Spencer and Darwin gave no proof of their statements, so that it devolves upon us physiologists to discover experimentally whether the intuition of these great philosophers is correct. Darwin was, as usual, very cautious, and in Chapter III., in which he treats of the general principles of expression, after having mentioned the above theory, says: 'Our present subject is very obscure, but, from its importance, must be discussed at some little length; and it is always advisable to perceive clearly our ignorance.’
III
I made a few experiments in order to see whether in reality, as Spencer assumes, there exists a difference of conductibility amongst the various nervous filaments which move the muscles of the face, and which, as we know, are united into one bundle called the facial nerve. I irritated this nerve at its point of departure from the skull, and near the ear, where it is most easily isolated. There is no need to describe the process of irritating the nerves by electric currents. Galvani began in the last century to produce contractions of the muscles by means of electric currents originated by the contact of two metals; and we all know that the contractions of the legs of some skinned frogs, observed by Galvani in Bologna while he was hanging them on the iron railings in his garden, were the beginning of one of the greatest conquests of science, and one of the discoveries which have exercised the greatest influence on civilisation.
The apparatus made use of to irritate the nerves is an invention of Professor Du Bois-Reymond. Many who are not physicians will yet know the apparatus, which is often used in the cure of diseases by electricity; its greatest advantage consists in the facility with which the intensity of the electrical stimulus may be increased or diminished.
After having produced such a profound sleep in a dog by means of chloral that he was insensible, I tried to irritate the nerve which moves the muscles of the face by a very weak electric current. At first the current was so weak that no effect was visible, but as it was increased, a slight contraction of the cutaneous muscle of the neck appeared, and a little movement at the corner of the mouth. It might perhaps be of use to explain here how, in the muscles situated under the skin of the neck, other facial muscles originate, amongst others the so-called risorial muscle; but in order not to interrupt the relation of this experiment, I shall reserve these remarks till the beginning of the next section. The intensity of the electric current when the slight movement of the mouth appeared was equal to 400 units.
If the current is increased, the movement of the lips becomes more apparent, and when the electric irritation of the facialis reaches the intensity of 700, a contraction of the orbicular muscle of the eyelids appears, which closes the eyes. At an intensity of 750 the muscles which elevate the upper lip contract. At 820 the nostrils are dilated and elevated. At 950 the contraction of the lips becomes so pronounced that the dog shows his teeth, and his face assumes an aggressive expression. At 1,250 there is a depression of the corners of the mouth, as though produced by pain and disgust. At 1,500 this expression becomes more intense, and the eye is forcibly shut. If the stimulus is still further intensified, the face assumes the fierce expression of an animal about to attack.
I obtained the same results with the animal soon after death.
These experiments show that Herbert Spencer’s hypothesis is correct; but we shall presently see that the matter is exceedingly complicated, and that we must take into account other factors no less important in the expression of the face.
IV
The muscles of the face have certainly not the office, as Duchenne de Boulogne thought, of expressing the passions of the soul. To speak frankly and without sentimentality, pedantry, or conventionality, we must recognise that the most important feature of the face is the mouth, and that the mouth is a funnel of flesh attached to the alimentary canal. Sometimes it only serves to seize the prey and receive the food before sending it to the stomach, as is the case in fishes, reptiles, and birds, in which the face is reduced to a minimum. As the apparatus for mastication becomes more complicated with the appearance of teeth for cutting and crushing the food, and of lips for sucking, drinking, and closing the mouth, the more complicated also does the structure of the face become.