=Basis of Habits.=--The ability of the organism to habituate an action, or make it a reflex is found to depend upon certain properties of nervous matter which have already been considered.
1. Nervous matter is composed of countless numbers of individual cells brought into relation with one another through their outgoing fibres.
2. This tissue is so plastic that whenever it reacts upon an impression a permanent modification is made in its structure.
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.