问Div背景图像Z索引问题

EN

//html
<div id="wrapper">
    <header>
        <div id="logo"></div>
        <nav>
            <ul>
                <li id="aboutNav"><a href="template.html">home</a></li>
                <li id="menuNav"><a href="">menu</a></li>
                <li id="specialsNav"><a href="">specials</a></li>
            </ul>
        </nav>  
    </header>
    <div id="content">
        content <br> goes <br> here <br>
        <a href="http://www.google.ca" target="_blank">google</a>
    </div>
</div>
<footer>
    <div id="thumbsDesc"></div>
    <div id="thumbs"></div>
</footer>



//css
header {
    width: 100%;
    height: 100px;
    background: url(../img/top.png) repeat-x;
    z-index: 110;
}

#wrapper #content {
    color: #FFFFFF;
    background: url(../img/body.png) repeat-y;
    width: 524px;
    padding: 25px 30px 25px 30px;
    position: absolute;
    bottom: 100px;
    top: 90px;
    margin: 0 0 0 150px;
    z-index: 1;
}

footer {
    margin: -107px 0 0 0;
    width: 100%;
    height: 107px;
    background: url(../img/bottom.png) repeat-x;
    z-index: 100;
}