Consider what took place in the simple action. In the first instance, a desire arose in your mind to take up the book. The brain is the organ of the mind; and having branches either proceeding from itself, or from the spinal cord, to every part of the body—branches that traverse like telegraphic wires throughout every part of the system,—it transmitted instructions along the nerves that proceed to the muscles of the arm and hand, directing them to take up the book. This was done instantly; and as soon as it was done you became conscious that your will had been obeyed—because the nerves sent back a sensation to the brain acquainting it that the book had been taken up, and that at the moment of the dispatch it was in the firm hold of the hand.
In all the varied motions of the body this double action of the nerves takes place. It is obvious that without an outward impulse from the brain, upon which the desire of the mind first made an impression, no motion of the muscles of the arm and the hand could have taken place; and it is also obvious that without an inward impulse from the nerves to the brain you would not have known that the muscles had fulfilled your instructions. The hand might have dropped by the side of the book, or have gone too far, or not far enough, and you would not have been aware of the result, but for an inward communication through the nerves.
We are not now speaking of the nerves which endow us with the sense of feeling, because they are regarded as separate and distinct from those nerves that produce in us consciousness of muscular response. When we walk, rise, or sit, we are made conscious, without any special feeling being exerted, that the muscles have placed the limb, or the body, in the desired position, that it is set down safely and firmly, and that we may repose upon it securely without further attention. We refer the impressions made by the book upon the nerves of the hand, and which enable us to tell whether it feels hot or cold, whether its surface is rough or smooth, and so on, to the special sense of feeling. The consciousness of muscular action is a separate and distinct function; and it is generally believed that the same nerves that convey the command of the will outward, bringing back the intimation that the will has been obeyed, but that different fibres of the nerves convey the outward and the inward impulses. A single nerve may therefore be likened to a double wire connected with the electric telegraph: one transmitting despatches in one direction, and the other in the opposite direction.
961. What are the nerves of special sense?
The nerves of special sense are those through which we hear, see, feel, smell, and taste.
"For the Lord seeth not as man seeth; for man looketh on the outward appearance, but the Lord looketh on the heart."—Samuel xvi.