Within the past few years many remarkable discoveries have been made in regard to the secretion of the various digestive juices. Until some ten or fifteen years ago it was believed that the secretion of the digestive juices depended wholly upon the presence of food in the alimentary canal. The recent discoveries in this branch of physiology are to be accredited chiefly to Professor Palloff, a Russian scientist, and his co-workers. The facts that are now known regarding this part of Nature's work are essentially as follows:

The secretion of the various substances which make up the digestive fluids of Recent discoveries concerning digestive juices the body depend upon two kinds of stimuli:

1 Direct nerve stimulus from the central nervous system.

2 The chemical stimulus on the walls of the digestive organs.

Depending upon either or both of these sources of stimulation, the digestive juices of the body are regulated in quantity, and what is much more worthy of note, in their actual chemical composition. Thus it will be readily seen how far-reaching in its effect upon scientific dietetic treatment is the knowledge of the influence of various foods, quantities, and combinations.

Professor Palloff's discoveries throw some very important light on the comparative digestibility of foods. The former method of estimating the digestibility of food was first to analyze the food, and then to analyze the intestinal residue, and subtract the undigested remnant of each particular class of food from the amount originally eaten. By such means it was possible to show that certain foods were, say 80 or 90 per cent digestible, as the case might be. By this method no allowance was made for the amount of nutrition or material that was consumed by the body in the digestion of these particular foods. According to these investigations, milk and meat were about equally digestible. It was not known that the digestion of milk requires only a small fraction of the energy that is necessary to digest meat, or proteids from vegetable sources. Thus it is obvious that when it is desirable to get a large amount of available nitrogen into the system, with as little expenditure of energy as possible, milk is a food par excellence. This is very logical inasmuch as the sole purpose of milk is food for animal life. The amount of acidity in gastric juice that must be secreted for the digestion of meat is much in excess of that required for a given amount of vegetable food. The amount of acidity required is greatest for milk, second for meat, and least for bread. The digestive energy required is greatest for bread, second for meat, and least for milk. From this we learn that starchy foods are unsuitable for those who are afflicted with hyperchlorhydria or supersecretion of hydrochloric acid, as the excess of acid prevents their digestion by neutralizing the alkali of the intestines.

Comparative digestibility of foods.

The saliva secreted when nitrogenous food is eaten does not contain as much ptyalin as that secreted when starchy food is consumed; for this reason the thorough insalivation of starchy foods is much more important than that of meat, milk, and eggs. Some authorities have recently advised that people should not chew meat at all, but should swallow it as do carnivorous animals. This advice, however, is not altogether sound. In the first place, man is not a carnivorous animal, and the gastric juice of the human stomach does not act as rapidly on flesh foods as does the gastric juice of meat-eating animals, but if meat be taken into the human stomach, either in large or in small quantities, decomposition may take place before digestion has proceeded far enough to prevent the action of micro-organisms. The mental influence upon the secretion of digestive fluids may originate from thought, or may be brought about reflexively by the sight, or by the smell of food. All are familiar with the experience of having one's mouth water at the sight of a particularly appetizing dish. Many of us have undergone the same experience by merely thinking of some particular food of which we are fond.

Comparative acidity and energy required in digestion.

Insalivation of starchy foods and meats.

Scientific investigation has shown that the secretion of saliva is only an example of what takes place in the other digestive organs. The experiments of the ingenious Russian scientist, heretofore mentioned, prove that the act of tasting and of swallowing food was the chief factor in determining the secretion of the juices from the stomach-walls. In a series of operations upon dogs, performed by skilled surgeons, certain interesting facts were observed. The esophagus was severed and made to open externally so that the food swallowed did not pass into the stomach. The secretion of gastric juice was then determined in the case of different foods which were taken into the dog's mouth and swallowed, but which did not reach the stomach. Not only did this act of pretended feeding start a flow of gastric juice, but the juice secreted in the case of different foods was especially adapted to the particular food, according to the general principle which we have already discussed.

Digestive juices vary with different foods.

These facts emphasize several important considerations regarding our diet:

1 We should eat slowly and get the whole taste out of food by thorough mastication, because taste largely controls the secretion of the digestive fluid

2 We should not disguise our food by high seasoning

3 Foods that do not require the same digestive principles should not be taken at the same meal

Fermentation is the term generally applied to changes that take place in such food substances as carbohydrates, due to the growth of bacteria, while the term putrefaction is applied in a similar way to "Fermentation" and "Putrefaction" compared the changes taking place in nitrogenous or proteid materials. Both of these chemical changes are exceedingly harmful.