我的导航栏怎么是这样的

.nav{

height:40px;

}

.nav_left{

width:10px;

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

float:left;

height:40px;

}

.nav_mid{

width:980px;

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

float:left;

}

.nav_right{

width:10px;

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

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-type:none;

width:100px;

text-align:center;

line-height:40px;

}