A pretty demonstration of this process has been worked out in the electrical stimulation of muscle. If a muscle, say the biceps, is irritated by an electric current, it will contract. As the strength of the current is increased, the degree of contraction becomes greater. A sort of stepladder effect of increasing contractions may be thus obtained. After a time, the electric shocks cannot cause a greater contraction, but only a lesser. And if continued, the muscle will cease to function because of fatigue. If now, when the muscle begins to lag in its response, and its contractions to decrease, one injects into a vein extracts of thyroid, parathyroid, or adrenal glands, they will immediately reinvigorate the failing contractions. The injections must be made before the fatigue is carried to the point of absolute exhaustion. It follows that these glands normally pour into the circulation substances which counteract the effect of fatigue substances, and in fact make possible muscular recuperation from fatigue throughout the day as well as in emergencies and crises. Fatigue, conventionally recognized, is something acute and urgent. As such it means a violent draining of the endocrine wells. But there is also a chronic fatigue, which has been dignified with the name of Fatigue Disease. Bernard Shaw once asked for someone to tell him the name of the germ causing the symptoms of overwork. That being impossible, he will have to be satisfied with the answer that it is not a germ, but an internal secretion, or rather a defect of internal secretion that is the cause. Whether or not the adrenals have been damaged by past experiences, and upon their capacity to respond to the necessities of an occasion, fatigue reactions primarily depend. A quotation from Sir James MacKenzie, most distinguished of modern English students of medicine, summarizes the matter neatly. "Abelous, and Langlois and Albanese have studied the relation of the adrenal bodies to fatigue.... They infer that the muscular weakness following removal of the adrenals is due to toxic substances. In view of our present knowledge of the physiological action of adrenaline in its various forms, it seems more probable that the weakness is to be explained by the absence of the normal tone producing internal secretions of the bodies in question." In other words, the adrenals regulate muscle tone. They produce nature's tonics for weary tissues. The chronic lassitude of thousands of our generation, suffering from "that tired feeling," may be put down to chronic adrenal insufficiency.