如何用for 循环得出多个span的值??

<body>
<form id="Form1" action="">
<span id ="sp1">Label1</span><br>
<span id ="sp2">Label2</span><br>
<span id ="sp3">Label3</span><br>
<span id ="sp4">Label4</span><br>
<span id ="sp5">Label5</span><br>
</form>
<script>
spans = document.getElementById("Form1").getElementsByTagName("SPAN")
for(i=0;i<spans.length;i++)
alert(spans[i].innerHTML)
</script>
</body>