Passing textarea value to variable

I'm trying to send the contents of a textarea to another page, as per the code below:

       <textarea id="mensagem" rows="8" cols="100" maxlength="500" style="border-radius: 6px; resize:none;width: 90%;height: 65px; margin:0 auto;">
    ESCREVA ALGO AQUI
      </textarea>

<button type="button" class="btn btn-success" id="sendMSG">Enviar</button>

                    <script>
                    $(document).ready(function(){
                    $("#sendMSG").click(function(){

                        var url = "enviamsg.php";
                        var email = "<?php echo $_SESSION['email']?>";
                        var msg = $('#mensagem').val();
                        alert("teste");
                        $.post(url,{postemail:email,postmsg:msg},
                        function(result) {
                            $("#result").html(result); // Só pra verificar retorno
                        });

                    });
                });

            </script>

Clicking on the button, nothing happens. I guess the problem is in the button call because I tried to comment a few lines and leave only a few alerts, and even then nothing happened.  Would anyone know where I'm going wrong?

Thank you.