Neurologically speaking, two systems are at work in the middle ear.
The first is under control of the facial nerve which innervates the muscle of the stirrup,
The second system is under the control of the trigeminal nerve, or forth cranial nerve, which innervates the muscle of the hammer.
This is extremely important when we use a neurological approach to understand how the ear functions; we realize that the human ear is decided into two parts and not three as usually thought. The first part includes the inner ear, which is regulated by the muscle of the stirrup. This muscle is located in the middle ear and is innervated by the facial nerve, which simultaneously controls all the muscles of the face and the platysma of the neck. The second part includes the external ear, in particular the tympanic membrane or eardrum which is regulated by the muscle of the hammer. This muscle is innervated by the trigeminal nerve, which also controls the muscles of mastication.
So, on one hand, the stirrup and its muscle maintain a stable pressure of the liquids within the ear. On the other hand, the hammer and anvil regulate the tympanic pressure in response to the sounds that one wishes to perceive.
—Alfred A. Tomatis, "The Ear and the Voice," Scarecrow Press, 2004: 59
The action of the muscles of the face and neck are integrated with the muscle of the stirrup? Really? Are you kidding me?
No, I am not.
The student who comes into the studio after a really bad day, and looks and sounds terrible is not making things up. Rather, the ear of the student had been impacted by events, auditory or otherwise, that keep the envelope of the ear from opening which it must for singing to occur.
Where am I going here? It's simple really: Many old school manuals indicate that the opening of the mouth must be towards a smile. In Tomatis terms this instruction can be viewed as an expression of an open ear. It's not any more complicated than that until the student imposes this position onto his face in mechanical fashion.
The truth is, the ear can't be fooled, which is easily deduced by anyone with eyes to see and ears to hear. The tell-tale sign? The conformation of the eyes. You've seen this when meeting someone who really doesn't want to meet you, and their mouth moves into a smile while their eyes stay dead. Listen carefully and you'll hear a dead tone too.
Believe it not, this illustrates the huge difference between the modern school which too often in relies on positioning and the old school which—literally— relied on the ear.
Helping the student's ear to open? That takes great skill and patience on the part of the voice teacher. Learning how to keep it open and give expression to real feeling in spite of the vicissitudes of life? That's the real art.