And just as a single occasion may lead to a passing digestive disturbance, so continual mental depression, worry, or grief may permanently impair the working of the tract and so undermine the vigor and capacity of the sufferer. It is believed that many acute attacks of indigestion in children are due to sharp rebukes administered at the table. _The Psychic Tone._—Just as there is a psychic secretion, so likewise there is a “psychic tone,” or psychic contraction of the gastro-intestinal muscles, as a result of taking food. And just as the secretory activities of the stomach are inhibited by strong emotions, so also are the movements of the stomach. And, indeed, the movements of nearly the entire alimentary canal are almost completely stopped during moments of great excitement. The condition of mental discord may thus give rise to a sense of gastric inertia, so that anxiety may be accompanied by a feeling of weight as if the food remained in the stomach, and every addition of food causes an increase of the distress. =The Importance of the Mechanical Factor in Digestion.=—The mechanical reduction of food in the alimentary canal is preliminary to its actual digestion. The first stage of digestion takes place in the mouth, and this is the only portion of the digestive canal which is under the control of the will. It is here that the food is ground into fine particles by the act of mastication, and the more thoroughly the food is pulverized in the mouth, the more rapidly and easily can it be acted on by the gastric juice, and the very act of chewing increases the flow of saliva as well as shortens the time that the food will remain in the stomach. [Illustration: Fig. 6.—The ribs removed, showing relation of thoracic to abdominal viscera: A, Trachea; B, heart; C, C, lungs; D, liver; E, stomach; F, small intestine; G, large intestine; H, bladder (after Masse). ] The _saliva_ not only protects the mucous membrane of the mouth, but it keeps it free from small food particles, which if allowed to remain would decompose, and thus injure the teeth by the action upon them by the acids produced. It also moistens the dry food, aids in the process of swallowing, and has some action on the starchy substances of the food. By the process of mastication, then, the food is divided into small particles and thoroughly admixed with the saliva until the whole is converted into a fine pulp. Besides favoring the mechanical part of digestion and its slight chemical action on starchy foods, saliva, being an alkaline fluid, is a distinct stimulation to the secretion of gastric juice. After the food has been reduced into a pulp in the mouth, and the change of starches into sugar has begun, it is swallowed and passes into the next compartment of the digestive apparatus, namely, the stomach.