Physiological Fatigue
The pupil’s ability to work is determined by certain physiological conditions which should be understood by every teacher. These conditions can be described in a brief study of the physiology of fatigue.
Any animal tissue, as, for example, a muscle, is a storehouse of energy. Through nutrition the muscle tissue is kept in condition to contract. Whenever the muscle contracts, it uses up its own substance; it burns up its tissues to a limited degree. In the process of thus consuming its material the muscle gradually becomes clogged with waste products. It is the business of the circulatory system in a living organism to carry away this waste material and thus free the muscle from the effects of its contraction. The circulatory system also brings new materials in the form of nutrition to restore the depleted tissues. The restoration of the tissue through nutrition is not the demand which is most urgent in the case of a muscle which is called on to contract for a long period of time. Sooner or later the muscle must, indeed, be brought back to its normal state of nutrition, but during actual contraction the most immediate physiological problem is to keep it clear of its own waste products. If these waste products are not removed, they tend to interrupt further contraction by preventing the nerve fiber which enters the muscle from discharging motor impulses into the muscle. If stronger nervous impulses are sent through the nerve fiber, the muscle, even though it is somewhat clogged, will be found in a condition to contract with its original vigor; but if the nervous impulses are not increased, the contractions gradually diminish in intensity. This is a condition of muscular fatigue, and is to be distinguished from exhaustion, which does not set in until the substance of the muscle has been used up to a point which endangers the tissue.
Fatigue is nature’s effort to protect tissues against any possibility of excessive use. Fatigue sets in at a period long before danger to the tissue is at hand. The overcoming of fatigue is dependent in all cases on the power to dispose of waste products. The athlete, for example, becomes a better runner chiefly through a training of his organism to carry away waste products. The untrained individual grows stiff and sore from exercise, not because his muscles are used up, but because his muscles are clogged with waste substances.
This description of muscular fatigue lays the foundation for an understanding of the problem of nervous fatigue. The nerve cells, like the muscles, get clogged by the products of their own action. They then fail to carry nervous impulses freely, and the individual can do his mental or physical work only with excessive effort. Fatigue of nerve cells means that nature has limits of work in these cells. Fortunately, the limit is reached long before exhaustion or other real dangers set in.