3. Not only are such modifications retained permanently, but they give a tendency to repeat the act in the same way; while every such repetition makes the structural modification stronger, and this renders further repetition of the act both easier and more effective.
4. The connections between the various nervous centres thus become so permanent that the action may run its course with a minimum of resistance within the nervous system.
5. In time the movements are so fixed within the system that connections are formed between sensory and motor centres at points lower than the cortex—that is, the stimulus and response become reflex.
An Example.—When a child strives to acquire the movements necessary in making a new capital letter, his eye receives an impression of the letter which passes along the sensory system to the cortex and, usually with much effort, finds an outlet in a motor attempt to form the letter. Thus a permanent trace, or course, is established in the nervous system, which will be somewhat more easily taken on a future occasion. After a number of repetitions, the child, by giving his attention fully to the act, is able to form the letter with relative ease. As these movements are repeated, however, the nervous system, as already noted, may shorten the circuit between the point of sensory impression and motor discharge by establishing associations in centres lower than those situated in the cortex. Whenever any act is repeated a great number of times, therefore, these lower associations are established with a resulting diminution of the impression upward through the cortex of the brain. This results also in a lessening of the amount of attention given the movement, until finally the act can be performed in a perfectly regular way with practically no conscious, or attentive, effort.
Habit and Consciousness.—While saying that such habitual action may be performed with facility in the absence of conscious direction, it must not be understood that conscious attention is necessarily entirely absent during the performance of an habitual act. In many of these acts, as for instance, lacing and tieing a shoe, signing one's name, etc., conscious effort usually gives the first impulse to perform the act. There may be cases, however, in which one finds himself engaged in some customary act without any seeming initial conscious suggestion. This would be noted, for instance, where a person starts for the customary clothes closet, perhaps to obtain something from a pocket, and suddenly finds himself hanging on a hook the coat he has unconsciously removed from his shoulders. Here the initial movement for removing the coat may have been suggested by the sight of the customary closet, or by the movement involved in opening the closet door, these impressions being closely co-ordinated through past experiences with those of removing the coat. When, also, a woman is sewing or kneading bread, although she seems to be able to give her attention fully to the conversation in which she may be engaged, yet no doubt a slight trace of conscious control is still exercised over the other movements. This is seen in the fact that, whenever the conversation becomes so absorbing that it takes a very strong hold on the attention, the habitual movements may cease without the person being at first aware that she has ceased working.
Habit and Nervous Action.—The general flow of the nervous energy during such processes as the above, in which there is an interchange between conscious and habitual control, may be illustrated by the following figures. In these figures the heavy lines indicate the process actually going on, while the broken lines indicate that although such nerve courses are established, they are not being brought into active operation in the particular case.
| A. Sensory Stimulus | A' Higher Motor Centre |
| B. Lower Sensory Centre | B' Lower Motor Centre |
| C. Higher Sensory Centre | C' Motor Response |
The arrows in Figure 1 indicate the course of sensory stimulation and motor response during the first efforts to acquire skill in any movement. No connections are yet set up between lower centres and the acts are under conscious control.
The arrows in Figure 2 indicate the course of sensory stimulus and motor response in an ordinary habitual act, as when an expert fingers the piano keys or controls a bicycle while his mind is occupied with other matters.