$(function() {	

	$.fn.minHeight = function(option) {
		var obj = $(".middle");
		
		obj.height("auto");
		var oldH = obj.height();
		
		var newH = $(window).height() - $(".head").outerHeight() - $(".footer").outerHeight();
		
		if ( newH > oldH ){
			obj.height(newH)
		}
		
	};
	
});
$(document).ready(function() {

	$(window).minHeight();
	$(window).resize(function(){
	  $(this).minHeight();
	});
	
});