short time; that is, until the gastric juice is secreted in sufficient quantity to stop its action. The salivary ferment only acts in an alkaline medium, but the gastric juice is acid, and the permeation of the mass of food in the stomach by this acid fluid effectually stops its action. It takes some time, however, to do this. The food is often swallowed in masses like large pills; and such masses may be sufficiently adhesive to prevent the gastric juice permeating them readily. The time usually stated for the action of the salivary ferment in the stomach is half an hour; and during that period a considerable proportion of the starchy material is transformed into sugar. But the time necessarily varies with the circumstances. The mastication may be very imperfectly performed and very little saliva mixed with the food. Many foods are swallowed without mastication and do not get mixed with saliva at all.

There is also a variation in the secretion of the gastric juice. Moreover, there are two periods for the secretion of the latter fluid. There is what is known as the appetite juice, or psychic juice, and the chemical juice. The thought or taste of food provokes a secretion of gastric juice; this is a mental effect, and therefore the secretion is called the psychic juice. It is an important part of the gastric secretion, and, like a crocodile's tears, flows at the thought, sight, smell, or taste of a pleasant meal. It is encouraged by mastication, hunger, the enjoyment of food, pleasant company, and agreeable surroundings. It does not occur when a person has no appetite; and it is checked by pain, fatigue, fear, worry, and mental distress. This portion of the gastric juice, therefore, would not be secreted if a person had already been fed; nor if the food is poured into the stomach through an oesophageal tube, or without the knowledge of the individual.

The second phase of gastric digestion depends upon the presence of food in the stomach. It is due to the formation of the chemical juice; and the secretion is believed to be independent of the nervous system. It begins some little time after the food has reached the stomach, and is especially provoked by the presence of meat, game, fish, eggs, soup, and similar substances. Why should these substances cause a secretion of gastric juice more than other foods ? It has been shown that there is a strong association between the various functions of the body, that the activity of one organ depends on the proper performance of function by others. Efficient mastication encourages the secretion of gastric juice; but imperfect mastication leads to a smaller secretion of gastric juice. This is chiefly due to the association of the nerves which give rise to the appetite or psychic juice. But it has also been shown that the association of functions is due to the formation of chemical bodies called hormones, or chemical messengers. Gastric digestion produces such a hormone among the early products of digestion. The substances which most easily provoke the second flow of gastric juice, the chemical juice, are derived from the digestion of proteins (meat, fish, game, eggs, etc.). It follows, therefore, that an abundant flow of the chemical juice depends upon the previous secretion of the psychic or appetite juice; for the materials necessary to provoke it arise from the action of the appetite juice. Substances, however, occur in meat-juice, meat-extract, soup, and suchlike articles, which have the same effect; when these are administered alone to dogs and men they provoke the secretion of gastric juice. This is the scientific basis for the custom of taking a small quantity of soup at the beginning of a dinner. When the chemical secretion is once started it goes on continuously for a period which depends more or less on the nature of the food and the time required for its digestion ; but the flow is not always at the same pressure, it gradually rises and falls, the secretion reaching its height from one to two hours after a meal.

The gastric juice is an acid fluid. It contains several ferments or enzymes, the chief one being pepsin, which digests meat, game, poultry, fish, eggs, milk, cheese, the gluten of bread, and other proteins. It also contains a ferment called rennin, which curdles milk; and another one which assists in the digestion of fat. It should be noted that pepsin only acts in an acid medium; and that the acid of the gastric juice is hydrochloric acid. The proportion of this acid is very small, only about two parts in 1000; but, little as there is of it, it is absolutely essential for the process of digestion in the stomach.

The passage of food through the alimentary canal has been studied by aid of the X-rays. A mouthful of fluid occupies five or six seconds in passing from the back of the mouth to the stomach. When solids are properly masticated they pass down to the stomach in about the same length of time. But when such food is not well masticated, or is swallowed in masses like pills, its passage is much slower. Swallowing is an involuntary act, when the food has passed through the mouth. It is due to a muscular movement, which passes the food downward. The opening into the stomach is guarded by the cardiac sphincter, which relaxes when the food passes through it, and closes again to prevent regurgitation.

The empty stomach is more or less pear-shaped, the wide end (at the left side of the body) containing some gas, the rest of the organ being contracted and its mucous membrane thrown into folds. The food which arrives in the stomach accumulates at the wide end, called the fundus. During digestion the stomach is divided into two parts - the fundus or cardiac portion and the pyloric portion. The fundus is the store-house of the food, and is not affected by peristaltic muscular movements. The pyloric portion is the active part, and it is affected by peristaltic muscular movements. The contents in the fundus remain alkaline in the centre, only the outer portion becoming acid; but all the food in the pyloric portion is acid.