Movements of the stomach slowly mix the food with gastric juice. This is a clear, colorless fluid, strongly acid in reaction and possessing a characteristic odor. It is secreted by about five million microscopic glands situated in the walls of the stomach, and contains an enzyme known as pepsin which acts upon proteins and acts only in an acid medium. Besides pepsin, it contains two other enzymes--renin, which coagulates the casein of milk, and gastric lipase, a fat-splitting enzyme. It also contains mineral matters and hydrochloric acid (commonly known as muriatic acid and used as a soldering acid) which is very powerful and literally eats to pieces the food it permeates. It would soon destroy the stomach except for the fact that its walls are continually protected by an alkaline secretion. This alkaline bath in which the stomach is kept is analogous to the water bath some furnaces have to be kept in to prevent them from melting.
Gastric Secretion: Gastric Secretion is divided into:
(1) Continuous secretion: Gastric secretion seems to be continuous, but the juice secreted in an empty stomach is less acid than that produced during digestion. Continuous secretion is absent during fevers, gastritis and other gastric inflammations. (Fasting is indicated.)
(2) Appetite juice: Gastric juice is poured out in response to hunger and the sight, smell, taste and thought of food. Miller and others have shown that the sight of food is a more powerful stimulus to gastric secretion than odor. It is more important that food is pleasing to the eye than to the nose. Unpalatable food produces little or no appetite juice, though it may ultimately be well digested. It is important that our food be palatable--that we relish it. When the tongue is coated, so that food flavors cannot be appreciated by the nerves of taste, the gustatory reflexes are destroyed, appetite juice is not formed and digestion is suspended. (A fast is indicated.) Appetite juice is either greatly diminished or entirely absent in gastritis or any inflammatory disorder of the gastric mucosa, as well as in fevers. It is also stopped by pain and strong emotions, and by fear and anger. (Fasting is indicated.) Worry and mental strain cause delay in the secretion of appetite juice and hinder digestion.
(3) Chemical Secretion: Gastric juice is poured out in response to the presence of food and to by-products of the process of digestion--particularly by gastrin, a hormone formed when protein is brought into contact with normal gastric juice. Chemical secretion is arrested by fever, especially by high fever. The injection of gastrin under the skin or into the vein of a healthy subject causes an active secretion of gastric juice. This does not occur if fever is present. (Fasting is indicated.)
Gastric juice is the product of six different sets of glands.
Three sets of glands secrete enzymes--pepsin, lipase and renin or chymosin.
One set secretes mucus.
One set secretes hydrochloric acid.
One set secretes a serous fluid, termed diluting juice, which serves to regulate the acidity and digestive activity of the juice.
About three pints of gastirc juice are secreted in twenty-four hours. About one and a half pints are required to digest a hearty dinner. The normal stomach produces about two thirds of an ounce of hydrochloric acid in twenty-four hours. The amount varies with the food eaten.
The amount of pepsin contained in a pint and a half of gastric juice produced in twenty-four hours is about seven and one-half grains. About four grains of pepsin are contained in the twenty or more ounces of gastric juice required to digest a hearty dinner, or enough to digest two-thirds pound of egg white, or three and a half pounds of dried albumen. The daily production of pepsin is sufficient to digest four or more times the amount of protein required by the body. Undigested starch tends to absorb pepsin and interferes with gastric digestion.
Pepsin is not active except in the presence of hydrochloric acid. Excessive gastric acidity prevents the action of pepsin--excess acid destroying the pepsin. Drug acids and fruit acids also demoralize gastric digestion.
The acidity of gastric juice is determined by the food eaten. Meat causes the production of a gastric juice similar to that in dogs. Pavlov, Rehfus and Hawk have shown that animal foods call for stronger acid juice than vegetable foods--the average acidity of beef is 120, eggs 80, vegetables 70. Milk calls for greater acidity than eggs, bread and cereals the lowest degree.
The secretion of gastric juice is in response to the higher centers, as these are set in motion by the taste and odor of food, and is poured into the stomach in advance of the food. It is poured out in response to substances requiring its action and variously modified to meet the requirements of various kinds of foods. If starch or other non-protein foods are eaten a gastric juice will be secreted which differs from that poured out upon proteins. As previously noted the taste lends aid in regulating its secretion, as do also the sight and smell of food.
Food taken into the mouth causes a flow of gastric juice, even if the food is not swallowed. Eager desire for food will do the same. But no amount of chemical and mechanical stimulation of the buccal membranes is capable of reflexly exciting the nerves of the stomach.
Gastric juice is not poured out in response to the presence of acid in the mouth. Salines, bitters, pepper, mustard, etc., taken into the mouth, do not result in the secretion of gastric juice. Mechanical and chemical stimulants applied to the mouth and its glands do not occasion any gastric flow.
The old fallacy that salt, pepper, mustard and other condiments and bitters aid or stimulate digestion is thus seen to be false. Active digestive juices are secreted only in response to and are modified to meet the requirements of the food substances requiring their action. Any juice that could possibly be excited by catsup, for example, after it reaches the stomach, would not be adapted to the digestion of meat, eggs or other substances upon which it is used. The precise and specific adaptation of the digestive juices to the particular food to be digested, renders it impossible that any "aid to digestion" can improve digestion in any way.
Carlson showed that bitters do not increase gastric secretion. Reichmann and Schoeffer showed that bitters actually lessen gastric secretion. Bitters hinder and do not aid digestion. Bitters taken into the mouth diminish the hunger contractions, as do other "stimulants" applied to the oral membrane.
Alcohol seems to increase gastric secretion, but the alcohol precipitates the pepsin thus destroying the activity of the juice.
In his classical research for the Committee, Prof. Chittenden, of Yale, showed that wines, as well as strong drinks, are decidedly detrimental to digestion. He showed that alcohol increases the flow of gastric juice, but found that an equal amount of water would increase gastric secretion equally as much. Upon further investigation it was found that the secretion induced by water possessed much more powerful digestive properties than that induced by alcohol.