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.
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.
The stomach may be felt at the lower extremity of the breast bone, in the triangular space formed by the divergence of the ribs. It is a large hollow, compound gland, the walls of which contain muscular fibers in addition to the tubules which elaborate the special secretions. Its cavity is lined with a thick mucous membrane, packed with tubular glands, into which is poured out a complex secretion called the gastric juice.
The Chemical Changes which Food Undergoes in the Stomach.—There are two chief phases or periods of gastric secretion: (a) The psychic or appetite juice, and (b) the chemical juice. Gastric juice is not constantly poured into the stomach to accumulate there, but is secreted only as it is needed under the influence of certain stimuli. These stimuli may be classified as psychic and chemical.
In the second phase, or period of gastric digestion, the exciting agent is the presence of food in the stomach. It has been shown that a correlation of the different organs of the body is brought about by hormones or chemical messengers. The hormones of gastric digestion are produced from proteins in the early stage of gastric digestion, and this shows the importance of the psychic or appetite juice. A similar effect is produced by meat extracts or infusions, which are well-known provocatives of gastric secretion.
Gastric Juice.—The principal active agents of the gastric juice are hydrochloric acid and the enzymes; pepsin, the proteolytic enzyme; rennin, the milk-curdling ferment; and lipase, the fat-splitting ferment. The uses of the gastric juice are digestive, activating, and disinfecting.