Evidently, the complexities attending digestive proteolysis are connected primarily with the complex nature of the proteids themselves, while proteolysis, as a process, is made possible through the natural tendency of the proteids to undergo hydration and cleavage. LECTURE II. PROTEOLYSIS BY PEPSIN-HYDROCHLORIC ACID, WITH A CONSIDERATION OF THE GENERAL NATURE OF PROTEOSES AND PEPTONES. PROTEOLYSIS BY PEPSIN-ACID. Gastric digestion is essentially an acid digestion. As a proteolytic agent, pepsin can act only in the presence of acid, and we have every reason for believing that the enzyme and the acid form a compound, which in turn combines with the proteid undergoing digestion; or, what amounts to much the same thing, that the acid perhaps forms first a compound with the proteid, to which the pepsin can then unite to form a still more complex compound capable of undergoing hydration and cleavage. Pepsin-proteolysis, therefore, is strictly the proteolysis produced by pepsin-acid. In view of this fact, we may well give a moment’s thought to the nature and origin of this acid. Without attempting any statement of the gradual development of our knowledge regarding the acid of the gastric juice, we may accept the now well-established fact that the acid is hydrochloric acid, and that it has its origin in the parietal, or so-called border-cells of the gastric glands. That the acid is derived from the decomposition of chlorides is practically self-evident, but Cahn[94] has added experimental proof which removes all shadow of doubt, through his study of the gastric secretion in animals deprived for many days of salt; the gastric juice in such cases being perfectly neutral in reaction, but normal as regards its content of pepsin. [94] Die Magenverdauung im chlorhunger. Zeitschr. f. physiol. Chem., Band 10, p. 522. The way in which the specific gland-cells manufacture free hydrochloric acid out of material contained in an alkaline medium is somewhat doubtful. There are, however, at the present day two theories worthy of special notice. The first is based upon observations made by Maly[95] many years ago, which tend to show that certain mineral salts present in the blood are capable of reacting upon each other with formation of hydrochloric acid. Thus, while the blood is an alkaline fluid, it really owes its alkalinity to the presence of two acid salts, viz., sodium bicarbonate (HNaCO_{3}) and disodium hydrogen phosphate (HNa_{2}PO_{4}). This latter compound, acted upon by the carbonic acid of the blood, is transformed into a dihydrogen sodium phosphate with simultaneous formation of acid sodium carbonate, as shown in the following equation: Na_{2}HPO_{4} + CO_{2} + H_{2}O = NaH_{2}PO_{4} + HNaCO_{3}. [95] Untersuchungen über die Quelle der Magensaftsäure. Annalen d. Chem. u. Pharm., Band 173, p. 227. This acid sodium phosphate dissolved in a fluid containing sodium chloride, gives rise to free hydrochloric acid by a very simple reaction: