jQuery怎么筛选出table里被check选中的行对象

<div class="field_line">
  <div class="field_content">
    <div class="st_con" style="width: 100%;">
      <table class="subtable">
        <thead>
          <tr>
            <td>
              <input type="checkbox" name="PRall">                   </td>
            <td width="90">姓名</td>
            <td width="90">年龄</td>
        </thead>
        <tbody id="">
          <tr>
            <td>
              <input type="checkbox"></td>
            <td></td>
            <td>小明</td>
            <td>20</td>
          <tr>
            <td>
              <input type="checkbox"></td>
            <td></td>
            <td>小红</td>
            <td>21</td>
        </tbody>
      </table>
    </div>
  </div>
  <div class="clear"></div>
</div>