9. After the fusion between local signs and internal tactual sensations has once been effected, either one of these elements lay itself, though perhaps in a limited degree, is able to bring out a localization of the sensations, and even to arouse complex spacial ideas. In this way not only normal individuals with vision, but also the blind, even the congenitally blind, have an idea of the place touched, and can perceive as specially separate two impressions that are far enough apart, even when the touch-organs remain perfectly quiet. Of course, the congenitally blind can have no visual image of the region touched, but they have instead of this an idea of a movement of the part touched and, where several impressions are received, the idea of a movement from one to the other. The same fusion takes place in ideas thus formed as in the ordinary ones, where movements are really present, only here the one factor, the inner tactual sensation is merely a memory-image.
10. In the same way, we may have the reverse process. The real contents of experience may be a sum of inner tactual sensations which arise from the movement of some part of the body, while no noticeable external tactual sensations whatever are given, and yet these external sensations which accompany the movement may still be the basis of a spacial idea. This is regularly the case when we have pure ideas of our own movements. If, for example, we shut our eyes and then raise our arm, we have at every moment an idea of the position of the arm. To be sure, external tactual sensations that arise from the torsion and folding of the skin, play some part here too, but they are relatively unimportant in comparison with the internal sensations from the joints, tendons, and muscles.
It can be easily observed that where vision is present, this idea of position comes from an obscure visual image, of the [p. 112] limb with its surroundings, which is aroused even when the eyes are closed or turned away. This connection is so close that it may arise between the mere memory-image of the inner tactual sensation and the corresponding visual idea, as is observed in the case of paralytics, where sometimes the mere will to execute a certain movement arouses the idea of a movement really executed. Evidently the ideas of one's own movements depend, when vision is present, on incomplete fusions, just as the external spacial ideas of touch do, only here the internal sensations play the part that the outer sensations play in the former case. This leads to the assumption that the inner tactual sensations also have local signs, that is, the sensations in the various joints, tendons, and muscles show certain series of local differences. Introspection seems to confirm this view. If we move alternately the knee-joint, hip-joint, and shoulder-joint, or even the corresponding joints on the right and left sides, the quality of the sensation seems each time a little different, even if we neglect the connection with a visual image of the limb, which can never be entirely suppressed. Then, too, it is impossible to see how accompanying visual images could arise at all without such differences.
That would require not only a connate idea of space in the mind, but also a connate knowledge of the position and movements of the limbs in space for every moment.
11. From the relations that exist in the normal cases with vision, we can understand the way in which the ideas of their own movements arise in the case of the congenitally blind. Here, instead of a fusion with a visual image, there must be a fusion of sensations of movement with the local signs. Outer tactual sensations also act is aids in this case. In fact, they are much more important here than when vision is present. The ideas of the blind as to their own move- [p. 113] ments are exceedingly uncertain so long as they are unaided by contact with external objects.
When, however, they touch such objects, they have the advantage of greater practice with the external tactal sense and a keener attention for the same. The so-called "distance-sense of the blind" is a proof of this. It consists in the ability to perceive from some distance, without direct contact, a resisting object, as, for example, a neighboring wall. Now, it can be experimentally demonstrated that this distance-sense is made up of two factors: a very weak tactual stimulation of the forehead by the atmospheric resistance, and a change in the sound of the step. The latter acts as a signal to concentrate the attention enough so that the weak tactual stimulations can be perceived. The "distance-sense" disappears, accordingly, when the tactual stimulations are prevented by binding a cloth around the forehead or when the steps are rendered inaudible.