// get the manual excerpt
$post = get_post( $page->ID );
$the_excerpt = $post->post_excerpt;

// if manual excerpt is present, show it otherwise first 40 words of the Page content
if ( empty( $the_excerpt ) ) {
	echo '<div class="excerpt">' . excerpt( '40' ) . '</div>';
} else {
	echo '<div class="excerpt">' . $the_excerpt . '</div>';
}