The activities of animals are carried on by a certain expenditure of energy which is set free as the result of a chemical breaking down of the living tissues of the body. In order to maintain the equilibrium of the body this waste must be replaced by new material which is taken in the shape of food, drink, and oxygen.

Digestion is the term applied to those changes in the food-stuffs which precede absorption; it is a refining process which separates the useless from the useful, and further prepares the latter to be used as building-stones for the repair of organs and tissues and to furnish fuel to supply the motor energy of the body.

In the human body the digestive processes are brought about by mechanical disintegration; by the action on the food-stuffs of acid and alkaline fluids; by changes produced by active substances called enzymes; and, lastly, decomposition is produced by the growth of microörganisms.

The digestive tract, or alimentary canal, begins at the mouth and ends at the anus. It consists of the mouth, the esophagus or gullet, the stomach, the small and large intestines. Two large glands, the liver and pancreas, pour their secretions into the small intestine to aid in the digestion of foods. The alimentary canal, liver, and pancreas together constitute the digestive system.

The contraction of the muscular walls of the digestive tract is the propelling power that carries the food downward and brings it into contact with the digestive fluids. These movements will be referred to later, under the Mechanics of Digestion.

The digestive tract is lined throughout its entire course by mucous membrane; that lining the mouth gives some idea of its character. It is from the mucous membrane lining the stomach and intestines, as well as from the liver and pancreas, that the digestive fluids are poured forth into its cavity.

In passing downward the food meets with five different digestive fluids: the saliva in the mouth; the gastric juice in the stomach; and the bile, pancreatic, and intestinal juices in the small intestine. Each digestive fluid acts only on some particular kinds of food. This action of the digestive fluids on food is called the chemical part of digestion.

The Care of the Mouth and Teeth.—It is of the first importance that the mouth should be kept clean and as nearly aseptic as possible, for as the food is rolled about in the mouth by the process of mastication it is obvious that it will carry all the germs and mucus with it into the stomach. A nasal spray containing some solution of an alkaline or antiseptic nature should form an essential part of the toilet of the mouth.

The Teeth.—From simply an esthetic point of view well-kept teeth are beautiful and greatly enhance the charm of the face. But the proper care of the teeth is also essential to good digestion and good health.

In perhaps the majority of people the teeth are sufficiently far apart for small particles of food to become lodged between them. If not promptly removed, the heat and moisture of the mouth, acting on these small particles, cause their decomposition and set up acid fermentation in the mouth; and it is this acid which causes the decay of the enamel and finally of the teeth. The decayed tooth gives still further lodgment to particles of food, and these, left to decompose, give rise to the most offensive gases, giving fetor to the breath, poisons the blood, and so injures the digestive and nervous systems.

Where possible a tooth-pick and dental floss should be used immediately after each meal; the waxed dental floss between the teeth, and the flat end of the tooth-pick to remove any particles that may have worked in about the roots of the teeth and gums.

A tooth-brush with good tooth-powder should be used twice a day. The brush should not be too broad, and is better if slightly curved. The bristles should not be too hard. The use of tooth-powder, which must be very fine, or tooth-paste is needed to produce sufficient friction to clean the teeth well. Warm water is a better solvent than cold, and, therefore, it is well to use it for cleansing the teeth.

While it is generally known that decay if neglected will destroy a tooth, it is not so well known that many teeth are lost as the result of the accumulation of tartar at the gum margin. There is a great tendency to this, especially about the necks of the lower incisors and upon the teeth that are not used in chewing. The deposit of tartar encroaches upon the neck of the tooth and presses upon the gum; the latter becomes irritated or inflamed, and recedes from the tooth; malnutrition and loosening of the tooth follows.

With very imperfect or decayed teeth proper mastication is impossible.

The teeth should be examined by a good dentist twice a year, so that small cavities shall be discovered at an early stage, the tartar removed, and the teeth kept in their best condition. This will prevent the early loss of the teeth. Lost teeth must be replaced, so that the teeth shall be opposite each other, for a very important factor in mastication is that the teeth shall strike properly.

Digestion.—The factors controlling digestion are psychic, mechanical, and chemical.

Briefly stated, the process of digestion consists first in the liquefaction of the solid portions of food and the conversion of the insoluble into soluble and diffusible, for no nutriment can be assimilated until reduced to a state in which it can pass through a cell membrane. These chemical changes are carried on by a series of enzymes.

Enzymes are unorganized ferments which possess the power of producing chemical changes in certain substances with which they come in contact under particular conditions without themselves suffering permanent alteration. The digestion of food is largely accomplished by the specific action of these enzyme bodies, of which every digestive fluid contains one or more.

But neither solubility nor diffusibility is adequate. Freely soluble substances like cane-sugar need to undergo digestive changes just as definite as those carried out in the case of fats or coagulated proteins. The changes which they undergo before absorption serve a more fundamental purpose than the mere hastening of their passage through the lining membrane of the intestine.

In the light of modern chemical knowledge we can be somewhat specific in regard to the molecular aspects of the digestive processes. They are probably always cleavages, large molecules giving rise to smaller ones. When the original molecule is of extraordinary size, as with proteins and starches, these cleavages have a serial character, and a number of intermediary products must accordingly be formed; that is to say, the earlier products are in turn subjected to digestion. Such cleavages are generally, if not always, hydrolytic; that is, water enters into the reaction and its elements are found combined in the products.

The Psychic Aspect or the Effect of the Emotions on Digestion.—The relation of the emotions to the activities of the alimentary canal are of very practical importance, because recent investigations have shown that not only are the first stages of the digestive processes normally started by the pleasurable smell, sight, and taste of food, but also the pouring out of the gastric juice.

The importance of the initial psychic secretion of the saliva for further digestion is indicated when we realize that materials can be tasted only when dissolved in the mouth, and thereby brought into relation with the taste organs. The saliva which waters the mouth assures the dissolving of the dry but soluble food even when it is taken in large amounts.

The importance of the initial psychic secretion of gastric juice is made clear by the fact that the continued flow of this juice during digestion not only stimulates the glands of the stomach to pour out the chemical gastric juice, but by its action on the mucous membrane of the duodenum it also starts the flow of bile and pancreatic juice.

These facts are of fundamental importance in the cooking and serving of food, especially when the appetite is fickle. A bright and cheery dining-room, the daintiness of the table service, the center-piece of ferns or flowers, all are factors in exciting the pleasurable emotions, and so stimulate the appetite.

On the other hand, the digestive processes may be wholly abolished by vexation, worry, and anxiety, or when such strong emotions as anger or fear are allowed to prevail. This fact has long been known in regard to the salivary secretion, and it has now been established in regard to the secretion of the gastric juice. It is necessary to avoid all circumstances likely to provoke emotional reactions.

The inhibitory influence of excitement on the flow of the gastric juice has been studied by Professor Cannon, of Harvard University, in an interesting series of experiments on dogs. And a most important point is that the cat, which was allowed to infuriate the dog being experimented on, was only in the room for five minutes, and in spite of the fact that the animal was hungry and ate eagerly; there was almost a complete suppression of the flow of gastric juice for a period of twenty minutes.

A similar experiment was tried on a boy with a gastric fistula. Food was shown to the boy and then withheld. He was so vexed because he could not eat at once that he began to cry and flew into a towering passion, with the result that no secretion appeared even after the child was calmed. In both the case of the dog and boy there was not only a temporary inhibition of the flow of gastric juice, but it is a noteworthy fact that the effects of the emotional excitement remained long after its cause had been removed.

The practical point is, that if the digestive processes have been inhibited by emotional disturbances any food taken will lie stagnant in the stomach; and not only will there be a suppression of the gastric but of the pancreatic secretion as well, with the consequence that there is likely to be an accumulation of unabsorbed organic material in the colon and perhaps higher up as well. Bacterial decomposition will be fostered and actual harm may be done the organism.

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.

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.

Gastric juice does not dissolve and chemically change all food that enters the stomach, but acts only on that class of foods called proteins, converting them into peptones.

The changes which food undergoes in the stomach are as follows: food is disintegrated; meat is broken up, gelatin dissolved; the muscle-fibers fall apart, and are split into disks and sarcous elements; the framework of fatty tissue undergoes similar dissolution of cellular membranes; milk is curdled; the caseinogen is transformed into casein, and the latter broken down by pepsin. Bread and other starchy foods are disintegrated, some of the starch being formed into sugar by ptyalin. The other vegetable foods are but little digested by this viscus. The whole is broken into irregular fragments by the disintegrating power of the stomach and gradually converted into chyme, the creamy emulsion which passes through the pylorus into the intestines. The passage from the stomach into the duodenum is aided by the motor activity of this organ.

The Mechanics of Digestion in the Stomach.—Recent investigations, especially those of Cannon, have thrown much light on this subject. The food introduced into the stomach from the esophagus is lodged first in the fundus or cardiac end of the stomach; from here it is moved by slow degrees toward the pylorus, from which it enters the small intestine. Formerly it was taught that this movement was brought about by a churning motion of the stomach throughout its entire length. Cannon has shown the error of this conclusion. From his observations it appears that the stomach is quiet at first. The waves of peristaltic constriction begin at the duodenal and middle portions and move the food toward the pylorus. In this way the constrictions that begin near the pyloric end gradually extend to the cardiac end. The latter part of the stomach is distended after a full meal, but gradually diminishes in size during digestion.

Moreover, there is a difference in the character of the gastric juice coming from the different areas of the stomach; that from the middle portion being acid, and that from the cardiac and pyloric being neutral or nearly so. These facts show that the food remains for some time in the fundus and meets there a neutral liquid; consequently, the alkalinity of the mass is retained for a time, and the saliva acts upon the starch for a much longer period than has been supposed.

It is believed that the length of time which food remains in the stomach varies with its kind. The digesting mass is not forced into the intestine until it becomes well saturated with the free acid of the stomach, a result that will be reached later with a meat than with a vegetable diet; for it is plain that much more acid will be required to combine with the proteins of meat than with the smaller amounts in the carbohydrate foods, and so free acid is longer in accumulating.

Digestion is aided by the movements of the food mass through the contractions of the walls of the stomach, so that anything which lowers the tone of the stomach or impairs the integrity of its lining membrane seriously interferes with the digestive processes.

The nature of the food influences the rate of its passage through the pylorus. It has been shown by means of the x-rays that when the meal consisted of lean meat, suet, and rice, properly cooked and prepared in such a manner that the constituents were properly mixed together, the carbohydrates (rice) began to leave the stomach in fifteen minutes, but the protein (lean meat) and fat did not begin to leave it under one-half hour, and were much longer in passing through.

It was further found that in a normal stomach a meal of rice and potatoes gets out of the stomach rapidly, no trace being left in three hours, while a meal of protein and fat meat was much slower, some being present six hours after the meal. Indigestible substances are rejected by the sphincter and returned to the digestive cavity over and over again, but in the course of time the sphincter relaxes and the peristaltic contractions increase, until the force is sufficient to drive through the opening hard substances like peas, plum-stones, and coins.

The Absorption of Food from the Stomach.—A very wide-spread misconception prevails in regard to the amount of absorption that takes place through the gastric walls. This is very limited indeed; some sugar, alcohol and other fluids, and a small proportion of nitrogenous matter are taken up, but that is all. The legitimate rôle of the stomach is a peptonizer and grinder only. Absorption of nutrients is the special work of the intestines, and occupies but a subordinate place in the duties of the stomach.

Proteins undergo peptonization, but that does not change them sufficiently. The mere splitting into peptones, formerly thought sufficient, is now known to be quite inadequate. Recent researches have shown that the complex protein molecule has to be quite crushed into a heterogeneous assemblage of fragments, from which those suitable for building up into the body protein are selected for that purpose, the remainder being used as fuel; that is, as a source of energy. In the case of the other food principles, fats and starches, the chemical processes are much simpler.

Intestinal Digestion.—The chemical changes which food undergoes in the small and large intestines are exceedingly complex. Digestion in the intestine is due to the combined action of the bile, pancreatic juice, and succus entericus. The material which enters the duodenum from the stomach is known as the chyme. The acid chyme provokes a flow of bile and pancreatic juice. It acts upon the prosecretin in the duodenal mucous membrane in such a manner that secretin, a hormone, is formed and carried by the blood to the cells of the pancreas which it stimulates.

Bile is a secretion of the cells of the liver and from the inner wall of the gall-bladder; after elaboration, bile is stored up in part at least in the gall-bladder. The secretion of bile is irregular in quantity, as is the case with the gastric juice, and appears to be induced by chemical excitants, of which acids, especially hydrochloric, seems to be especially effective. Of the nutrients, the proteins exert the most influence. Less secretion follows the ingestion of carbohydrates than of proteins.

Formerly it was supposed that fatty foods checked the secretion of bile, but later experiments have proved that, to the contrary, they increase its secretion; and olive oil is a powerful cholagogue.

Bile precipitates the proteins of the chyme, neutralizes the acidity of the gastric juice, dissolving the fatty acids, and activates the lipase or steapsin of the pancreatic juice and assists in the absorption of fats. When for any reason the discharge of bile is retarded and the organism attempts to eliminate it through the kidneys, the tissues become charged with its compounds and take on a yellowish coloration.

Pancreatic Juice.—The second action upon the food in the intestine is that of the bicarbonate of soda contained in the pancreatic and intestinal juices, which neutralizes its acidity; and pancreatic digestion can only take place in an alkaline medium.

The pancreatic juice has the most comprehensive action of all of the digestive fluids; it contains a great variety of enzymes and acts upon all classes of nutrients. Its flow is intermittent, being induced by the action of the acids in the partially digested food from the stomach.

The pancreatic secretion attains its maximum pressure about three hours after a meal, but this varies according to the character of the meal. The enzymes of the pancreatic fluid are trypsin, amylopsin, lipase or steapsin, and pancreatic rennin.

Trypsin.—Pancreatic juice alone has but little influence on proteins, but when mixed with succus entericus its influence is great. Trypsin differs from pepsin by acting in an alkaline or neutral solution, and a free mineral acid, like hydrochloric, stops its operations; it acts much more powerfully and rapidly than pepsin. In conjunction with other enzymes it splits protein foods into simpler compounds, which may be regarded as the building stones of the original proteins.

Amylopsin.—The digestion of starch in the intestine is mainly effected by means of a diastatic ferment or enzyme in the pancreatic juice, called “amylopsin.” This enzyme has the power of hydrolyzing the starch mostly into maltose.

Steapsin or Lipase.—The pancreatic secretion acts vigorously on fats, not only splitting them into fatty acids and glycerin, but, in conjunction with the bile, also effects their emulsification. This latter result is doubtless aided by the soaps which form a union of the fatty acids and alkaline bases (mostly sodium) in the bile. The cleavage of the fats is due to the enzyme steapsin.

The succus entericus is the secretion of the glands of Lieberkühn. It contains quite a number of enzymes, including four which act on carbohydrates, namely, amylase, maltose, lactase, and invertase; the proteolytic enzyme is erepsin, which seems to supplement the action of trypsin. It has no action on native proteins except casein, but it breaks down proteoses and peptones into amino-acids.

In other words, the discovery of erepsin in the succus entericus and its effects has led to the now almost generally accepted view that the digestion of the proteins is carried further than the stage of proteoses and peptones; in fact, that this enzyme hydrolyzes them into amino-acids, in which form they are taken up by the cells of the intestinal mucous membrane.

The Absorption of Food from the Intestines.—It was formerly believed that the main stream of the nutrient passed out of the intestines through the lacteals and thoracic duct into the circulation. But it is now known that only the fats take this course, and that the dissolved proteins, carbohydrates, some fats and salts find their way into the circulation through the portal system and the liver.

The blood contains a constant proportion of serum-albumin and serum-globulin, which are constructed out of the amino-acids resulting from the digestion of protein foods. The reconstruction of proteins takes place chiefly in the cells. Fuchs believes that the amino-acids from the food are carried to all the tissues; and that the cells of the muscles and glands pick out from the blood the “building stones” necessary for the construction of their special proteins.

Fat is absorbed for the most part in the form of an emulsion and as a solution of soap. Only about 60 per cent. of the fat in the food is absorbed by the lacteals; the remaining 40 per cent. gets directly into the blood. The fat is carried directly by the blood-stream to the tissues all over the body; it is oxidized in the tissues to produce heat and energy; what is not immediately used for this purpose is stored up in the cells for future use.

Carbohydrates.—Practically all of the carbohydrates digested are absorbed in the form of sugar. The absorption takes place more rapidly in the upper than in the lower bowel. A portion of the ingested carbohydrates is destroyed in the alimentary canal by bacteria, whose enzymes transform them into various acids and gases.

The Absorption of Sugar.—The sugars pass into the blood through the portal circulation, and not through the lymphatic vessels. The question is, Where does sugar go after it gets into the blood? The proportion of sugar in the general circulation is not greater after a meal than before it. It is necessary that there should be some means of storing it, for it cannot all be immediately used for the production of energy. Is it stored as glycogen? Bunge believes that a considerable amount of carbohydrate is stored as fat, since the total amount of glycogen in the liver never exceeds 150 grams, and there is a similar store in the whole mass of muscles. It is evident, therefore, that only a small proportion of carbohydrates is laid down as glycogen, and we must assume that the greater part of it is converted into fat. That fat accumulates in the body on a purely carbohydrate diet is proved beyond a doubt, and the formation of fat from sugar is considered proved by experimental evidence.

Sugar is an important source of energy for the muscles, and provision is made for a sufficiency of it always to be in the blood circulating through them, and the storehouse from which it is derived is the liver. When the liver and muscles contain enough glycogen to keep the blood supplied with it, the excess of sugar is converted into fat, and is reconverted into sugar when there is a demand for it.

Mechanics of Digestion in the Intestines.—The intestinal movements, like those of the stomach, have recently been studied by means of the x-rays. There are three kinds: A pendulum-like motion, consisting of a gentle, swaying, rhythmic movement occurring in all parts of the intestines. These movements do not affect the whole of the gut at one time, but occur in successive segments of it, and are more obvious in those parts which are distended with food, at a period of three or four hours after a meal. They are most energetic in the upper and least so in the lower part of the gut, and proceed from above downward with the course of the food.

The movement consists in lengthening and narrowing, followed by shortening and widening, of the canal; the contraction involves both the longitudinal and circular muscular coats, and in the course of its progress divides the bowel into many segments. This movement breaks up the food, sways it backward and forward, diffuses the digestive fluids through it, and drives chyle into the lacteals.

The second kind of movement consists of peristalsis, or wave-like propulsive movements; also localized dilatation, followed by contraction of the canal, progressing from above downward at the rate of 1 or 2 inches per second, and is from three to four hours traveling the entire length of the intestines.

Under pathologic conditions a third movement is observed, consisting of a swift venicular movement, starting at the pylorus and traveling the entire length of the gut in about one minute. It is produced by toxins, gases, and other irritants.

Digestion is practically completed in the small intestine.

The colon consists of a proximal portion, consisting of the cecum, ascending colon, and one-half of the transverse colon; a mesial part, formed by the other half of the transverse colon and part of the descending colon and a distal portion, formed by the remainder of the descending colon and of the rectum. The contents of the proximal portion are fluid; food accumulates in this portion; it is gradually concentrated by the absorption of water. It has been shown by the x-rays that a meal reaches the cecum in four or five hours; the hepatic flexure in six or seven hours; the splenic flexure in nine hours; and the rectum in about eighteen hours.

The feces consist of the residue of the food, digestive secretions, etc. The contents of the alimentary canal become altered in character as they descend its course. The mass attains a greater density as it descends, more and more nutrient being absorbed from it, and even in the colon it becomes more solid; its color is brownish and its odor characteristic. The amount of feces from an average mixed diet represents from one-seventh to one-eighth of the food consumed.

Defecation.—The waste matter which collects in the lower bowel must be evacuated every day. Allowed to remain longer than this, the digestive system is clogged by the non-removal of worn-out material, and the blood is constantly absorbing matter which is poisonous to the body. Decomposition goes on without being suspected by the sufferer.

Intestinal bacteria or organized ferments are present throughout the entire intestinal canal and play a part in the food changes. They are most abundant in the lower part of the small intestine and the upper part of the large. They act upon the proteins, causing putrefaction, dissolve cellulose, and cause a decomposition of the carbohydrates. The products of these fermentations include indol and skatol, which have the characteristic fecal odor; volatile fatty acids and gases, some of which are carbon dioxid, hydrogen, marsh-gas, and hydrogen sulphid.

Fermentations of this character up to a certain extent are normal and may be beneficial, but they may proceed so far as to be deleterious to health. Anything which retards digestion, such as imperfect mastication, excessive eating, abnormal amounts of meat in the diet, and failure of the organs secreting the digestive fluids to supply these secretions in sufficient quantity, gives these bacteria a better opportunity to act on the food residues and so increases their bad effects.

Some foods, especially vegetables of the leguminous class, appear to be provocative of excessive intestinal fermentation. Flatulence and even toxic poisoning may be the result of great bacterial activity in the digestive tract. It is hardly possible to check this by administering septics, but purging is of value by removing the fermentative material. Particular foods, especially soured milk and kephir, have been shown to have a preventive action on putrefaction.

Influence of Food on Secretion.—The more recent investigations reveal the fact that the kind of food has an influence not only on the abundance, but also on the kind of digestive secretions; this is most important because an abundant supply of digestive juices is necessary for good digestion. Broths, meat extracts, and milk stimulate the secretion of the gastric juice, which makes rational the taking of soup or bouillon as the first course at dinner, or the eating of toast or zwieback by persons with weak digestions. Fats, on the other hand, tend to inhibit gastric secretion, so that an excessive proportion of fat in the meat might hinder digestion in the stomach.

Food may exert an indirect influence on the pancreatic secretion. The acid in the chyme stimulates the flow of pancreatic juice. One is almost inclined to speak of the physiologic education of the digestive glands, and to conceive them as being trained for fat, protein, or carbohydrate digestion.

Metabolism.—Life consists, so far as material phenomena are concerned, in the transformation of matter into energy. To these transformations the term metabolism is applied. In the metabolism of matter the changes are chemical; in energy the changes are physical. It is commonly assumed that the laws of conservation of matter and energy are conformed to or obeyed in animal bodies.

The body converts potential into kinetic energy by metabolism in the body. The potential energy of food is transformed into the actual energy of heat and mechanical labor. In this respect the only difference between man and other vertebrates is the nervous and intellectual processes, which are not yet understood.

Metabolism is anabolic and katabolic. Anabolism is constructive; it includes growth and the act of the tissues in selecting, appropriating, and making substances absorbed from the alimentary canal a part of themselves. The body is never stable; while growth and nutrition progress, destruction or demolition takes place, and this is called katabolism. To ascertain the exact amount of matter and energy used daily by the body a balance sheet of the exchange of material is necessary. The income consists of (a) matter—food, drink, and the oxygen of the air; (b) energy, the potential energy of the food and drink. The outgo consists of (a) matter in the feces, urine, perspiration, and breath; (b) energy—the potential energy of feces, urine, products of respiration, and perspiration. A complete account would show the amount of C, N, H, O, P, S, Cl, Na, Ca, K, Mg, and Fe in the income and outgo, it would also show the compounds in the excreta, including proteins, carbohydrates, water, and carbonic acid.

Experiments show that the body requires a given quantity of given energy producing substances for sustenance, and something more to meet the demands for muscular work.

The metabolism of nitrogen is usually measured by the amount of urea in the urine, taking into account the small proportion of uric acid and other nitrogenous bodies. In fasting the organism lives on its own flesh and fat.

The Effects of a Nitrogenous Diet on Metabolism.—The most striking effect of a purely nitrogenous diet is a large increase in the nitrogenous metabolism, but it also increases the metabolism of the non-nitrogenous elements of the body. With an ordinary mixed diet the normal excretion of urea varies from 33 to 37 grams a day, while with a meat diet the urea may rise to 50 or even 80 grams daily.

Carbohydrates as a Protein Protector.—It has long been observed that when there was a deficiency of protein in the food the metabolism of nitrogen will be spared and the tissues protected if the food contains plenty of carbohydrates and fat. The subject was fully investigated by Lusk. When the diet contains an abundance of protein, carbohydrates, and fat the organism gains a little nitrogen; when the diet contains the same amount of protein but no carbohydrates, the body loses considerable nitrogen. Again, when the food was the ordinary mixed kind, and contained a sufficiency of energy, but was of a low protein character, the excretion of nitrogen was normal. These results led to the conclusion that the carbohydrates were a protein protector.

Fat as a Protector of Proteins.—The metabolism of nitrogenous tissue and elimination of nitrogen is not prevented by the consumption of fat, but the consumption of fat reduces the metabolism of protein so much that one-quarter or one-third as much meat will suffice to maintain the nitrogen equilibrium as would have to be consumed if only lean meat was used.

The nutritive value of meat extracts is comparatively small; their chief value is in promoting digestion and metabolism of other foods.

Water and Metabolism.—The body consists of 530 parts of water per 1000. It is of greatest importance as a component of the tissues to assist in the exchange of nutritive substances, the discharge of the products of metabolism, the regulation of temperature, and other vital functions. If the supply of water is stopped the body will die, and it may die sooner from the deprivation of water than from starvation.

A reduction in the amount of water consumed accelerates the decomposition of protein and fat to replace the water essential for the bodily functions. As the result of experiments it has been found that the average income of water is 4 pints, and the excretion is 4½ pints; so that at the lowest estimate ½ pint of water is formed in the tissues by the oxidation of hydrogen in the food and tissues; and during ordinary work it was found that 17 ounces of water was excreted daily in excess of that consumed in food and drink.

The intense suffering entailed by prolonged thirst is to be accounted for by the absolute necessity for preserving the normal aqueous dilution throughout the body.

Water is also essential for removing the effete materials arising from metabolism, cell growth, and other disintegration.

Chlorids.—About 15 to 18 grams of sodium chlorid (common salt) are excreted daily in the urine, and smaller quantities in the feces and perspiration. It is, therefore, a most important food. The tissues retain common salt most tenaciously, and when there is none in the food it gradually disappears from the urine. It facilitates the absorption of the protein foods and increases tissue metabolism. And, further, an insufficiency of common salt is followed by a diminution of hydrochloric acid from the gastric juice, and consequently a failure of nutrition. There can be no doubt that the hydrochloric acid of the gastric juice originates from the chlorids of the blood.

On the other hand, a very large number of people consume more salt than they require. Salt is not only a food, it is a condiment, and as such it is liable to abuse. It has been estimated that from 2 to 4 grams of salt a day is sufficient, whereas most people take from 20 to 30 grams daily in one way or another. The kidneys have to excrete the excess of salt, which they seem to do in health without any difficulty; but in nephritis the kidneys do not excrete it so readily, and this may lead to a retention of salt in the body.

Iron is essential to the human body. It is taken into the body in the form of food, and is excreted from the body in the bile and feces. It has been estimated that typical food contains 10 mgm. of iron daily. The hemoglobin of the blood contains 0.04 per cent. of iron.

Constipation.—It must be remembered that the intestine is a permanent source of poisons, which, under certain conditions, cause grave alterations in the principal organs, notably in the liver, kidneys, and skin, and serious functional disturbances of the nervous system.

Normally, the organism manages to protect itself against the microörganisms which are found in the intestines; given, however, certain conditions, the toxic products can be generated in excess of the powers of the organism to dispose of them, such as errors of diet, quantitative or qualitative, atony of the muscular walls of the intestine, and, above all, constipation. The higher up in the intestine the stasis occurs, the more serious is the result. Constipation is considered by many surgeons as the most important factor in the production of appendicitis.

There is frequently a condition present which is known as semiconstipation, and which is most deceptive to the patient. There may be a bowel movement every day, but the bowel is never emptied; only the lower portion of the hardened fecal matter is broken off. This is repeatedly found to be the case in examining women for some form of pelvic trouble, and in a woman who says that she has a daily evacuation of the bowels, and has had one that very morning, the sigmoid flexure will be found to be packed with hardened feces.

Besides the clogging up of the digestive system by the non-removal of the waste-products of digestion, the formation of hemorrhoids, which is caused by the mechanical pressure on the veins, and so interferes with their emptying out in the normal way, and the general autointoxication of the entire system, there are also caused in women serious displacements of the pelvic organs together with their congestion and inflammation.

The general symptoms of autointoxication are: headache, vertigo, palpitation of the heart, a feeble and irregular pulse, irritability of temper, melancholia, numbness and tingling in the hands and feet, and the emaciation and loss of weight are sometimes so marked as to lead to the suspicion of malignant disease.

Treatment of Constipation.—This should always be preventive, and the diet is a most important factor. The food should be of a coarse quality, that is, such as directly stimulates the walls of the intestine to contraction by their constitutents, or by the large amount of the indigestible bulk. Corn and Graham bread should be substituted for white bread. Toast is always constipating. Plenty of fresh fruit and vegetables should be eaten. For those who can digest them, raw apples, eaten just before retiring, are a great aid. The drinking of a sufficient quantity of water daily is also essential, and this quantity must be 3 pints. A glassful of cold water, taken the first thing in the morning on rising, is often very effective. If this is insufficient, the phosphate of soda, one teaspoonful to one tablespoonful, may be added.

Habit.—Nowhere is the effect of habit more conspicuous than in the matter of a daily evacuation of the bowels. There should be a fixed time every day for this, and the very best time is in the morning, directly after breakfast. Such a habit, once established, will enforce itself upon the attention and make regularity a necessity. It not infrequently happens that constipation is the result of irregularity in going to the toilet. The school-girl or woman gets up a little late, and, although she may feel the inclination to empty the bowels, she is able to defer it.

If the movement is sufficiently large, one stool daily is sufficient, but where the stool is slight in quantity, there may be two or three during the day, entirely consistent with health, and in a run down state of the system there are apt to be several small movements rather than one full stool. So long as the stools are not watery, the individual may rest assured that there is no looseness of the bowels.

Constipation should never be allowed to become chronic. It is, as has already been shown, the progenitor of myriads of the most serious diseases; and, after the bad habits of years have been established, it is one of the most obstinate of diseases to cure. In every case a good physician should be consulted at once, and the treatment should be persevered in until the cure is complete. It is a well-known fact that all medicines for this trouble lose their effect, the dose has to be increased, and a frequent change made from one laxative to another. When everything else fails, electricity may be resorted to. It is one of our most valuable remedies, since it brings about a cure through the toning up of the muscular walls of the intestine.

The constant use of hot clysters to empty the rectum is one of the most pernicious habits; in this manner the bowel becomes overdistended and loses its tone, and the fecal mass is not sufficiently large to cause the distention of the rectum, which is the normal stimulus leading to the desire to defecate.

The Amount of Food Required.—Food is required for growth and for repair; that is, to build up the organism and make good the loses sustained by physiologic processes, to maintain the heat of the organism, and to supply it with mechanical energy.

It has become an established custom to compare the human body to a machine. Both derive their power from fuel; in both instances the potential energy of the fuel is transformed into kinetic energy or mechanical power which works the machine; in both cases the energy which is not used in work escapes in the form of heat.

The human body uses the mechanical power chiefly in muscular work; the heat is used in warming the body and causing the evaporation of moisture from its surface. The animal organism is much superior to the mechanical engine. It is more economic in the use of fuel; it has a nervous organization rendering it sensible to impressions and capable of directing its energies. The human machine is capable of adapting itself to many circumstances and changes in the demands upon it.

But to enable the body to continue to perform these functions indefinitely it must be properly fed, and a proper feeding of the body requires a knowledge of its composition and the exchanges which are constantly going on. This knowledge is to be derived from a study of its metabolism, the analysis of foods, and a determination of their heat values.

The Classification of Foods.—Foods are divided into proteins, carbohydrates, fats, mineral salts, and water. The body is composed of materials belonging to the same groups. Proteins form the principal part of muscles, bones, and many other tissues of animal bodies; they also constitute some of the most important vegetable structures.

Proteins are probably the most complex compounds in nature; all contain carbon, hydrogen, oxygen, and nitrogen, generally sulphur, and sometimes phosphorus also. They are, with rare exceptions, colloids, that is, glue-like, non-crystalline bodies, which even in solution cannot pass through animal membranes.

The building stones of the body are the amino-acids. All proteins contain them, but the kinds and proportions are not always the same. It has been shown by biologic experiments that life and growth cannot be maintained when certain amino-acids are deficient.

Proteins are of use to the human body as tissue formers, and, secondly, as producers of energy, but they also have a food value as flavoring agents, rendering the food appetizing, and so are to a certain extent stimulants. The palatability of meats and soups are due to their presence. The amino-acids have decided heat value.

The necessity for proteins in the diet has been abundantly demonstrated. Many investigations have shown that when the food contains no protein the waste of nitrogen continues, no matter how abundant the supply of carbohydrates and fats. In other words, a continuous protein cleavage is demanded by the animal organism, and no other nutriments can serve as a substitute for protein to meet this demand. If the food contains no protein, the body tissues will be depleted. It cannot be said that carbohydrates or fats are an essential part of the diet in the sense that protein is, because it is possible to substitute one for the other to produce energy, or to substitute proteins for both.

In spite of these facts, it is safe to assert that the welfare of the human organism is best promoted by a mixed diet, including all three classes of food. The larger part of man’s food is used for the production of energy, and it is physiologically economic that this energy be supplied by the non-nitrogenous nutrients, particularly the carbohydrates, and to allocate to protein, so far as practicable, its special rôle of building material.

Nitrogenous Waste Products.—The end-product of proteins is urea, which is formed from protein decomposition products in relatively large amounts in the liver cells, and, being readily soluble and diffusible, is easily eliminated by the kidneys. Besides urea there are other smaller quantities of nitrogen compounds, the one most deserving of notice being uric acid.

When the proteins are broken down to supply energy, there is always a definite proportion of urea and uric acid residue that must be eliminated through the kidneys. An excessive protein diet would burden these organs beyond their accustomed habit, and flooding the system with these nitrogenous wastes increases the tendency to rheumatism and gout.

Uric acid is of general interest, and when present in the system in abnormally large amounts, as in gouty persons, has a pathologic significance. It is more complex in its composition, and, what is of particular importance, is that it does not readily dissolve. It forms very insoluble salts which have unpleasant tendencies to settle in the joints, causing great pain. Did its metabolism proceed properly, it should be resolved into urea and carbon dioxid. The quantity of uric acid that appears in the urine is the residue that has escaped this oxidation.

Uric acid is built up from purin bodies in the food, so that it would be well for those with a gouty diathesis to abstain, at least when an attack of gout threatens, from all aliment in which purin is at all abundant. The most valuable of the purin-free foods are eggs and milk, and to these the uric-acid sufferer has to turn in times of trouble.

Carbohydrates Physiologically Economic.—This group of compounds comprises starches, sugars, and gums; the elements of which they are composed—namely, carbon, hydrogen, and oxygen—are so balanced that if all of the carbon were removed the residual hydrogen and oxygen would be in the proportions to form water.

Carbohydrates are usually characterized as the fuel portions of the food, or, in other words, that part which is burned to produce the various forms of energy. They are also essential for the well-being of the organism; reduction of the intake below the essential point frequently leads to acetonuria.

When the metabolism is perfect, any carbohydrates consumed in excess of the ordinary requirements are converted into glycogen and fat and stored for providing fuel at a future date. When stored as fats they are completely oxidized in the simplest compounds, carbon dioxid and water, and are eliminated through the lungs and skin, possibly part of the water so formed acting as a solvent for the urinary compounds. Investigations seem to prove that the body has a preference for the carbohydrates over fats or other nutrients as a source of energy. There is every justification for the abundance of starches in man’s diet.

Fat is essential in the food of mankind; it is absorbed ready formed from the food, or manufactured in the body from the proteins and carbohydrates. Neutral fats and fatty acids are valuable foods; their primary function is to supply the body with fuel for heat and energy.

The mineral substances form 5 or 6 per cent. by weight of the human body, and are constantly leaving it by different channels; they are indispensable elements of food. They give solidity and stability to the organism, constituting a considerable proportion of the bones. They keep various proteins in solution and confer upon them the property of electric conductivity. They are necessary for all of the secretions and assist in the general metabolism. The carbonates of soda, potash, iron, and other minerals render the blood and secretions alkaline. The removal of carbon dioxid is performed mainly by the alkaline carbonates, which take it from the blood and surrender it to the lungs. We have already considered the need of sodium chlorid. The total daily requirements of salts is estimated at about 360 grains.

Vitamins a New Factor in Nutrition.—But recent investigation has proved that something more is essential for the maintenance of growth and well-being than protein, carbohydrates, fats, and salts; that food contains a minute portion of accessory bodies, and that when they are deficient or absent from the diet the immature body does not grow, the mature body does not maintain its condition, and there are manifestations of more or less serious disease. These accessory bodies have been called “vitamins,” and they are essential to maintain the normal metabolism of the body.

It is recognized that although vitamins are undoubtedly widely distributed in food products, they occur for the most part in very minute amounts, and the various foods differ in the proportion which they contain. If the diet is made up of substances which are poor in vitamins, or rendered so by their mode of preparation, abnormal metabolic processes have been found to follow.

The study by many scientists during the past few years of the enzymes and their striking specificity; of the salts; of the insufficiency of many proteins, and of the vitamins is epoch making, and has caused a corresponding advance in dietetics. These discoveries are so important as to raise the question whether nutritive failure or success does not depend as much on the accessory bodies—the vitamins, the enzymes, and lipoids—as on the primary element of the diet.

In addition to these food principles, there exists in our food a number of compounds which, while not indispensable, act beneficially as flavorings, stimulants to digestion, etc.; these are termed food adjuncts, and comprise such bases as caffein, essential oils, organic acids, etc.

Standard Dietaries: The Maintenance Diet.—There is great need of standardization and of knowledge regarding the maintenance diet, first among physicians and then among the people in general, or scientific dietaries based on the nutritive value of foods. These are given in terms of proteins, carbohydrates, and fats, together with the aggregate energy of the nutritive value in each. This is the corner-stone of dietetics.

Dietetics is the science of feeding. It has to do with the necessities of the body and the ability of the food to meet these necessities in the various circumstances and conditions of life. The ultimate scientific knowledge concerning human nutrition should be to promote the healthful and economic use of food.

The problems to be dealt with are quite complex. These are: (1) changes in the economic conditions of the population; (2) changes in food production and food supply; (3) changes in the methods of preparing food.

In regard to the influence of the economic conditions of the people on the composition of their diet, it might be expected that a considerable decrease in the earning capacity of the poorer people, or an increase in the cost of foods, would be followed by a change in their diet. Everyday experience teaches that under such conditions the more expensive foods—meat, eggs, and milk—are reduced in the diet. These same foods are also rich in vitamins, so that a reduction here would, therefore, reduce the vitamin content of the dietary unless other dietary complements rich in vitamins, such as legumes, were introduced.

The value of any food as a source of heat and energy is measured by a bomb-calorimeter. The heat given off during the combustion is a measure of the latent or potential energy of the food. The kinetic energy of the food is the amount of heat developed by the proportion which is digested. The unit commonly used is the calorie, or the amount of heat which would be required to raise the temperature of 1 kilogram of water 1° C., which is about equal to that required to raise the temperature of 1 pound of water 4° F.

Heat Value and Digestibility of Foods.—One of the chief functions of food is to supply the body with heat and energy; the food must be capable of digestion and absorption. Herein lies the exact value of any food to the consumer. In science the figures that are given for the digestibility of the various foods refer to the completeness or extent to which the food is dissolved and transferred to the circulation, and an indigestible one is that of which a considerable portion passes out of the system into the feces without being disintegrated and absorbed.

Animal food is more completely digested than vegetable food, as shown by the difference of nitrogen in the feces. In meats 97 per cent. of the protein and 98 per cent. of the fat are absorbed. Lean meat is more rapidly digested than fat, and the flesh of young animals than that of older ones.

The breast of chicken, fresh beef, and mutton are among the most digestible of the solid foods. Raw and rare meats are more easily digested than well-done meats; in other words, cooking lessens the digestibility of meats. Steak should be broiled and never fried; all fried foods are difficult to digest. Veal and pork are both difficult to digest.

Eggs are almost as nutritious as meat; their digestibility is unsurpassed and only equalled by a few foods, such as milk and oysters. They are most easily digested when soft boiled or poached. Dry toast finely broken up and mixed with a soft-boiled egg aids in its digestion. Soft-boiled eggs are more easily digested than raw eggs, but the latter are less irritating to the stomach, probably because they are digested in the intestines. It has been found that two poached or soft-boiled eggs leave the stomach in from two to three hours; that is, in the same time as milk, oysters, white bread, and light fish.

Milk.—Although one of the most completely digested of foods in a mixed diet, milk is not quite so completely digested as meat and eggs. When milk is the sole food (milk diet) the proportion digested depends partly on the amount consumed. With the consumption of 3½ pints of milk daily the loss of milk solids varies from 10 to 11.16 per cent. Young children digest milk more completely than adults.

The addition of aërated waters or crackers broken up in the milk prevent the formation of tough clots, and hence render it more digestible. Hot (not boiled) milk is more digestible than cold. Boiling increases the toughness of the curd, but it destroys the bacteria. Buttermilk and koumiss are more easily digested than cows’ milk.

Wheat flour enters largely into the diet of every family. In producing it the outer coating of the wheat kernel is removed, thus throwing into the milling offals that part of the kernel which is most heavily charged with mineral ingredients and vitamins. The proportion of digestible proteins in white flour is not less than in whole wheat flour, as is so often claimed, but because the latter is richer in mineral ingredients its use is recommended.

Bread is readily digestible; white bread digests more rapidly than brown or black; and crackers more rapidly than either.

Rice is another cereal of great economic importance. As a food for invalids it possesses a high value on account of its digestibility, especially in intestinal diseases. Ordinarily polished rice contains only 0.5 per cent. of cellulose, and almost all of the substance of the grain is absorbed.

There is a perfect analogy between the well-known relation of the polishing of rice to its nutritive value, and the milling of wheat and corn to the nutritive value of wheat, flour, and cornmeal; that is, it loses the mineral ingredients and vitamins.

Potatoes.—In European countries potatoes rank next to bread; this arises from the ease of their digestion. From 92 to 95 per cent. of the starch is absorbed, but there is a loss of 23 per cent. of the protein.

The manner of preparation of vegetable foods determines the proportion of loss. When potatoes are baked or boiled in the skins the loss is negligible. The greatest amount of loss occurs when the skins are removed, the potatoes then allowed to soak in cold water, and placed in cold water to boil. The potatoes should be pared just before cooking, and the water should be boiling hard before they are put in. Mealy potatoes digest more rapidly than waxy ones, and mashed quicker than unmashed.

Oatmeal, barley meal, and other cereals which are not ground very fine do not digest so easily as wheat flour, but much depends on the mode of cooking. When oatmeal-gruel is consumed with a sufficient amount of milk it forms a complete diet.

Nuts are valuable as a source of protein and fat, but they are rather difficult of digestion.

Fats.—As a rule, children do not thrive whose diet is deficient in fats, and even adults are prone to tuberculosis and nervous diseases when fat in the body is deficient.

Cheese is one of the most indigestible of foods. All fried foods are highly indigestible because the fat envelope of the foods has to be melted off before the gastric juice can act on the food substance itself. Pastry is also very indigestible. Of the vegetables, beans, while highly nutritious, are exceedingly difficult of digestion; also boiled cabbage, cauliflower, hot breads, iced drinks, ice-cream, and water-ices.

The Relation of Diet to Various Conditions of Life.—The chief factors influencing bodily needs are age, height, weight, occupation, idiosyncrasies, and atmospheric conditions.

All activity of the human body, whether in the maintenance of its functions or in the performance of labor, is work. These two forms of work may be classified as physiologic and mechanical. Nothing in nutrition is more important than the relation of food to work.

Children are practically in constant motion during their waking hours, and their demand for food energy is from two to three times as much per unit of weight as that for adults.

Sex.—Men and women of the same age and weight, doing the same kind and amount of work, require the same amount of food. The fact that men usually require more food than women is because, as a rule, they weigh more, are more active physically, and perform more external work.

Temperament.—Persons of a nervous type, being more active, use more energy than the phlegmatic, and, therefore, require more food.

Brain Workers.—A man whose work is sedentary and chiefly mental does not need so much food as a man doing muscular work. The amount of carbohydrates required is less, and the amount of fat rather more than for the man doing light muscular work.

Ranke’s diet for the brain worker is: Protein, 100 grams; fats, 100 grams; carbohydrates, 240 grams; giving an energy value of 2310 calories.

The diet should not be bulky, but light and easily digestible. Excess of food and heavy foods are especially bad for brain workers because they produce heaviness, dulness, and drowsiness. Spiced and rich foods upset the alimentary functions, whereby the circulation is flooded with the products of imperfect metabolism to the detriment of the brain.

Trained workmen will do a given amount of labor on less food than untrained, because when persons take up mechanical operations with which they are unfamiliar, or undertake work which exercises a new set of muscles, a unit of work accomplished costs more in food energy than when the muscles have been trained to do a particular thing.

Very strenuous exercise, like athletic contests and unnaturally slow movements, are both wasteful of food energy.

Again, a continuance of the same labor for hours, and a state of fatigue, whether it comes after a longer or shorter time, causes an increase in the energy expenditure per unit of work performed.

Economy in the use of energy that food supplies to the body is most fully secured when the movements of the body are at the natural rate; when periods of intense effort do not occur, and when the labor is not too long continued and is not carried to the point of fatigue.

Standard for Daily Dietaries (American)—Atwater.