Sending input by onclick

Is this possible to add to onclick the comment that is in the textarea in the same form? and how do i get it in javascript function?

        <form id='comments' method='post'>
        <textarea rows='8' cols='80' name='comments'></textarea> <br />
        <input type='submit' name='send' onclick='sendcomment(".$photoid.",".$mynick.",".$_POST['comments'].")' value='Wyślij'>
        </form>

 <script>   
 function sendcomment(photoid, mynick,comments){ }  
 </script>