为什么我的怎么改导航栏都不对？

.wrapper{

width:1000px;

margin:0px auto;

}

.logo{

height:80px;

background-color:#FFF;

}

.logo_left{

width:200px;

float:left;

}

.logo_right{

width:300px;

float:right;

height:28px;

margin-top:30px;

}

.logo_right img

{

vertical-align:middle;

margin-right:5px;

}

.nav

{

height:40px;

}

.nav_left

{

width:10px;

height:40px;

background:url(../images/nav_left.jpg) no-repeat;

float:left;

}

.nav_mid

{

width:980px;

height:40px;

background:url(../images/nav_bg.jpg) repeat-x;

float:left;

}

.nav_right

{

width:10px;

height:40px;

background:url(../images/nav_right.jpg) no-repeat;

float:left;

}

.nav_mid li

{

float:left;

list-style:none;

}