THE MECHANISM OF HABIT. The implications of habitual behavior can better be understood after a brief analysis of the mechanism of such action. An instinct has been defined as a tendency to act in a given way in response to a given stimulus. What happens when a stimulus prompts the organism to respond in a given way, is that some sensory nerve, whether of taste or touch or sound, sight, smell, or muscular sensitivity, receives a stimulus which passes through the spinal cord to a motor nerve through which some muscle is "innervated" and a response made. In the simplest type of reflex action, such as the winking of an eye in a blinding light, or the withdrawing of a hand from flame, such is the physiology of the process. But where an immediate adjustment cannot be made by an instinctive response, where satisfaction is not secured by the passage of a sensory stimulus to an immediate motor response, the nervous impulse is, as it were, deflected to the brain area, auditory, visual, or whatever it may be, which is associated with that particular type of sensation. The path to the brain area is far from simple; the nervous impulse, which might be compared to an electric current, must pass through many nerve junctions known as "synapses," at which points there is some not completely understood chemical resistance offered to the passage of the nerve current. On passing through the network of nerves in the brain area, the current passes back again through a complicated maze of connections to a motor nerve which insures a muscular response. The first time a stimulus passes through this network the resistance offered at the nerve junction or synapse is very high; at succeeding repetitions of the stimulus the resistance is reduced, the nerve current passes more rapidly and fluently over the paths it has already traveled, and the action resulting becomes as direct and automatic as if it were an original reflex action.[1]
[Footnote 1: See McDougall: _Physiological Psychology_.]
THE ACQUISITION OF NEW MODES OF RESPONSE. Expressed in less technical language this means simply that human beings can learn by experience, and that they tend to repeat actions they have once learned. Where an animal is perfectly adjusted to its environment, all stimuli issue in immediate and nicely adjusted responses. This happens only where the environment is very simple and stable, and where in consequence no complexity of structure or action is necessary. In the clam and the oyster, and in some of the lower vertebrates, perhaps, instinctive activity is almost exclusively present. But in the case of man, so complicated are the situations to which he is exposed that random instinctive responses will not solve his problems. He must, as with his highly modifiable nervous system he can, acquire new modes of response which will, in the complexity of new situations serve as effectively as his original tendencies to act would serve him in a simpler and stabler environment. A human being in a modern city cannot live by instinct alone; he must acquire an enormous number of habits to meet the variety of complex situations he meets in daily life. A monkey exists with fairly fixed native tendencies to act. But civilization could never have developed if in man new ways could not be acquired to meet new situations, and if these new ways could not be retained and made habitual in the individual and the race.