Accessing Instances within a MC

Not sure where the difference lies here, but I am trying to understand it. In this piece of code that is a public function of a MC with instances of other MCs in it, I cannot seem to access the instances except in a function within the public function (confused? so am I but here’s the code):

  public function TitleFlight(stageX=800, stageY=800):void{
   var dTimer1:Timer = new Timer(5);
   x = -600;      
   y = stageY + 600;
   gotoAndStop(100);
   [COLOR=red]head.teeth.gotoAndStop(5);[/COLOR]
   scaleX = 2.5;
   scaleY = 2.5;
   dTimer1.start();
   dTimer1.addEventListener(TimerEvent.TIMER, SpinDude);
   function SpinDude(e:TimerEvent):void{
    rotation += 1;
    [COLOR=lime]head.teeth.gotoAndStop(5);[/COLOR]
    armL.rotation += Math.random()*2 + 1;
    armR.rotation += Math.random()*2 + 1;
   }
  }

[COLOR=black]the line in green works fine (like the rotation calls below it), but if I try the same thing (as above in the red font) out side of the SpinDude function, I get this error:[/COLOR]

TypeError: Error #1009: Cannot access a property or method of a null object reference.
 at Actors::TheDude/TitleFlight()
 at Main/::TitlePage()
 at Main$iinit()

Can anyone tell me why?