The varying resistance which the different nerves of the organism oppose to the nervous currents is an important factor in expression. The proximity of the muscles of the face, and especially of the eyes, to the brain renders the nervous discharges easier. Death always begins in the parts furthest removed from the centre, the legs grow rigid sooner than the arms, and the eye is the last to be extinguished.
The subject at present under consideration is a field of study which physiologists have perhaps too much neglected. Johannes Müller,[20] the father of modern physiology, in speaking of those 'movements which depend upon mental conditions,’ expresses himself in the following manner: 'The extremely varied expression of the lineaments of the face in different passions, shows that, according to the various states of the mind, entirely different groups of fibres of the facial nerve are brought into activity. The reasons of this phenomenon, of these relations of the facial muscles to special passions, are totally unknown.’
Wishing to make a few experiments on the facial nerve, to see whether I should succeed in discovering anything in this obscure field of physiology, I laid bare the facial nerve, at its point of departure from the skull, in a dog rendered insensible with chloral, and then fixed two electrodes in such a manner that I was sure of being able to irritate the whole nerve by means of an electric current. While using irritants so weak that they were imperceptible on the tongue, I observed that they could cause a contraction of the muscles of the forehead and make the ears move, while the whole muzzle remained still as in an animal in an attentive attitude. When I used a slightly stronger stimulus, the muscles of the nose and eyelids and the zygomatic muscle moved; when the irritant was still more intensified, the muscles of the under-lip contracted and the mouth opened; while under very strong irritations, the dog assumed the fierce expression of one about to attack.
There is something fantastic in those experiments on decapitated animals of which the brain has been destroyed, and the face of which may be taken up in the hand like a mask of flesh. While applying an electric current to the motor nerves, one sees the features reanimate themselves, and a series of expressions pass over them one after another—attention, joy, rage, as though the electric apparatus applied to the facial nerve represented the commands of the brain or psychic impressions which in reality no longer exist.
The mechanical part of expression is, therefore, much simpler than one thinks. When a psychic operation takes place in the nerve-centres, the tension propagates itself along the nervous lines of least resistance. The more sensitive we are, the more graceful, beautiful, expressive, and fascinating is the curving of the lips produced by a smile. Peasants and coarse, dull persons cannot smile, with them the stimulus increases until it bursts out in a noisy, vacant laugh.
The nerve-paths are constructed in such a way that the brain does not need trouble itself about the muscular movements. It is the intensity of the excitement which produces the expression; the stronger it is, the more numerous are the paths through which the nervous tension forces its way; as it increases, it overcomes all obstacles and resistance confronting it in other paths impassable till that movement, and contracts muscles which till then had remained neutral.
The effects of the passions are reflected principally in the muscles of the face and respiration. No other function has to adapt itself more continuously than this last to the needs of the organism, standing as it does in close connection with all changes taking place in the nerve-centres. The muscles most vividly expressing passion are nearly all respiratory muscles.
IV
Our nervous system is so constituted that during violent emotion its activity discharges itself in all directions, and herein must we seek the reason of the resemblance between such different conditions as laughter and weeping, pain and pleasure.
It is the quantity, not the quality, of the stimulus which has weight on the scale of the expressions. This statement of mine will appear clearer if we study the phenomena produced by tickling.