着急！怎么得到表格里某个单元格内的控件对象？？？

Question

<table width="100%" border="0" cellspacing="0" cellpadding="0" id=t1>
  <tr>
    <td>Test</td>
    <td>&nbsp;</td>
    <td>&nbsp;</td>
  </tr>
  <tr>
    <td>&nbsp;</td>
    <td>
        <input name="Q" type="checkbox" id="Q" value="1">
</td>
    <td>A</td>
  </tr>
  <tr>
    <td>&nbsp;</td>
    <td>
        <input name="M" type="text" id="M">
</td>
    <td>&nbsp;</td>
  </tr>
</table>
<button onclick="alert(t1.rows[1].cells[1].children[0].value)" >测试值</button>
<button onclick="alert(t1.rows[1].cells[1].children[0].checked)" >测试是否选中</button>