23. The stomach has also two openings; that by which food enters, being situated near the heart, is called the _cardiac_, or heart orifice; the other is the _pylorus_, or "gatekeeper," which guards the entrance to the intestines, {91} and, under ordinary circumstances, permits only such matters to pass it as have first been properly acted upon in the stomach. Coins, buttons, and the like are, however, readily allowed to pass, because they can be of no use if retained. The soft and yielding texture of this organ--the stomach--indicates that it is not designed to crush and comminute solid articles of food. [Illustration: FIG. 23.--THE ORGANS OF DIGESTION. O, Oesophagus. S, Stomach. L, Liver. M, Pylorus. C, Large Intestines. P, Pancreas. I, Small Intestines. N, Spleen. G, Gall-bladder. ] [Sidenote: 24. What is meant by the gastric juice?] 24. THE GASTRIC JUICE.--We have seen how the presence of food in the mouth excites the salivary glands, causing the saliva quickly to flow. In the same manner, when food reaches the stomach, its inner lining, the mucous membrane, is at once excited to activity. (At first its surface, which while the stomach is empty presents a pale pink hue, turns to a bright red color, for the minute blood-vessels which course through it, are filled with blood. Presently a clear, colorless, and acid fluid exudes, drop by drop, from its numerous minute glands or "tubules," until finally the surface is moistened in every part, and the fluid begins to mingle with the food. This fluid is termed the gastric juice. {92} [Sidenote: 25. What is the office of the gastric juice? Acidity of the gastric juice? Quantity of gastric juice used? What becomes of it?] 25. The gastric juice is the proper solvent of certain articles of food, especially those belonging to the albuminoid class. This solvent power is due to its peculiar ingredient, _pepsin_; in digestion, this substance acts like a ferment; that is, it induces changes in the food simply by its presence, but does not itself undergo change. The acidity of the gastric juice, which is due to _lactic acid_, is not accidental; for we find that the pepsin cannot act in an alkaline solution--that is, one which is not acid or neutral. The quantity of gastric juice secreted daily is very large, probably not less than three or four pints at each meal. Though this fluid is at once used in the reduction of the food, it is not lost; since it is soon re-absorbed by the stomach, together with those parts of the food which it has digested and holds in solution. [Sidenote: 26. Muscular coat of the stomach? Expansion and contraction of its fibres? Action of the fibres?]