This article was originally published in Positive Health issue 80 – Sept 2002
When it comes to our senses, we usually only speak of five of them: sight, hearing, touch, smell and taste. But in fact we have many more. For example, embedded in our muscles, tendons and joints, are special sensory organs that form what Sherrington called “our secret sense, our sixth sense”, which he termed ‘proprioception’ as it informs us, among other things, about the position of the different parts of our body in relation to one another (from the Latin proprius, own).
There is also another little known, albeit important, sense which deals with equilibrium and orientation. Its receptors are in the vestibular organs (semicircular canals [SCCs], utricle and saccule), a structure sheltered in the inner ear. The vestibular system is also known as the labyrinthine system or apparatus. It is aptly named as it is a rather complex affair in both structure and function. Made of tubes (the SCCs) and sacs (the utricle and the saccule) containing fluid (the endolymph), it works a bit like a spirit level except that you don’t have to look at a bubble to know where you are in relation to gravity. Every time you move your head, the liquid and the otoliths (microscopic particles of calcium carbonate floating in the endolymph of the utricle and saccule) in the canals and chambers stimulate sensory hair cells situated in these tubes and sacs. The hair cells are bent by the moving fluid and send impulses to the central nervous system. The SCCs measure rotational movements of the head while the saccule and utricle act as gravity-orientating organs that measure linear acceleration and deceleration of the head (a whiplash is an acute example of these two movements) and its angles of inclination in relation to gravity. Ears were not evolved for the sole purpose of hearing!
Like all mammals, we possess hidden, and nestled deep in our inner ears, three SCCs on each side, one for each plane of space, each at right angles to the other two, which is handy as they thus inform us that space has three dimensions. But they do much more than that.
Although the more recent – evolutionally speaking – visual sense has usurped to some extent the functions of the older labyrinthine, the latter is still the main source of sensory information for balance.
Labyrinthine disturbance can indeed be very crippling as anyone who suffers from Menière’s (NB not Mézières!) or Parkinson’s disease could witness. But when all is well, our mechanics of equilibrium, posture and movement depend on the harmonious co-ordination of different senses – the visual, the proprioceptive and the vestibular – and when one of these is deficient the others can compensate to a certain extent.
The horizontal or lateral SCC is roughly on the same plane as an imaginary line drawn from the external corner of the eye to the external auditory meatus (the opening of the ear). Now, “In animals, the lateral SCC is in the horizontal plane whatever is the habitual carriage of the head, which varies with each species”, says André Thomas. For example, the camel, who holds its head high up (which gives it a certain arrogant air), and the bison who holds it towards the ground, both have their lateral canals situated on the horizontal plane.
Not in humans though! According to Cunningham’s Textbook of Anatomy, “The lateral SCC… slopes upwards and forwards at approximately 30º to the horizontal when the head is erect.” And, according to Delattre and Fenart, when the sight line points to the horizon, the canal is at an angle of 28º to 37º to this plane. This is why, when physiologists want to study the labyrinthine mechanics of equilibrium in humans, prior to carrying out their tests they tilt the head of their subjects forwards to around 30º in order to restore their SCC to the horizontal.
The question is whether this human exception to the horizontality of the lateral SCC is normal or abnormal. L Girard, who studied the canals of many animals, thinks that the habitual poise of the head which in humans gives sloping lateral SCCs might not be the normal one. Delattre and Fenart also are of the opinion that it is highly improbable that at any stage of its evolution the human species would have had sloping lateral SCCs in the neutral position of the head. Two observations seem to corroborate this point of view:
If you look at babies from the time they can hold their heads up you will notice that they hold them in a more forward position than adults, and I have been told that the lateral SCCs of babies and toddlers are horizontal when the head is erect. This apparently poses a problem in the case of glue ear (it just shows that ears haven’t been designed to secrete ‘glue’);
The head of an astronaut in a zero gravity environment, where there is no need for muscular effort to keep it in a balanced position, is tilted forward around 25º relative to the floor.
Because the centre of gravity of the head is located forward of its fulcrum, the head would drop on the chest (as when we fall asleep in a sitting position) if it were not for the posterior neck muscles that counteract gravity. “This… explains the constant tone in these… muscles”, says Kapandji. Unfortunately, the muscles of the nape of the neck, like all posterior uscles, are too zealous in their action and end up hypertonic and shortened with the head pulled back into an incorrect position. No wonder the ‘horizontal’ SCCs in humans are not horizontal. This is no trivial matter. It “…puts a strain on the nervous system and generates chronic muscular tension and disturbed bodily balance. It also hinders the capacity of the muscle system to react readily and adequately to a change in head position…”, says Jean Hiernaux.
F M Alexander got it right as his method induces a proper head poise. Head forwards and up wins, head backwards and down loses.
Notes and References
1. Sherrington Sir Charles Scott. 1857-1952. English physiologist, awarded the 1932 Nobel prize for physiology or medicine jointly with Lord Adrian for the isolation and functional analysis of the neurone.
2. Thomas André. Equilibre, équilibration. Masson. Paris. 1940.
3. Romanes GJ (ed.). Cunningham’s Textbook of Anatomy. Oxford University Press. 1981.
4. Delattre A and Fenart R. L’hominisation du crâne étudiée par la méthode vestibulaire. Paris. CNRS. 1960.
5. Girard L. Atlas d’Anatomie et de Médecine opératoire du labyrinthe osseux. 1939.
6. Kapandji A. The Physiology of the Joints. Vol. 3. Churchill Livingstone. 1974.
7. Hiernaux J. Natural head poise and urban-industrialized life. Current Anthropology. 25(3). June 1984.