求大神指点一下，搜索框不在同一水平上

  <div class="nav">

   <div class="nav_left"></div>
   <div class="nav_mid">
     <div class="nav_mid_left">
      <ul>
       <li><a href="#">首页</a></li>
       <li><a href="list.html">关于慕课</a></li>
       <li><a href="list.html">新闻动态</a></li>
       <li><a href="list.html">课程中心</a></li>
       <li><a href="list.html">在线课程</a></li>
       <li><a href="list.html">人才招聘</a></li>
      </ul>
     </div>
     <div class="nav_mid_right">
      <form action="" method="post">
       <input type="text" class="search_text"/>
      </form>
     </div>
   </div><!--nav_mid结束-->
   <div class="nav_right"></div>
  </div><!--nav结束-->
</div><!--wrop结束-->
</body>
</html>

css代码

.nav{
 height:40px;
 }
 
.nav_left{
 width:10px;
 background:url(../image/nav_left.jpg) no-repeat;
 /*background-image:url(../image/nav_left.jpg);
 background-repeat:no-repeat;*/}
 
.nav_mid{
 width:980px;
 background:url(../image/nav_bg.jpg) repeat-x;
}
 
.nav_right{
 width:10px;
 background:url(../image/nav_right.jpg) no-repeat;
 float:left;}
 
.nav_left,.nav_mid,.nav_right{
 float:left;
 height:40px;}
 
.nav_mid_left,nav_mid_right{
 float:left;}
 
.nav_mid_left{
 width:680px;}
 
.nav_mid_right{
 width:300px;}
 
.nav_mid_left li{
 float:left;
 list-style:none;
 width:100px;
 text-align:center;
 line-height:40px;}
 
.nav_mid_left a:link,.nav_mid_left a:visited{
 text-decoration:none;
 color:#FFF;
 font-size:16px;
 font-family:Fixedsys, "Microsoft YaHei UI";}

.nav_mid_left a:hover,.nav_mid_left a:active{
 text-decoration:none;
 color:#FF0;
 font-size:16px;
 font-family:Fixedsys, "Microsoft YaHei UI";}
 
search_text{
 width:190px;
 height:25px;
 margin-top:5px;
 background:url(../image/search.jpg) no-repeat right center;
 }