As has been indicated, pepsin and hydrochloric acid are the important constituents of gastric juice. It is noteworthy, however, that it is the combination of the two that is effective in digestion. Pepsin without acid is of no avail, and acid without pepsin can accomplish little in the digestion of food. Pepsin and acid are secreted by different gland cells in the stomach, and gastric insufficiency, or so-called indigestion, may arise from either a condition of apepsia or from hypoacidity. It is worthy of comment that the amount of hydrochloric acid secreted during 24 hours by the normal individual, under ordinary conditions of diet, amounts to what would constitute a fatal dose of acid if taken at one time in concentrated form. At the outset of gastric secretion, the fluid shows only a slight degree of acidity, but as secretion proceeds, the acidity rises to 0.2–0.3 per cent of hydrochloric acid. The main action of gastric juice is exerted on proteid foods, which under its influence are gradually dissolved and converted into soluble products known as proteoses and peptones. It is a process of peptonization, in which the proteid of the food is gradually broken down into so-called hydrolytic cleavage products. The enzyme, like the ptyalin of saliva, is influenced by temperature, maximum digestive action being manifested at about 38° C., the temperature of the body. Further, a certain degree of acidity is essential for procuring the highest degree of efficiency. Ordinarily, it is stated that digestive action proceeds best in the presence of 0.2 per cent hydrochloric acid, but what is more essential for vigorous digestion is a certain relationship between the acid, pepsin, and proteid undergoing digestion. As pepsin and the amount of proteid are increased, the amount of acid, and its percentage somewhat, must be correspondingly increased if digestion is to be maintained at the maximum.