It will be simpler, and will answer the purpose equally well, to combine the third and fourth elements, and to consider as one element the idea of the movement and the fiat of will to execute the movement.
The Idea of a Movement
The mental picture of a purposed movement is simple and direct. No reference is involved to the muscles concerned in the performance of the movement. When you will to raise your hand, the action is pictured to your mind as the raising of the hand, and nothing more. Certain muscles are to be contracted. But the mental picture of the movement does not indicate what these muscles are, in what order they are to be brought into play, nor the relative degrees of strength to be exerted by each muscular fiber. You do not consciously direct the muscles in their contractions.
The Discharge to the Muscles of the Nerve Impulse
How then are the muscles informed that their contraction is called for? They have no independent volition; each muscular fiber obeys the impulse transmitted to it by the nerve, from the nerve center governing its action. These nerve centers are in their turn controlled by the central nervous mechanism. And in complex voluntary movements the central nervous mechanism is under the control of the higher cerebral centers. The wish to raise the hand appears to the mind as an idea of the hand being raised. This idea is translated by the central nervous mechanism into a set of motor nerve impulses. Does consciousness or volition come into play here? Not at all. On this point Prof. Ladd remarks: "As to the definite nature of the physical basis which underlies the connection of ideas of motion and the starting outward of the right motor impulses, our ignorance is almost complete."
Is it necessary for the performance of a complex muscular action that the individual know what muscles are involved and how and when to contract them? No; this knowledge is not only unnecessary, it is even impossible. Prof. Ladd says of this: "It would be a great mistake to regard the mind as having before it the cerebral machinery, all nicely laid out, together with the acquired art of selecting and touching the right nervous elements in order to produce the desired motion, as a skilful player of the piano handles his keyboard."
How then are the muscles informed of the service required of them? Or more precisely, how does the central nervous mechanism know what distribution of nerve impulses to make among the different nerve centers governing the muscles? As Prof. Ladd says, our ignorance on this point is almost complete. There resides in the central nervous mechanism governing the muscles something which for lack of a better name may be called an instinct. When a purposeful movement of any part of the body is willed, the mental picture of the movement is translated by the central nervous mechanism into a succession of nerve impulses; these impulses are transmitted through the lower centers to the muscles. The instinct informing the central nervous mechanism how to apportion the discharges of nerve impulse among the various muscular centers is to a high degree mysterious. The present purpose will not be served by carrying the analysis of this instinct further.[7]
There is therefore no direct conscious guidance of the muscles, in any movement, simple or complex. So far as the command of voluntary muscular actions is concerned, the first simple statement of the process sums up all that for practical purposes need be determined;—we know what we want to do, and we do it. The mind forms the idea of an action and the muscles instinctively respond.
But the fact remains that the muscles need to be guided in some way. We do not perform instinctively many complex actions,—writing, dancing, rowing, swimming, etc. All these actions, and indeed most of the activities of daily life, must be consciously learned by practice and repeated effort. How are these efforts guided? To arrive at an answer to this question let us consider how a schoolboy practises his writing lesson.
The boy begins by having before him a copy of the letters he is to write. Under the guidance of the eye the hand traces these letters. At each instant the eye points out to the hand the direction in which to move. As the hand occasionally wanders from the prescribed direction the eye immediately notes the deviation and bids the hand to correct it. The hand responds to the demands of the eye, immediately, without thought on the boy's part of nerve impulse or of muscular contraction. By repeated efforts the boy improves upon his first clumsy attempts; with each repetition he approaches nearer to the model.