Having A Checkbox Send Information To A Textarea

I am setting up a form which will ask the user for several questions after which they will then be saved to a database. It all works well with exception of one thing. I am trying the following and can't get it to work.

What I want to do is to to have multiple checkboxes, each of them activating the same textarea and then add information to the textarea. For activating the textarea I am using the following code:

<script>
function SetState(obj_checkbox, obj_textarea){
if(obj_checkbox.checked){obj_textarea.disabled = false;}
else{ obj_textarea.disabled = true;}}
</script>

And the checkbox has the following attribute:

onclick="SetState(this,this.form.NAMEOFTEXTAREA)"

This works, however, how do I add information to the textarea when each of the checkboxes is checked? I tried obj_textarea.value='VALUE' but this will only allow one checkbox to display its value into the textarea. Code: