Xml/flash gallery arrows issue

I’ve been using kirupa’s flash and xml tutorial to build a little photo gallery, and everything is coming out great. The only thing left is to make the next and previous arrows disappear when displaying the last and first images. In the same way that the arrows don’t move along a node when clicked, I would like to actually make them disappear.
I’m sure I can plug in a little function with next_btn._visible = false or something, but I don’t know where to put it! Could someone look at my code?
Much appreciated.

function loadXML(loaded) {

if (loaded) {

xmlNode = this.firstChild;
image = [];
description = [];
total = xmlNode.childNodes.length;
for (i=0; i<total; i++) {

image* = xmlNode.childNodes*.childNodes[0].firstChild.nodeValue;
description* = xmlNode.childNodes*.childNodes[1].firstChild.nodeValue;

}
firstImage();

} else {

content = “file not loaded!”;

}

}
xmlData = new XML();
xmlData.ignoreWhite = true;
xmlData.onLoad = loadXML;
xmlData.load(“images.xml”);
/////////////////////////////////////
listen = new Object();
listen.onKeyDown = function() {

if (Key.getCode() == Key.LEFT) {

prevImage();

} else if (Key.getCode() == Key.RIGHT) {

nextImage();

}

};
Key.addListener(listen);
previous_btn.onRelease = function() {

prevImage();

};
next_btn.onRelease = function() {

nextImage();

};
/////////////////////////////////////
p = 0;

this.onEnterFrame = function() {

filesize = picture.getBytesTotal();
loaded = picture.getBytesLoaded();
preloader._visible = true;
if (loaded != filesize) {

preloader.preload_bar._xscale = 100*loaded/filesize;

} else {

preloader._visible = false;
if (picture._alpha<100) {

picture._alpha += 10;

}

}

};
function nextImage() {

if (p<(total-1)) {
p++;
if (loaded == filesize) {
picture._alpha = 0;
picture.loadMovie(image[p], 1);
desc_txt.text = description[p];
picture_num();
previous_btn._visible = true;

}

}

}

function prevImage() {

if (p>0) {

p–;
picture._alpha = 0;
picture.loadMovie(image[p], 1);
desc_txt.text = description[p];
picture_num();
next_btn._visible = true;

}

}
function firstImage() {

if (loaded == filesize) {

picture._alpha = 0;
picture.loadMovie(image[0], 1);
desc_txt.text = description[0];
picture_num();
previous_btn._visible = false;

}

}
function picture_num() {

current_pos = p+1;
pos_txt.text = current_pos+" / "+total;

}