Psychology · William James
CONSCIOUSNESS AND MOVEMENT.
There are probably no exceptions to the diffusion of every impression through the _nerve-centres_. The _effect_ of a new wave through the centres may, however, often be to interfere with processes already going on there; and the outward consequence of such interference may be the checking of bodily activities in process of occurrence. When this happens it probably is like the siphoning of certain channels by currents flowing through others; as when, in walking, we suddenly stand still because a sound, sight, smell, or thought catches our attention. But there are cases of arrest of peripheral activity which depend, not on inhibition of centres, but on stimulation of centres which discharge outgoing currents of an inhibitory sort. Whenever we are startled, for example, our heart momentarily stops or slows its beating, and then palpitates with accelerated speed. The brief arrest is due to an outgoing current down the pneumogastric nerve. This nerve, when stimulated, stops or slows the heart-beats, and this particular effect of startling fails to occur if the nerve be cut.
In general, however, the stimulating effects of a sense-impression proponderate over the inhibiting effects, so that we may roughly say, as we began by saying, that the wave of discharge produces an activity in all parts of the body. The task of tracing out _all_ the effects of any one incoming sensation has not yet been performed by physiologists. Recent years have, however, begun to enlarge our information; and we have now experimental proof that the heart-beats, the arterial pressure, the respiration, the sweat-glands, the pupil, the bladder, bowels, and uterus, as well as the voluntary muscles, may have their tone and degree of contraction altered even by the most insignificant sensorial stimuli. In short, a _process set up anywhere in the centres reverberates everywhere, and in some way or other affects the organism throughout, making its activities either greater or less_. It is as if the nerve-central mass were like a good conductor charged with electricity, of which the tension cannot be changed at all without changing it everywhere at once. Herr Schneider has tried to show, by an ingenious zoölogical review, that all the _special_ movements which highly evolved animals make are differentiated from the two originally simple movements of contraction and expansion in which the entire body of simple organisms takes part. The tendency to contract is the source of all the self-protective impulses and reactions which are later developed, including that of flight. The tendency to expand splits up, on the contrary, into the impulses and instincts of an aggressive kind, feeding, fighting, sexual intercourse, etc. I cite this as a sort of evolutionary reason to add to the mechanical _a priori_ reason why there _ought_ to be the diffusive wave which _a posteriori_ instances show to exist. I shall now proceed to a detailed study of the more important classes of movement consequent upon cerebromental change. They may be enumerated as-- 1) Expressions of Emotion; 2) Instinctive or Impulsive Performances; and 3) Voluntary Deeds; and each shall have a chapter to itself.
Text from Project Gutenberg, public domain in the United States.