal-dialog" style="overflow-y: scroll; max-height:85%;  margin-top: 50px; margin-bottom:50px;" > 
        <div class="modal-content"> 
            <div class="modal-header"> 
                <h3 class="modal-title"></h3> 
            </div> 
            <div class="modal-body"></div> 
            <div class="modal-footer"></div> 
        </div> 
    </div> 
</div> 
<div class=