One of the most curious things discovered by anatomists is, that many muscles of the face, of great importance in the expression of the emotions, were originally, that is, in the inferior animals, muscles serving a very different purpose. I shall develop this fact briefly, so that the reader may see how difficulties continually increase the nearer science approaches to the origin of things.
We all know that the hedgehog rolls itself up on the approach of danger, its body having the appearance of a ball covered with bristles. This movement is executed by means of a muscle under the skin which covers nearly the whole body, and which contracts in a similar manner to a bag drawn together by a string. Many other animals are similarly furnished with a fine muscular layer which covers the body. In the mole, for instance, this muscular system is well developed. Amongst domestic animals we may mention the dog, the cat, and the horse, in which, although these layers of cutaneous muscles are less compact, they are still sufficiently developed to be noticeable when they enter into action.
We have all remarked the rapid twitching of the skin by which dogs and horses rid themselves of flies. This movement is due to the rapid contraction of one of these muscles. It is easily proved that this is not in reality their office, for they are well developed in birds, fish, and reptiles that do not need to defend themselves from flies in this way.
In all higher animals the traces of organs exist, which remind us of our kinship with the lower animals. Sometimes these organs retrograde from want of use, at other times they remain in existence but fulfil a very different office, and one always much less useful than the original one. Thus the cutaneous muscles still exist under the skin in many parts of the human body, as an inheritance and a sign transmitted to us by generations of animals that have preceded us on the earth.
But the contractions of these muscles are no longer of any actual use. When the nerve-force in emotion spreads from the centre towards the periphery, these muscles, from their position in the skin, produce effects which are more easily seen than in other parts of the body, but which serve no effectual purpose in the struggle for existence and in the preservation of life. In the dog and cat, for instance, the contraction of these muscles in strong emotions causes the erection of the hair on the back, and gives the animal the characteristic expression of attack or defence, of fear or pain. In man, on the other hand, a forcible contraction of the cutaneous muscles of the neck which extend under the skin near the lips gives to the mouth the expression so characteristic of children when about to cry, or when they are trying to restrain their tears. Duchenne de Boulogne studied the function of this muscle, and showed that, when irritated by electric currents, it opens the mouth in the manner of one under the influence of terror.
From Ehlers’ observations on the facial muscles of the gorilla and chimpanzee, we learn that these animals have the same muscles of the face as we have. Ehlers maintains that the statement made by certain authors is not true, namely, that the single fasciculi in the muscular system of the face of these animals are less thick and compact than in man. Only the wrinklings of the brow are less developed, and the muscles round the eye are finer, while those distributed to the nostrils and lips are more highly developed.
It is not true that laughing and weeping are exclusively human. One need only observe attentively the face of a sensitive and faithful dog in order to see the first traces of expressions betraying an altered state of the nervous system. In joyful emotion, as, for instance, when he meets his master, the lips are lifted in such a manner as to uncover the teeth, the head inclines in a caressing attitude, the rhythm of the respiration is modified, and the eyes glisten. Notwithstanding the difference of anatomical structure and the wide dissimilarity of parts, we can yet trace in the dog the rudiments of those involuntary muscular movements which attain their supreme expression in man, in whom a slight movement of the muscles curves the lips into a smile which sheds a ray of benevolence over the whole face and increases the charm of beauty, as though with the breath of love itself.
Darwin wrote many interesting pages about the way in which monkeys laugh. Humboldt observed the eyes of a monkey fill with tears when it was overcome by fear, and Brehm relates that seals weep with pain, and that young elephants when ill-treated shed tears as abundantly as man.
V
Their reasons why changes in the psychical state are reflected are numerous with such facility by the muscles of the face. Besides that of proximity to the nerve-centres propounded by Spencer and Darwin, there is the anatomical fact that the facial muscles have, for the most part, no antagonists. We know that in the hand, for instance, a slight contraction of the muscles which serve to open the hand and extend the fingers, is opposed by the action of the flexors which bend and contract the fingers. In the face the majority of the muscles can act freely, hence a slight nervous shock produces effects far more intense than in the other muscles of the body, in which the slight contraction of muscles acting in a contrary sense must always be overcome.