Food is forced by peristaltic action through the œsophagus into the cardiac portion of the stomach, where it comes in contact with the gastric juice. The gastric juice is a fluid which contains hydrochloric acid (HCl) and two ferments, pepsin and rennin. The flow of gastric juice is intermittent, but about the same quantity is secreted, daily, as of saliva.

Pepsin acts upon proteid foods, changing some to albumoses and peptones, while by far the largest part is simply swollen in gastric digestion. Pepsin is the principal ferment which acts upon gelatin. Rennin is a milkcurdling ferment.

Cut showing the division of the stomach into two portions.

The digestion of starch continues for about one-half hour after entering 'the stomach; by that time the food material is sufficiently mixed with the gastric juice to render the whole acid, thus destroying the alkaline reaction. Fats are set free, and to some extent melted in the stomach. About six per cent of proteids, twenty per cent of sugar, and some salts are absorbed through the walls of the stomach. Water passes on with the partially digested food. If the food is liquid, the water leaves the stomach very quickly, and in drinking water some leaves the stomach before the last swallow is taken.

The stomach has two muscular motions. The first is a turning movement, which takes place in the larger or cardiac portion, mixing the food with the gastric juice, thus bringing the whole to a semi-fluid consistency.

The second is a wave-like movement which takes place in the pyloric end, by means of which the food is allowed to pass by intervals into the duodenum, which is the entrance to the small intestine.

The juice poured out in the pyloric portion contains no hydrochloric acid, but is neutral or slightly alkaline; pepsin is present.

The quantity of gastric juice varies not only in different individuals, but in the same individual according to the diet. Extremes in temperature exert an influence on gastric digestion. Pawlow has made many very interesting experiments along this line, and has discovered that a diet composed chiefly of meat produces a large flow of gastric juice poor in ferments; bread produces a small flow of gastric juice rich in ferments; while milk produces a moderate flow of gastric juice and a moderate amount of ferments. To keep in good normal condition without gain or loss of body weight, a plain, wholesome, mixed diet is the most satisfactory.

There is great danger, especially in the young, of becoming addicted to digestive habits. Each food calls forth a special gastric juice, and if the diet is limited to a few foods the power to assimilate others becomes lessened; therefore if the diet is increased, gastric disturbances are apt to occur. When a patient has been kept for some time on a milk diet, other foods must be introduced gradually, and in small quantities, for the comfort of the individual.

The stomach being capable of great distension, often gives rise to the taking of too much food at a single time. Three meals daily meet the needs of the average person. Dinner should be the heartiest meal, and should be served after the work of the day is over, when sufficient time may be allowed for eating, which may be followed by rest.

In cases of impaired digestion, fifteen or twenty minutes is recommended for rest after each meal. Where a light breakfast is taken, a lunch should be indulged in in the middle of the forenoon. There are frequently found people of small stomach capacity who seem to require food at frequent intervals in small quantities; whereas if a meal is taken which would serve the needs of the average person, gastric disturbances follow.

Appetite has a marked effect on gastric digestion, and it is often necessary to stimulate the appetite. Attractive surroundings (plants, flowers, music, singing birds, etc.) are provided in institutions where money is not the first consideration. The sanitariums and hospitals in Germany are far in advance of ours in this respect. Good cooking plays a far more important part than surroundings, and it is the duty of the cook to stimulate the appetite by appealing to the sense of hearing, smell, sight, and taste.

While the stomach plays but a small part in digestion, the digestibility of foods is calculated by the length of time they remain in this organ. The average meal leaves the stomach in about four or five hours. The following table will be found of value in considering the ease or difficulty with which certain foods are digested.

Table showing Time required for the Digestion of some Important Foods.

Digestion principally takes place in the small intestine. The stomach acts as a reservoir for food, playing but a small part in digestion. Many instances are recorded where people have been well nourished after the removal of the stomach. There was, however, a radical change in the diet, the food being taken in a liquid or semi-solid state. Food in the small intestine comes in contact with two fluids, the pancreatic juice and the bile (which is poured out from the liver), both of which are alkaline fluids. The flow of pancreatic juice is suspended except during digestion, while the flow of bile is constant but greatest during digestion.

The pancreatic juice contains four ferments, -amylopsin, trypsin, steapsin, and invertin.

Amylopsin acts upon starches and completes their digestion. Trypsin completes the digestion of proteids. Its action is similar to the action of pepsin in the gastric juice, but it is able to act in an alkaline medium. The proteids which were simply swollen in the stomach are now penetrated by this juice and their digestion is completed. Steapsin splits the fats into glycerine and fatty acids. The fatty acids combine with an alkaline solution

and form soap. The bile salts also play an important

part in the digestion of fats, but affect neither proteids nor carbohydrates. They, too, combine with fatty acids to form soap, and soap forms an emulsion. Fats thus emulsified are ready for absorption. Invertin acts upon cane sugar, changing it to levulose and dextrose.

The liver acts as a storehouse for the body, to be called upon as needed. Some of the carbohydrates which during digestion have been converted into sugar, on reaching the liver are changed into glycogen, and glycogen is reconverted into sugar before entering the general circulation.

The digested food is now ready for absorption, although, as has been stated, the digestion of all foods need not be completed before the absorption of some foods take place. For example, alcohol, sugar, and some proteids and salts are absorbed in the stomach.

Food is moved along from the small to the large intestine by peristaltic muscular contraction. Absorption takes place to a small extent in the small intestine, but to a much larger extent in the large intestine.

Bile salts, on account of their great value, are nearly absorbed before reaching the rectum, and are used over and over again. Salts, bile pigments, connective tissue, and cellulose are not digested (although some authorities affirm that the cellulose in young vegetables is partially digested); these, with the waste products of metabolism, are excreted through the rectum as fæces.