Definition. Digestion is the breaking up, changing, and liquefying of the food in the various chambers of the alimentary canal designed for that purpose. The mechanical breaking up is done principally by the teeth in the mouth, the chemical changes and liquefying by the various digestive fluids.[19]
Digestive Fluids. The digestive fluids are true secretions. Each is formed from the blood by a special gland for the purpose which never does anything else; they do not exist in the blood as such. Their flow is intermittent, taking place only when they are needed. The liver, however, is an exception to all the others. It is both secretory and excretory, and bile is formed all the time, but is most abundant during digestion.[20]
Saliva. The fluid which is mixed with the food in the mouth is secreted by a considerable number and variety of glands, the principal of which are the parotid, submaxillary, and sublingual. Smaller glands in the roof and sides of the mouth, in the tongue, and in the mucous membrane of the pharynx contribute to the production of saliva, the digestive fluid of the mouth. The flow from the parotid gland is greatest. The flow from all the glands is greatly increased when food is taken, especially if it be of good flavor. Sometimes the amount is increased by smell alone, as when a nice steak is cooking, or a savory soup, and sometimes the saliva is made copious by thought, as when we remember the taste of dishes eaten in the past, and we say, "It makes the mouth water just to think of them."
Amount of Saliva. According to Dalton the amount of saliva secreted every twenty-four hours is 42½ oz. Its reaction is almost constantly alkaline. It is composed of water, organic matter, and various mineral salts. Ptyalin is its active principle, and is called by some authors animal diastase, or starch converter.
Gastric Juice. Gastric juice is the digestive fluid of the stomach. It is acid. Its flow is intermittent, occurring only at times of digestion. Its active principle is pepsin.
It is worthy of notice here that the character of the digestive fluids when food is taken is different from what it is when the organs are at rest. For instance, the gastric juice which flows in abundance under the stimulus of food, is not like the fluid secreted when the stomach is collapsed and empty.
Pancreatic Juice. Pancreatic juice is the digestive juice of the pancreas, and is poured into the small intestine a short distance below the pyloric opening. Its reaction is alkaline. Its flow is entirely suspended during the intervals of digestion.
Bile. Bile, the fourth in order of the digestive liquids, is the secretion of the largest gland of the body—the liver. It is poured into the small intestine by a duct which empties side by side with the duct from the pancreas. The flow of bile is constant, but is greatest during digestion.
Intestinal Juice. Intestinal juice has been to physiologists a difficult subject of study. It is mingled with the salivary and gastric juices at the times of digestion, when it is most desirable to notice its action. Nearly all authorities agree that it is alkaline, and that its function is to complete the digestion of substances which may reach it in an undigested condition.