When food first reaches the stomach, the alkaline saliva which accompanies it neutralizes the acidity of the gastric juice. For some time, probably about half an hour, the conversion of starch into sugar is still carried on by the ptyalin of the saliva, owing chiefly to the difficulty which the gastric juice encounters in permeating the masses of masticated food. The _Bacillus acidi lactici_ is always present in the stomach. It converts some of the sugar into lactic acid; of this a small quantity is further changed into butyric and acetic acids, with the formation of carbonic acid and hydrogen gas. After a while the lactic acid is absorbed, and hydrochloric acid alone remains. The secretion by the gastric glands of so powerful a mineral acid as hydrochloric has always aroused interest. How is it possible for the gland-cells to produce it without injury to them selves, or for the stomach to contain it without self-digestion? Many chemical and physical theories have been advanced in the belief that they rendered the process of its production less difficult to understand. All such theories are, however, inadequate to explain the secretion as a discontinuous process, which occurs only as a response to demand. That the source of the acid is the sodic chloride which the gland-cells take from the blood does not need assertion, but we cannot picture the process by which this exceedingly stable compound is decomposed otherwise than on the assumption that weaker acids, or, rather, acid salts, are also absorbed by the cells, and that, in accordance with the laws which govern the composition of salts in solution, an exchange of acids occurs. If sodic chloride and any acid salt—acid phosphate of sodium, for example—are in solution in water, the salts do not retain their form as we know them when isolated by crystallization. The mixture contains “free” hydrochloric as well as “free” phosphoric acid. It may be assumed that within secreting cells a similar exchange of acids takes place. By a process which we term “vital,” the acids are kept apart, and the hydrochloric acid is extruded by the cells. In the present state of knowledge this vital action is mysterious; but it is no more mysterious than the isolation of pepsin, or any other metabolic event which occurs within a cell.