The most important parts of the face are the apertures of the mouth and nostrils. These alone never disappear, however the form of the head may alter in different animals. The lips, nose, and chin may become unrecognisable, as in birds; the eye may become a mere point, as in the mole, or may disappear altogether as in certain animals living in caves; but the mouth always remains, because the alimentary canal is the most useful organ of the body. It appears even in animals that have neither heart nor lungs, and is formed like a funnel at its upper end. It is this end of the alimentary canal which we call the face. However grotesque such a mode of expression may seem, it is yet the expression of truth.
The development of the facial muscles is proportioned to the need of seizing prey and crushing the food. In frogs, fish, reptiles, birds, that swallow their food whole, one may say that the face is wanting; they have no expression except in the eye. In birds, the functions of the facial nerve are restricted to a little filament distributed to the cutaneous muscles of the neck, which produces that ruffling of the feathers and erection of the crest which is the characteristic expression of their feelings. The more complex the movements of seizing and devouring the prey become, the more complicated becomes the formation of the mouth. The lips must be mobile in order to suck the nipple of the breast, in the manner of a cupping-glass. Later, they serve to bring the fragments which must be masticated between the jaws, and further, they must be capable of being drawn upwards, as in the dog when he shows his teeth in preparing to bite.[18]
Then come the movements of the jaws furnished with fangs for tearing, crushing, breaking, gnawing, and again the very complex movements of the tongue in drinking, licking, collecting the food in the mouth, forming it into a bolus, and finally despatching it.
Of all animals, monkeys possess the greatest development of the facial muscles. This is owing principally to the circumstance that they eat everything, being half carnivorous, half herbivorous, and make use of the mouth as an organ for seizing the prey, and assisting the hands in tearing, skinning, and continually preparing the food.
The countenance of the monkey is of unexampled mobility; in a few minutes one sees all expressions pass over it, from desire to contempt, from cunning to innocence, from attention to carelessness, from love to rage, from aggression to fear, from joy to sadness.
III
One of the reasons why the facial muscles move more easily, is their diminutive size. It was Spencer who first clearly developed this idea, and I know of nothing more fundamental in the language of the emotions. 'Supposing,’ he says, 'a feeble wave of nervous excitement to be propagated uniformly throughout the nervous system, the part of it discharged on the muscles will show its effects most where the amount of inertia to be overcome is least. Muscles which are large, and which can show states of contraction into which they are thrown only by moving limbs or other heavy masses, will yield no signs; while small muscles, and those which can move without overcoming great resistances, will visibly respond to this feeble wave. Hence must result a certain general order in the excitation of the muscles, serving to mark the strength of the nervous discharge and of the feeling accompanying it.... It is because the muscles of the face are relatively small, and are attached to easily moved parts, that the face is so good an index of the amount of feeling.’[19]
This law, however, is in my opinion insufficient to explain the expressions of the face, because we have very fine, small muscles in the ear, the skin, and elsewhere, that yet take no part in the expression, although the resistance they offer is very small.
Great importance must, I think, be attached to the continual use of certain muscles, and to the different excitability of their nerves. The muscles which we most frequently put into movement are also those which most easily betray the excitement of the nerve-centres. It is so with the ear of the horse and dog, which is a faithful mirror of everything they feel, of all their emotions; while the ears of man, although possessing the same muscles, remain immovable even during the strongest emotion, and solely because we never make use of them.
The facial muscles are agitated by every little shock which the nervous system receives, because they are already perpetually in movement in respiration, speaking, chewing, and in the defence and use of the organs of sense situated in the head. We very often meet people who, in consequence of increased irritability of the nerve-centres, suffer from nervous contractions of the face, which make them wink rapidly, contort the mouth and frown, but we never notice similar disturbances in hands or feet, or in any other part of the body.