function nav_over(care){
    var id_but = "b" + care;
    var new_src = "./images/" + id_but + "_ov.png";
    document.getElementById(id_but).src = new_src;
}
function nav_out(care){
    var id_but = "b" + care;
    var new_src = "./images/" + id_but + ".png";
    document.getElementById(id_but).src = new_src;
}