This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
The Use Of Automatic And Reflex Centres is to relieve the thinking centres of the vast amount of work which would be thrown upon them if every action of the body each moment had to be planned and willed. Were not the unconscious regulating nerve-centres always at work the mind would be overburdened by the mass of business which it would have to look after every minute. No time would be left for intellectual development if we had to think about and to will each heart-beat, each inspiration and expiration, and the swallowing of each mouthful of food. Moreover, during sleep, so necessary for the rest and repair of the psychic centres, the automatic and reflex centres carry on the actions essential for the nutrition of the body and the maintenance of life. If we had to reason concerning each beat of the heart and decide if it was time for it to occur and what force it should have, and then to make up our minds whether to will it or not, we could never sleep.
Habits are Acquired Reflex Actions, distinguished from primary or those born with us, such as sneezing, coughing, and winking. Every time a nerve-centre acts in a given way it tends to more easily act in that manner again; as a result many actions which are at first only performed with trouble and thought are after a time executed easily and unconsciously. The act of walking is a good instance; each of us in infancy learned to walk with much pains and care, thinking about each step. But the more we walked the closer became ingrained in the nervous system the connection between the stimulation of nerves in the sole when a foot touched the ground, and the sending out by the reflex nerve-centres with which they were in connection, of impulses to those muscles which had to make the next step. At last the contact of the foot with the ground, stimulating some sensory nerves, acts so readily on the " nerve-centres of walking" that the cerebral hemispheres need take no heed about it: we walk ahead while thinking of something else. In other words we have acquired a reflex action not born in us. Other instances will readily come to mind: as the difficulty with which we learned to ride, or swim, or skate, thinking about and willing each movement; and the ease with which we do all these things after a little practice. The trained lower nerve-centres then do all the co-ordinating work and the Will has no more need to trouble about the matter. A habit simply means that the unconscious parts of the nervous system have been trained to do certain things under given conditions, and can only be restrained from doing them by a special effort of the conscious Will. A practised rider will keep his seat unconsciously under all ordinary circumstances, and can only fall off his horse by taking some trouble to do so, by willing it in fact; an unskilled rider, on the other hand, must exert all his attention to avoid falling. So with what in every-day language are called " habits": once we have repeated an action so often that our bodies almost unconsciously do it, it becomes a habit, and needs special exercise of Will to deviate from it. We thus find, in the tendency of the nervous system to go on doing what it has been trained to do, a physiological reason for endeavoring to form good and to avoid bad habits of whatever sort, physiological, business, social, or moral. Every thought, every action, leaves in the nervous system its result for good or ill. The more often we yield to temptation the stronger effort of the Will is required to resist it. The knowledge that every weak yielding degrades our nerve-organs and leaves its trail in the brain, through whose action man is the "paragon of animals," while every resistance makes less close the bond between the feeling and the act for all future time, ought surely to "give us pause"; on the other hand, every resistance of temptation helps to make subsequent resistance easier.
Give other examples of reflex actions.
What is the main use of the automatic and reflex nerve-centres? What would result if the unconscious nerve-centres were not always at work?
What are habits? What happens when a nerve-centre acts in a given manner? What is the result?
Illustrate by the act of walking. Give other examples. What does a " formed habit" really mean? Illustrate.