A reservoir for the bile is provided by a small, membranous sack, called the _gall bladder_, located on the underside of the liver. The bile pa.s.ses from the gall bladder, and from the right and left lobes of the liver, by three separate ducts. These unite to form a common tube which, uniting with the duct from the pancreas, empties into the duodenum. Though usually described as a digestive gland, the liver has other functions of equal or greater importance (Chapter XIII).

*The Bile* is a golden yellow liquid, having a slightly alkaline reaction and a very bitter taste. It consists, on the average, of about 97 per cent of water and 3 per cent of solids.(60) The solids include bile pigments, bile salts, a substance called cholesterine, and mineral salts. The pigments (coloring matter) of the bile are derived from the hemoglobin of broken-down red corpuscles (page 27).

Much about the composition of the bile is not understood. It is known, however, to be necessary to digestion, its chief use being to aid in the digestion and absorption of fats. It is claimed also that the bile aids the digestive processes in some general ways-counteracting the acid of the gastric juice, preventing the decomposition of food in the intestines, and stimulating muscular action in the intestinal walls. No enzymes have been discovered in the bile.

*The Pancreas* is a tapering and somewhat wedge-shaped gland, and is so situated that its larger extremity, or head, is encircled by the duodenum.

From here the more slender portion extends across the abdominal cavity nearly parallel to and behind the lower part of the stomach. It has a length of six or eight inches and weighs from two to three and one half ounces. Its secretion, the pancreatic juice, is emptied into the duodenum by a duct which, as a rule, unites with the duct from the liver.

*The Pancreatic Juice* is a colorless and rather viscid liquid, having an alkaline reaction. It consists of about 97.6 per cent of water and 2.4 per cent of solids. The solids include mineral salts (the chief of which is sodium carbonate) and four different chemical agents, or enzymes,-trypsin, amylopsin, steapsin, and a milk-curding enzyme. These active const.i.tuents make of the pancreatic juice the most important of the digestive fluids.

It acts with vigor on all of the nutrients insoluble in water, producing the following changes:

1. It converts the starch into maltose, completing the work begun by the saliva. This action is due to the _amylopsin_,(61) which is similar to ptyalin but is more vigorous.

2. It changes proteids into peptones and proteoses, completing the work begun by the gastric juice. This is accomplished by the _trypsin_, which is similar to, but more active than, the pepsin.

3. It digests fat. In this work the active agent is the _steapsin_.

The necessity of a milk-curding enzyme, somewhat similar to the rennin of the gastric juice, is not understood.

*Digestion of Fat.*-Several theories have been proposed at different times regarding the digestion and absorption of fat. Among these, what is known as the "solution theory" seems to have the greatest amount of evidence in its favor. According to this theory, the fat, under the influence of the steapsin, absorbs water and splits into two substances, recognized as glycerine and fatty acid. This finishes the process so far as the glycerine is concerned, as this is soluble in water; but the fatty acid, which (from certain fats) is insoluble in water,(62) requires further treatment. The fatty acid is now supposed to be acted on in one, or both, of the following ways: 1. To be dissolved as fatty acid by the action of the bile (since bile is capable of dissolving it under certain conditions). 2. To be converted by the sodium carbonate into a form of soap which is soluble in water.

The emulsification of fat is known to occur in the small intestine. By this process the fat is separated into minute particles which are suspended in water, but not changed chemically, the mixture being known as an _emulsion_. While this is believed by some to be an actual process of digestion, the advocates of the solution theory claim that it is a process accompanying and aiding the conversion of fat into fatty acid and glycerine.(63)

*The Intestinal Juice* is a clear liquid with an alkaline reaction, containing water, mineral salts, and certain proteid substances that may act as enzymes. It a.s.sists in bringing about an alkaline condition in the small intestine and aids in the reduction of cane sugar and maltose to the simple sugars, dextrose and levulose. Since it is difficult to obtain this liquid in sufficient quant.i.ties for experimenting, its uses have not been fully determined. Recent investigators, however, a.s.sign to it an important place in the work of digestion.

*Work of the Small Intestine.*-The small intestine is the most important division of the alimentary ca.n.a.l. It serves as a receptacle for holding the food while it is being acted upon; it secretes the intestinal juice and mixes the food with the digestive fluids; it propels the food toward the large intestine; and, in addition to all this, serves as an organ of absorption.

Digestion is practically finished in the small intestine, and a large portion of the reduced food is here absorbed. There is always present, however, a variable amount of material that is not digested. This, together with a considerable volume of liquid, is pa.s.sed into

*The Large Intestine.*-The large intestine is a tube from five to six feet in length and averaging about one and one half inches in diameter. It begins at the lower right side of the abdominal cavity, forms a coil which almost completely surrounds the coil of small intestine, and finally terminates at the surface of the body (Figs. 2, 71 and 73). It has three divisions, known as the caec.u.m, the colon, and the r.e.c.t.u.m.

[Fig. 73]

Fig. 73-*Pa.s.sage from small into large intestine.* At the ileo-caecal valve is the narrowest constriction of the food ca.n.a.l.

_The caec.u.m_ is the pouch-like dilatation of the large intestine which receives the lower end of the small intestine. It measures about two and one half inches in diameter and has extending from one side a short, slender, and blind tube, called the _vermiform appendix_. This structure serves no purpose in digestion, but appears to be the rudiment of an organ which may have served a purpose at some remote period in the history of the human race. The caec.u.m gradually blends into the second division of the large intestine, called the colon.

_The colon_ consists of four parts, described as the ascending colon, the transverse colon, the descending colon, and the sigmoid flexure, or sigmoid colon. The first three divisions are named from the direction of the movement of materials through them and the last from its shape, which is similar to that of the Greek letter sigma (S).

_The r.e.c.t.u.m_ is the last division of the large intestine It is a nearly straight tube, from six to eight inches in length, and connects with the external surface of the body.

The general structure of the large intestine is similar to that of the small intestine, and, like the small intestine, it is held in place by the peritoneum. It differs from the small intestine, however, in its lining of mucous membrane and in the arrangement of the muscular coat. The mucous membrane presents a smooth appearance and has no villi, while the longitudinal layer of the muscular coat is limited to three narrow bands that extend along the greater length of the tube (Fig. 74). These bands are shorter than the coats, and draw the large intestine into a number of shallow pouches, by which it is readily distinguished from the small intestine (Fig. 71).

[Fig. 74]

Fig. 74-*Section of large intestine*, showing the coats. 1. Serous coat.

2. Circular layer of muscle. 3. Submucous coat. 4. Mucous membrane. 5.

Muscular bands extending lengthwise over the intestine.

*Work of the Large Intestine.*-The large intestine serves as a receptacle for the materials from the small intestine. The digestive fluids from the small intestine continue their action here, and the dissolved materials also continue to be absorbed. In these respects the work of the large intestine is similar to that of the small intestine. It does, however, a work peculiar to itself in that it collects and retains undigested food particles, together with other wastes, and ejects them periodically from the ca.n.a.l.

*Work of the Alimentary Muscles.*-The mechanical part of digestion is performed by the muscles that encircle the food ca.n.a.l. Their uses, which have already been mentioned in connection with the different organs of digestion, may be here summarized: They supply the necessary force for masticating the food. They propel the food through the ca.n.a.l. They mix the food with the different juices. At certain places they partly or completely close the pa.s.sage until a digestive process is completed. They may even cause a reverse movement of the food, as in vomiting. All of the alimentary muscles, except those around the mouth, are involuntary. Their work is of the greatest importance.

*Other Purposes of the Digestive Organs.*-The digestive organs serve other important purposes besides that of dissolving the foods. They provide favorable conditions for pa.s.sing the dissolved material into the blood.

They dispose of such portions of the foods as fail, in the digestive processes, to be reduced to a liquid state. A considerable amount of waste material is also separated from the blood by the glands of digestion (especially the liver), and this is pa.s.sed from the body with the undigested portions of food. Then the food ca.n.a.l (stomach in particular) is a means of holding, or storing, food which is awaiting the processes of digestion. Considering the number of these purposes, the digestive organs are remarkably simple, both in structure and in method of operation.

HYGIENE OF DIGESTION

Many of the ills to which flesh is heir are due to improper methods of taking food and are cured by observing the simple rules of eating. Habit plays a large part in the process and children should, for this reason, be taught early to eat properly. Since the majority of the digestive processes are involuntary and the food, after being swallowed, is practically beyond control, careful attention must be given to the proper mastication of the food and to such other phases of digestion as are under control.

*Necessity for Thorough Mastication.*-Mastication prepares the food for the digestive processes which follow. Unless the food has been properly masticated, the digestive fluids in the stomach and intestines cannot act upon it to the best advantage. When the food is carefully chewed, a larger per cent of it is actually digested-a point of importance where economy in the use of food needs to be practiced.

A fact not to be overlooked is that one cannot eat hurriedly and practice thorough mastication. The food must not be swallowed in lumps, but reduced to a finely divided and pulpy ma.s.s. This requires time. The one who hurries through the meal is necessarily compelled to bolt his food. Thirty minutes is not too long to give to a meal, and a longer period is even better.

Perhaps the most important result of giving plenty of time to the taking of food is that of _stimulating the digestive glands to a proper degree of activity_. That both the salivary and gastric glands are excited by the sight, smell, and thought of food and, through taste, by the presence of food in the mouth, has been fully demonstrated. Food that is thoroughly masticated and relished will receive more saliva and gastric juice, and probably more of other juices, than if hastily chewed and swallowed. This has a most important bearing upon the efficiency of the digestive processes.

*Order of Taking Food.*-There has been evolved through experience a rather definite order of taking food, which our knowledge of the process of digestion seems to justify. The heavy foods (proteids for the most part) are eaten first; after which are taken starchy foods and fats; and the meal is finished off with sweetmeats and pastry.(64) The scientific arguments for this order are the following:

1. By receiving the first of the gastric flow the proteids can begin digesting without delay. Since these are the main substances acted on in the stomach, the time required for their digestion is shortened by eating them first.

2. Sugar, being of the nature of predigested starch, quickly gets into the blood and _satisfies the relish_ for food. The result of taking sugar first may be to cause one to eat less than he needs and to diminish the activity of the glands.

3. Fat or grease, if taken first, tends to form a coating over the walls of the stomach and around the material to be digested. This prevents the juices from getting to and mixing with the foods upon which they are to act.

4. Starch following the proteids, for the most part, does not so quickly come in contact with the gastric juice. This enables the ptyalin of the saliva to continue its action for a longer time than if the starch were eaten first.

*Liquids during the Meal.*-Liquids as ordinarily taken during the meal are objectionable. They tend to diminish the secretion of the saliva and to cause rapid eating. Instead of eating slowly and swallowing the food only so fast as the glands can supply the necessary saliva, the liquid is used to wash the food down. Water or other drinks should be taken after the completion of the meal or when the mouth is completely free from food.

Even then it should be taken in small sips. While the taking of a small amount of water in this way does no harm, a large volume has the effect of weakening the gastric juice. Most of the water needed by the body should be taken between meals.

*The State of Mind* has much to do with the proper digestion of the food.

Worry, anger, fear, and other disturbed mental states are known to check the secretion of fluids and to interfere with the digestive processes.

While the cultivation of cheerfulness is important for its general hygienic effects, it is of especial value in relation to digestion.

Intense emotions, either during or following the meal, should if possible be avoided. The table is no place for settling difficulties or administering rebuke. The conversation, on the other hand, should be elevating and joy giving, thereby inducing a desirable reactionary influence upon the digestive processes.

*Care of the Teeth.*-The natural teeth are indispensable for the proper mastication of the food. Of especial value are the molars-the teeth that grind the food. The development of the profession of dentistry has made possible the preservation of the teeth, even when naturally poor, as long as one has need of them. To preserve the teeth they must be kept clean.

They should be washed at least once a day with a soft-bristled brush, and small particles of food, lodged between them, should be removed with a wooden pick. The biting of hard substances, such as nuts, should be avoided, on account of the danger of breaking the enamel, although the chewing of tough substances is considered beneficial.

Decayed places in the teeth should be promptly filled by the dentist. It is well, even when decayed places are not known to exist, to have the teeth examined occasionally in order to detect such places before they become large. On account of the expense, pain, and inconvenience there is a tendency to put off dental work which one knows ought to be done.

Perhaps in no other instance is procrastination so surely punished. The decayed places become larger and new points of decay are started; and the pain, inconvenience, and expense are increased proportionately.

*The Natural Appet.i.te* should be followed with reference to both the kind and the amount of food eaten. No system of knowledge will ever be devised which can replace the appet.i.te as an aid in the taking of food. _It is_ _nature's means of indicating the needs of the body_. The natural appet.i.te may be spoiled, however, by overeating and by the use of highly seasoned foods, or by indulging in stimulants during the meal. It is spoiled in children by too free indulgence in sweetmeats. By cultivating the natural appet.i.te and heeding its suggestions, one has at his command an almost infallible guide in the taking of food.