The next effect of acts of expression is to be found in the modifications made in the sensory areas of the brain. You will recall that every movement of a muscle produces nervous currents which go back to the brain and register there in the form of kinaesthetic sensations. To demonstrate kinaesthetic sensations, close your eyes and move your index finger up and down. You can feel the muscles contracting and the tendons moving back and forth, even into the back of the hand. These sensations ordinarily escape our attention, but they occupy a prominent place in the control of our actions. For example, when ascending familiar stairs in the dark, they notify us when we have reached the top. We are still further impressed with their importance when we are deprived of them; when we try to walk upon a foot or a leg that has gone "to sleep"; that is, when the kinaesthetic nerves are temporarily paralyzed we find it difficult to walk. But besides being used to control muscular actions, they may be used in study, for they may be made the source of impressions, and impressions, as we learned in the chapter on memory, are a prime requisite for learning. Each expression becomes, then, through its kinaesthetic results, the source of new impressions, when, for example, you pronounce the German word, _anwenden_, with the English word "to employ," in addition to the impressions made through the ear, you make impressions through the muscles of speech (kinaesthetic impressions), and these kinaesthetic impressions enter into the body of your knowledge and later may serve as the means by which the word may be revived. When you write the word, you make kinaesthetic impressions which may later serve as forms of revival. So the movements of expression produce sensory material that may serve as tentacles by means of which you can later reach back into your memory and recall facts.