急判断是否起讫图片！

急~~判断是否首尾图片！！

代码如下：

<div class="spec-scroll"> <a class="prev">&lt;</a> <a class="next">&gt;</a>

      <div class="items">

        <ul>

          <li><img bimg="../images/facility/siremobil.jpg" src="../images/facility/siremobil.jpg" onmousemove="preview(this);"></li>

          <li><img bimg="../images/facility/20150129140714.jpg" src="../images/facility/20150129140714.jpg" onmousemove="preview(this);"></li>

          <li><img bimg="../images/facility/20150129140324.jpg" src="../images/facility/20150129140324.jpg" onmousemove="preview(this);"></li>

          <li><img bimg="../images/facility/20150129140905.jpg" src="../images/facility/20150129140905.jpg" onmousemove="preview(this);"></li>

          <li><img bimg="../images/facility/20150129140124.jpg" src="../images/facility/20150129140124.jpg" onmousemove="preview(this);"></li>

         

        </ul>

      </div>

    </div>

判断图面是否首末张，如果是的话则替换该方向箭头的class
------解决思路----------------------
判断数量
if($(".items ul li img").length > 5)