The gastric juice contains three principal enzyms or digestive principles. These are hydrochloric acid, pepsin, and rennet. The hydrochloric acid and the pepsin are secreted by different cells, and could be considered as separate digestive juices, but as the action of one is dependent upon the other, I will consider these actions as one. Pepsin, in the presence of hydrochloric acid, acts on proteids, and changes them into proteoses and peptone. Comparatively little food is completely peptonized in gastric digestion. Proteoses are intermediate products between food proteids and peptone, being the principal product of the action of the gastric juice. Thus it is seen that this stomach-action is only preparatory for the digestive processes of the intestines.

Action of pepsin on pro-teids.

The gastric juice does not act on fat, but in the case of animal food, in which the membranes or connective tissues that enclose the fat-cells are formed of pro-teid material, the gastric juice sets the fat-globules free by dissolving these enclosing membranes.

The chief action of hydrochloric acid in the stomach is to aid the action of the pepsin. Pepsin alone has no digestive power. There are no other acids produced by the secretive glands of the stomach. If other acids are found in the contents of the stomach, it is because they have been taken in with the food, or produced by abnormal fermentation.

Peptone and proteoses.

Action of gastric juice on fat.

The source of hydrochloric acid is from.

the sodium chlorid or common salt of the blood. The secreting cells of the stomach-glands are thought to have the power to form hydrochloric acid by uniting the chlorin of the salt with the hydrogen of the water. This is a very unusual chemical process, and has not yet been successfully produced in a laboratory.

One of the chief functions of hydrochloric acid in the stomach is that of an antiseptic. In other words, hydrochloric acid kills bacteria. This is not true of all bacteria, for some germs can live in an acid medium, while others may live best in an alkaline solution. The alternation of the digestive juices from alkali to acid is a provision of Nature which has a dual purpose:

Purpose of hydrochloric acid.

How hydrochloric acid is formed.

Hydrochloric acid as an antiseptic.

1 To reduce food to the finest possible solution; that is, to subdivide or to digest food elements into a form that will admit of assimilation and use

2 To destroy bacteria and enzyms of plant and animal origin that are taken into the digestive tract with food

(These two facts constitute additional reasons for the thorough mastication of food)

By such plan Nature provides for the digestion of food only by such enzyms and ferments as will produce a finished product wholly suited to the particular requirements of the body. When we at-

Object of alternating digestive juices tempt by artificial processes to digest our food with other enzyms than those of our own digestive organs, or take into the stomach large quantities of food without proper mastication, which causes fermentation, we may expect that the nutritive material supplied to our tissues will not be perfectly adapted to the needs of human cell-growth, and, as a natural result, consequent derangement of the body-functions will take place.

The rennet of the gastric juice is primarily for the purpose of digestion. Other than this it has no particular function that has yet been discovered.

The problem as to why the stomach does not digest itself has puzzled scientists for many years. Investigations of the twentieth century have at last solved this fascinating question. The walls of the human stomach are composed of proteid material, and should be dissolved by the gastric juice according to all known chemical laws. The explanation formerly given was that the stomach did not digest itself because it was alive. This answer did not satisfy scientists.

Rennet.

Why stomach does not digest itself.

There has recently been discovered an enzym, known as antipepsin, which is secreted by the cells in the stomach-walls. This anti-pepsin destroys the action of the pepsin, thus in turn preventing its action on the stomach-wall itself. Were antipepsin secreted in sufficiently large quantities to mix with the food in the stomach-cavity, no digestion could take place. The presence of this antipepsin in the stomach-walls has been proved in the following manner: The arteries leading to a portion of the stomach-wall of a dog was severed. This portion, receiving no blood supply, did not form the usual amount of antipepsin. The secretion of pepsin went on in the remainder of the animal's stomach, but digested that portion of the stomach-wall which was receiving no blood supply; that is, secreting no antipepsin.

Antipepsin in the blood.

Bile

The bile is a juice secreted by the liver and is alkaline in character. It is collected by the biliary ducts to be conveyed into the duodenum. The most important constituents of bile are bile salts and sodium glycocholate. The chief purposes of bile are to emulsify fats, thus aiding them to pass through the intestinal walls, and to stimulate intestinal peristalsis.