When animals look attentively at some object, they turn their ears towards it. This movement, which they make in order to collect the sounds, must be preceded by a contraction of the muscles of the forehead and of those serving to turn the auricle of the ear. It is very probable that these movements, noticeable also in monkeys, have been preserved in man, although in attention he no longer moves the ears but only the muscles of the forehead.
In our nature psychic processes are so closely connected with their external sensory manifestations that it is impossible to check the manifestation of nervous activity in the muscles whenever the ideas appear to which these external movements stand in a permanent relation, even when this external communication is quite unnecessary. Thus we see that a man lost in thought gesticulates, making a hundred involuntary movements, and sometimes speaks, although no one is with him to whom he need communicate his ideas. And so it happens that we reproduce the characteristic movements of attention in forehead and eye whenever, in the various contingencies of life or in the development of ideas, an obstacle hinders the progress of thought. As soon as we begin any work which demands greater force of attention and reflection, we immediately and involuntarily put into action the mechanism of forehead and eye, which has always been made use of in intently scrutinising objects.
IV
All will have noticed that when we look intently at anything, all other objects become the more indistinct the further they are removed from the point of attention. This is because we have only one point in our eye in which vision reaches a maximum of acuteness. This point is called the fovea centralis, because it looks like a little dimple or funnel of two-tenths of a millimetre in diameter. If the image of an object falls at a distance of only a few millimetres from the fovea centralis, the eye can no longer accurately distinguish the colours. Red and green give an impression of palish yellow, violet appears blue. A little further distant, yellow and blue disappear completely, and only light and dark are perceived. This anatomical disposition of the elements destined to perceive the image and colour of objects obliges us to move the eye and bring it into relation with all parts of an object if we wish to examine it minutely. On this account no organ has such precise movements as the eye. If we look at our eye in a mirror, and move our head up and down, to the right and to the left, we see to our astonishment that the eye can remain fixed and motionless. Let the reader repeat this experiment in order to conceive the facility and precision with which the eye fixes on one point which we wish to look at attentively. The restlessness of the eye contemplating an unknown figure, the agitation which is visible in a man when he is afraid of another, and therefore examines him from head to foot in order to be ready to defend himself, or escape an impending danger, is an inevitable consequence of the structure of the eye, which cannot contemplate and embrace a wide field without moving.
When the object is not small enough to be embraced by the simple movements of the eye in its orbit, we bend and turn the head, or move the trunk to right or left; if that does not suffice, we move the whole body. Actors represent fear by exaggerating the attitude peculiar to one intently observing an object.
These movements are so spontaneous and natural, that it costs an effort to keep head and body still when looking at an object situated on one side of us. A feeling of profound contempt, hatred, or pride is necessary before we can pass close to a man with head stiff and erect.
V
Anyone who studies the parts of a machine can judge of the accuracy of its movements, because the structure of a machine represents its function; the dead organism is, therefore, no less important a field for observation and study to the physiologist than the living organism.
When we see, on opening the skull, that three nerves leave the brain to move the eye, and that six muscles are attached to this little ball weighing on an average seven grammes, we may conclude at once that perhaps no other organ has the same variety, independence, and rapidity of movement.
The eye is indeed unrivalled in the complication of its muscles and the number and variety of its nerves by any other organ except the tongue. This explains why both have their own language, and how they are able, by the infinite variation of their movements, to express every emotion of the mind.