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Foods, as we receive them from the bountiful hand of Nature, are not fitted for entrance into the blood and lymph or for the cells. They must undergo a disorganizing and refining process by which the structure of the food is broken down and the useful is separated from the useless in food. This process is called digestion.
The process by which apples, corn, beans, celery, are transformed into blood, bones, nerves, muscles, skin, hair and nails, is both complicated and intensively interesting. Digestion, the first step in this wonderful process, is the process by which food is prepared (in the mouth, stomach and intestines) for absorption into the blood and lymph to be used by the body. Digestion is carried on partly by mechanical, partly by chemical means.
The chemical part of digestion is performed by a series of digestive juices, alternating between alkalies and acids. The active principles in these juices or fluids are ferments known as enzymes. All true digestive juices contain enzymes. These are substances which possess the power of instigating chemical reactions, without themselves being transformed or destroyed in course of the process. Strictly speaking, an enzyme is an organic compound formed by a living cell, while other substances which bring about chemical changes are called by the broader term, catalyzer. An enzyme is simply a special kind of catalytic agent, or a catalyst produced by a living organism.
Digestive enzymes bring about chemical changes in the food eaten. They are known as protein-splitting or proteolytic, fat-splitting or lipolytic, and starch-splitting or amylolytic, according to the type or food-stuff upon which they act. They are specific in their action, by which is meant, they are not capable of inciting several different reactions but each enzyme acts upon but one class of food. If a digestive juice affects two distinct types of food it is considered to contain two enzymes. Enzymes are destroyed by heat short of boiling and are prevented from acting by cold, although as a rule this does not prevent them from resuming their activity upon being warmed. The enzymes in the human body are most active at body temperature (about 98° F.) and begin to break down at a higher temperature. Fever prevents their action.
If they are compared with other chemicals a very striking peculiarity is disclosed. This is, enzymes are not used up in proportion to the work they do. If one is pouring hydrochloric acid upon iron to make hydrogen gas he is forced to continue pouring the acid if he is to continue evolving the gas. But if starch is being converted into sugar by pytalin the amount of sugar formed depends less upon the amount of saliva present than upon the time the enzyme acts upon the starch. A small amount of digestive juice, containing a much smaller amount of enzyme, may, under favorable conditions, act continuously with but the most gradual loss of power.
Digestion begins in the mouth where the food is subjected to the mechanical process of grinding to break it up into smaller particles thus enabling the digestive juices to get at the food more readily. This also aids in mixing the salive of the mouth with the food. (Chewing or mastication is the only conscious work of digestion and all the subconscious processes depend upon how well this has been performed.)
Simultaneously with the chewing of the food the digestive juice of the mouth is poured out upon and thoroughly mixed with it. Saliva, as it is called, is a colorless, tasteless ropy fluid secreted chiefly by the parotoid, submaxillary and sublingual glands. Secretions from the bucal, palatine, lingual, molar and tonsillar glands also contribute to the saliva. In man it is normally alkaline in reaction, although, during fevers, while fasting, when there are digestive disturbances, and between midnight and morning it may become acid. About 1500 grams, or between one and two quarts are secreted in twenty-four hours.
Its secretion is not a simple filtration due to blood pressure but is accomplished by the action of the cells composing these glands. In common with all the cells of the body, these exercise a selective power by which they select from the blood stream the elements needed in the manufacture of saliva and reject the rest. The salivary glands are under nerve control which secures coordination.
The active principle in saliva is an enzyme known as pytalin which acts upon starches (polysaccharides), converting these into a form of sugar known as dextrines (disaccharides).
If saliva is put into a test tube with starch it will convert this into sugar. At low temperatures this process goes on slowly, the velocity increasing as the temperature increases until it reaches its maximum at about 37° C. Above this temperature the velocity again decreases, the enzyme being destroyed at about 70° C.
Pytalin is lacking in the saliva of all carnivorous and some other animals. In these the saliva is not a true digestive juice, but acts, solely to moisten the food thus enabling the animal to swallow it.
Pytalin is not present in the saliva when food that does not contain starch is taken into the mouth. The tongue contains various sets of taste buds among which are proteid and starch buds. The function of taste not only affords us pleasure, but is an all important element in the subconscious process of digestion. Particularly it serves to stimulate the flow of the digestive juices, especially those of the stomach, and to suit their character to the food eaten. The nerve impulses set into motion by the taste of foods set the mechanism into action necessary to digestion. The character of food eaten determines, through the taste buds, the character of the digestive juices released to act upon it. Saliva will be poured into the mouth but no pytalin will be present if the food eaten contains no starch. Even sugar, if put into the mouth will not occasion the release of ptyatin, although, the mouth will quickly fill with saliva.
Edinger showed by experiment that potassium rhodanate is the antiseptic principle in saliva. He found that three parts of saliva to the thousand will kill the "bacilli of cholera morbus" in one minute, while nine parts to the thousand will kill diptheria germs in the same time. Here is a constantly produced antiseptic powerful enough to destroy any germ, yet harmless to the body. Chew your food well and the saliva will aid in preventing decomposition. A pint of human saliva was collected and exposed in an open jar to the sunshine and heat of June, July and August, and at the end of the experiment showed no sign of infection or disintegration.
Pytalin is destroyed by acid in a minute percentage. Tannic acid in tea and coffee interferes with the digestion of starch. Drug acids do likewise. One half of one per cent of acid stops the action of pytalin.
Tart (acid) fruits taken with starches completely neutralize the alkalinity of the saliva, the only secretion in the body able to initiate the digestion of starches, and paralyzes the ptyalin. Besides being wet, they are also acid and thus there is both a mechanical and a chemical reason why they should not be taken with cereals or other starch.
After food is masticated it is swallowed and enters the stomach where the work of digesting the starch continues until sufficient gastric juice has been poured into the stomach cavity to render its contents acid.
Dr. Cannon of Harvard University Medical School, demonstrated that if starch is well-mixed with saliva, it will continue to digest in the stomach for up to two hours. If proteins, which require an acid stomach juice in which to digest, are eaten at the same meal, nature deluges the food in the stomach, including the starch, with acid gastric juice which neutralizes the alkaline saliva and destroys the ptyalin, and starch digestion ceases shortly.
When starches are soaked with any kind of fluid--water, milk, fruit juices, etc.--very little saliva is poured out, no matter how long one chews them. Dry starches excite a copious flow of saliva rich in ptyalin. Dry starch increases in bulk upon being masticated; soaked starch does not. Dry starches, taken into the mouth with fruits, milk, water, coffee, or tea, etc., do not excite the flow of saliva.
If starches are to digest, they must be eaten dry. Starches put into soup are never digested. When starches are not digested they lie in the stomach and produce much trouble. Soaked starches are also likely to be swallowed without chewing. Unmasticated starches, even if they were insalivated would not digest. Boiled starches do not digest.
Experiments carried on by the Defensive Diet League showed that oatmeal is never digested in the stomach. The same was shown to be true of every other cooked and soaked cereal. The stomach does not digest starches--does not secrete a starch digesting fluid--and when starches are soaked so that they do not receive saliva and ptyalin, they cannot be digested in the stomach.
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.
The secretion of hydrochloric acid is only temporarily increased, after which its secretion is diminished, while the alcohol hinders the formation of pepsin. It also causes the mucous glands to pour such large quantities of alkaline fluid (mucus) into the stomach that this upsets gastric digestion.
It has been definitely established that tea and coffee both retard gastric digestion. Coffee is considered to have less effect than tea, providing they are both of the same strength. Since, however, coffee is customarily used in a stronger infusion than is tea, the effects of coffee in actual practice are about the same as those of tea. Their inhibiting effects are largely due to their modifying influence on the chemical processes of digestion.
The effects of these two poisonous infusions do not end with retarding the processes of digestion. They affect the stomach itself. Tea in particular, rich in tannic acid, and other astringent agents, acts as a strong irritant to the lining membrane of the stomach. Caffeol, and other substances produced by the roasting of coffee, are greater irritants even than tea. Chronic gastric catarrh and other disorders of the stomach may easily be produced and maintained by the effects of these two popular drinks.
Aside from these effects upon the stomach and the effects upon the nervous system and kidneys, produced by these two drugs, they undoubtedly affect the intestine and colon as well. There are many people upon whom coffee produces a laxative effect and this indicates that its irritating effects extend to the intestine and colon. Perhaps they also retard intestinal digestion.
As de-caffeinized coffee is not decaffeinized, and since, if it were, the coffee would still possess its tannic acid, caffeol and other poisons, and would in addition to its other effects, continue to retard digestion and injure stomach and kidneys, there seems to be no rational excuse for continuing its use.
In the well-known experiments upon Saint Martin it was found that a piece of metal could be introduced into the stomach but it would not occasion any flow of gastric juice. If, however, someone entered the room with a platter of steaming steak, the instant the man's eyes fell upon this the gastric juice would begin to flow into the stomach. When no gastric juice was needed none was supplied. Pavlov, introduced into the stomach of a sleeping dog (through a fistula) 100 grams of flesh. After an hour and a half the flesh was withdrawn by means of a string that had been tied to the meat. The loss to the meat was only six grams. This same amount of meat (100 grams) was again introduced into the stomach through the fistula, after the dog had been allowed to see and smell the meat. Under these conditions the weight of the meat was reduced by 30 grams in the same time. The reader will readily perceive the importance of such facts in diet. They teach us that food must be seen, smelled and tasted if digestion is to proceed normally. But the food must not be so disguised by condiments, spices, etc., as to deceive the senses as this will hinder the setting into motion, through the nerves, of the mechanism necessary to digestion.
The flow of gastric juice into the stomach is apparently in advance of the actual arrival of food and seems to be proportioned to the pleasure afforded by eating. This should teach us that the pleasure we derive from eating is only a means to an end, not the end itself.
The secretion of gastric juice is hastened and retarded by a number of factors the chief of which are here given:
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| (1) Hunger | (1) Fear, worry, anxiety, anger and other destructive emotions |
| (2) Pleasurable taste | (2) Failure to taste food |
| (3) Sight and smell of food | (3) Absence of hunger |
| (4) Thought of food | (4) Lack of proper salivary digestion |
| (5) Joy, happiness, etc. | (5) Pain, fever, etc. |
| (6) Effects of food on lining of stomach | |
| (7) Ingestion of water | |
| (8) Secretagogues arising as by-products of the process of digestion |
Pepsin, the protein-splitting enzyme of the gastric juice, converts proteins into peptones. Beyond coagulating the casein of milk, renin appears to have no other function. Gastric lipase has but little effect upon fats.
Pavlov, the renowned Russian physiologist, has shown (see his The Work of the Digestive Glands) that the first secreted portions of the gastric juice are not always stronger in digestive power than that secreted an hour or so later. The strongest juice is poured out when it is most needed--when the quantity of food is large and when its structure is coarse. His experiments have proved that each kind of food calls forth a particular activity of the digestive glands and that the powers of the juice vary with the quantity of the feeding. Khizhin, one of his co-workers, performed experiments which have shown that feeding mixed diets, or separated administrations of milk, bread and meat, calls forth each time special modifications in the activity of the gastric glands. The secretion response is not "limited to the powers of the juice but extends to the rate of its flow, and also its total quantity." This proves that the character of the food not only determines the digestive power of the gastric juice, but also its total acidity. The acidity is greatest with meat and least with bread.
Prof. Pavlov says: "On proteid in the form of bread, five times more pepsin is poured out than on the same quantity of protein in the form of milk, and that flesh nitrogen requires more pepsin than that of milk. These different kinds of proteids receive, therefore, quantities of ferment corresponding to the differences in their digestibility."
Comparing equivalent weights, Pavlov found that flesh requires the most and milk the least amount of gastric juice;but comparing equivalents of nitrogen, he found that bread needs the most and flesh the least juice. The gland work per hour is almost the same with milk and flesh diets, but far less with bread. The last, however, exceeds all the others in the time required for its digestion, and consequently, the flow of juice is somewhat prolonged.
"Each separate kind of food," he says, "determines a definite hourly rate of secretion and produces characteristic limitations in the powers of the juices. Thus with a fish diet, the maximum rate of secretion occurs during the first and second hour, and the quantity of juice in each being approximately the same. With a broad diet, we have invariably a pronounced maximum in the second hour; and with milk a similar one during the second and third hours."
The acidity of gastric juice is determined by the food eaten, by the length of time that has elapsed since the food was consumed, and by the familiarity or unfamiliarity of the system with the food. Physicians persistently ignore these facts in making gastric tests and in feeding in hyper- and hypo-acidity. They invariably feed foods in hyper-acidity that increase the acidity and feed foods in hypo-acidity that decrease acidity. They make the same mistakes with regard to pepsin, for what is true of hydrochloric acid is true also of the secretion of pepsin.
"On the other hand," says Pavlov, "the most active juice occurs with flesh in the first hour; with bread in the second and third; and with milk in the last hour of secretion. Thus the period of maximum outflow, as well as the whole curve of secretion, is characteristic for each diet."
Pavlov also says: "The work of the gastric glands, in providing juice for the different food stuffs, must be recognized to be also purposive in another sense. The vegetable protein of bread requires for its digestion much ferment. This demand is supplied less by an increase in the volume of the juice than by and extraordinary concentration of the fluid poured out. One may infer from this that it is only the ferment of the gastric juice that is here in great requisition, and that large quantities of hydrochloric acid would be useless, or possibly injurious. We see from the following, that during gastric digestion of bread, an excess of hydrochloric acid is actually avoided. The total quantity of juice secreted on bread is only a little larger than that secreted on milk. It is distributed, however, over a much longer time, so that the mean hourly curve of juice with the bread diet is one and one-half times less than after taking milk or flesh. Consequently, in the digestion of bread but little hydrochloric acid is present in the stomach during the period of secretion. This harmonizes well with the facts of physiologic chemistry, namely, that the digestion of starch is impeded by an excess of acid.
"From clinical observation, we know further that, in cases of hyperacidity, a large part of the starch of bread escapes unused from the gastro-intestinal canal, while the flesh is excellently digested."
Are we not fully justified, by these facts, in assuming that the variations observed in gland activity during the course of digestion have some essential meaning? Each kind of food produces a special curve of secretion, and there must be a definite purpose for it, and a special significance to the secretory reaction. Pavlov holds that the work of the digestive glands, while elastic, is at the same time specific, precise and purposive. These facts are useful in working out proper food combinations, as we shall see later.
There are foods, like the starches, which, so far as stomach digestion is concerned, can only be digested in an alkaline medium--saliva--and others, like the proteins, which can only be digested in an acid medium--gastric juice--and if eaten together, interfere with the digestion of each other. We are justified in calling these incompatible foods.
From these facts it becomes obvious that the digestion of carbohydrates and of proteins is quite different. Indeed they are almost incompatible, for the requirements of each are so different that, when taken together, one forestalls the proper gastric digestion of the other. Another important observation should be taken into consideration at this point. Carbohydrates, proteins and fats are always mixed in the diet of most people. Fats have no stimulating effect on the gastric glands; whether the oil or fat is consumed before a meal, during the meal, or after the meal, an inhibitory influence upon gastric secretion becomes apparent immediately. If consumed after the meal and the gastric juice has begun to flow, it exerts an inhibitory influence which lasts usually for one or two hours.
Fat depresses, or inhibits, the normal activity of the secretory processes and this inhibitory effect while, perhaps partly mechanical, is for the most part chemical, as is shown by the results of administering milk with an increased amount of fat. The amount of juice secreted upon cream is less in amount and weaker in power than the small amount of weak juice poured out upon milk.
Nor is the effect of fat on the secretion of gastric juice limited to the depression of the flow of the gastric juice. Its preventive influence may last from one-half to two hours; only to be followed in the third hour, if the meal of fat be at all large, by a renewed secretion of gastric juice. This late secretion is much prolonged and furnishes a considerable quantity of gastric juice and it seems to be an explanation for many cases of hyperacidity which follow the taking of oils, butter fats and meat fats with a protein meal.
Bile precipitates pepsin, so that its presence in the stomach stops protein digestion even though the contents of the stomach remain acid. (Fasting is indicated in such cases). Trypsin (pancreatic) digests pepsin so that its action does not long continue in the intestine. Bile also stops its action, as does alkalinity.
When the work of digestion is completed in the stomach the food is poured through the pyloric orifice into the small intestine where it undergoes further changes.
In negroes the small intestine is shorter and the large intestine longer than in the white man of similar build. There are also differences due to sex--extremes in males running from 15 ft. 6 inches to 31 feet, 10 inches and in females, from 18 feet 10 inches to 29 feet 4 inches. The tall thin type of person, with trunk of small circumference, has a shorter intestine than the heavy rugged type. There are three digestive juices which are poured into the intestine--bile, pancreatic juice and intestinal juice; all of these are alkaline in reaction.
The pancreatic juice is secreted by the pancreas and enters the intestine just below the union of the stomach and duodenum or upper portion of the small intestine. This juice, the secretion of which is excited by the action of the acid contents received from the stomach upon the walls of the intestine, is poured out about the time the contents of the stomach pass through he pyloric valve.
Pancreatic juice contains four enzymes. One of these known as diastase or amylase resembles ptylain and continues the work of digesting starches and sugars, converting these into a form of sugar known as monosaccharides. It is not destroyed by the acid contents of the stomach as is ptyalin. A second, known as trypsin, is a protein-splitting enzyme, but unlike pepsin, does not require the cooperation of an acid to accomplish its work. In fact, it is destroyed by a strong acid. By its action the peptones are converted into amino-acids. The third enzyme known as liapase causes fat to undergo cleavage forming fatty acids and glycerine. The fourth, chymosin, or pancreatic renin, coagulates milk.
Pavlov discovered that the pancreatic juice, as it leaves the pancreas, has no appreciable action upon proteins, but becomes rapidly active when a small quantity of the intestinal juice, which Pavlov has called enterokinase, which converts the inactive trypsinogen, from the pancreas, into active trypsin. Pavlov regarded enterokinase as itself an enzyme. Active trypsin in the pancreas and pancreatic duct might destroy these organs. Nature seems to have safeguarded the body by arranging that it cannot become active until it is in the presence of food in the intestine, where it is activated by the enzymic effect of the intestinal juice upon it.
Considering the pancreatic secretion, we find the same marvelous adaptation of the digestive properties to the class of food to be acted upon. Each kind of food calls for its own particular kind of juice. The character of these juices is often the direct opposite of that seen in the stomach. In the stomach the weakest juice is poured out upon milk and the strongest upon meat; in the duodenum the weakest juice is poured out on flesh and the strongest on milk. This statement, of course, has reference to the protein-splitting character of the juice.
With regard to the starch-splitting enzyme, this is present in greater quantity in the "bread juice" and in lesser quantity in "milk juice." The fat-splitting enzyme is very scarce in "bread juice" abundant in "milk juice," and intermediate in "flesh juice." "The work of the pancreas, like that of the gastric glands," to quote Pavlov, "is specialized both as regards the quantity and property of its juice, and the rate of its progress which the secretion takes for the different classes of food."
The second of the juices poured into the intestines is bile or gall. This is secreted by the liver and enters the intestine at about the point where the pancreatic juice enters. Its secretion goes on continuously but is accelerated after meals. It contains no enzyme and is, therefore, not a true digestive juice; but acts chiefly by producing a favorable environment for the action of the pancreatic enzymes. If it is prevented from entering the intestines, the ability to digest and absorb foods, particularly fats, is reduced. Bile increases the solubility of the fatty acids by emulsification, accelerates the action of the pancreatic liapase, stimulates intestinal activity, counteracts putrefaction, and assists in the union of water and oils.
Bile, secreted in the liver and conveyed by a duct to the duodenum, is not regarded as a true digestive juice because it contains no enzyme. But by alkalizing the acid bolus from the stomach, when this enters the duodenum, it provides a suitable environment for the operation of the pancreatic and intestinal enzymes. The hydrochloric acid, of the stomach, upon entering the intestine, acts as a powerful stimulus to the flow of pancreatic juice, intestinal juice and bile, but is antagonistic to the action of their enzymes. The bile counteracts the acid and produces a favorable medium for the action of these enzymes.
Bile is a powerful disinfectant and prevents putrefaction in the intestine. It also serves to prevent the formation of gas and helps to maintain the alkalinity of the intestine.
The third or intestinal juice is secreted in abundance by small glands in the walls of the small intestine. It contains an enzyme known as crepsin which cooperates with trypsin in the final stages of protein digestion. This juice also completes the preparation of carbohydrates for entrance into the blood.
Intestinal juice--succus-intericus--is elaborated by many microscopic glands embedded in the walls of the intestine. There are four kinds of glands which secrete intestinal juice--Crypts of Lieberkuhn, Brunner's glands, solitary glands and Pyer's patches or glands. The glands of Liebekuhn. secrete an intestinal juice containing several enzymes--erepsin (Proteolytic), lactase, invertase, (Amolytic), Maltase (Amolytic), and lactase which digests milk sugar. Brunner'sglands secrete a juice containing the enzyme, enterokinase, which, acting upon the trypsinogen of the pancreatic juice, as it enters the duodenum, converts this into the powerful protein-splitting enzyme, trypsin. Chymosis, which coagulates milk, is also contained in succus-interucus.
Having considered the processes of digestion let us now get some idea of the work accomplished by these. First, it is a refining process, breaking down the structure of the food and separating the nutritive portions from the waste or useless parts. Second, it splits up the large and complex molecules of food into smaller, less complex ones, in this way adding to the diffusibility of the food. Diffusibility is the capability of spreading and enabling substances to pass through ordinary membranes. Lastly, it standardizes our food. By this we mean it obliterates many of the characteristics of the various foods consumed and gives us, finally, practically the same set of products whatever the meal eaten. From the many strange and foreign compounds that are taken into the mouth, as food, are formed a few acceptible compounds.
When, finally, the work of digestion is completed in the intestine the carbohydrates have been reduced to a form of sugar known as monosaccharides, the fats have been converted into fatty acids and glycerol and the proteins have been reduced to amino-acids. Water and salts undergo no change. The waste portions of the food are separated from the usable portions and are sent on into the large intestine or colon to be expelled.
While fats, starches, sugars and proteins undergo several changes in the process of digestion, the mineral elements of our food are absorbed unchanged. They do not require to be digested.
So long as the body is normal, the digestive secretions are sufficient protection against the fermentation and putrefaction of food, which would otherwise be set up by microbes. If, however, the vital powers are lowered so that the secretions are deficient in quality or are insufficient in quantity, or, if there is disease, which impairs the digestive powers, bacterial fermentation sets in and we have indigestion. The fermentation produces toxins of various kinds, which, when absorbed into the blood and lymph, serve to poison the body. Some of these poisons are the ptomains and leucomains; phenol, cresol, leucin, tryson, ammonia, sulphurated hydrogen, fatty acids, oxalic and uric acids, alcohol and the xanthin bodies. Of these, indol is the most easily absorbed and is most readily recognized in the urine.
The chief causes of gastro-intestinal indigestion are overeating, enervation and bad food combinations. Enervating influences are anything that lowers nerve force and include such things as overwork, underwork, extremes of cold and heat, use of stimulants, sexual excesses, etc. Anything that enervates lessens digestive power and becomes an indirect cause of indigestion.
Overeating overworks the digestive organs, as well as introduces more food into the system than is needed. Food eaten in excess is bound to accumulate as waste and decompose as poison.
Other things being equal, digestion is more efficient when but one food is eaten. A single article of food will digest more quickly and perfectly than will the same food if mixed with other foods. The more foods one takes together, the less efficient is the process of digestion.
From the differences in the results of fermentation and those of digestion, it should be apparent that, although, the enzymes are spoken of as ferments, they do not produce fermentation. Rather, the digestive juices and their enzymes act as powerful solvents--for (and keep this fact in mind), digestion reduces food-stuffs to the diffusible state without depriving them of their organic qualities, while fermentation renders them diffusible by reducing them to the inorganic and, therefore, useless state. Digestion is solution; fermentation is disintegration.