Introductory observations, 1 Early history of gastric digestion, 3 The proteolytic power of the pancreatic juice, 7 The general nature of proteolytic enzymes, 8 Origin of proteolytic enzymes, 8 Preparation of pepsin, 10 Reactions and composition of proteolytic enzymes, 13 The proteid nature of enzymes, 15 Conditions modifying the action of enzymes, 17 The influence of temperature on proteolytic action, 18 The influence of acids, alkalies, and other substances on the activity of enzymes, 20 Action of chloroform on pepsin, 21 Theories of enzyme action with special reference to catalysis, 22 The general nature of Proteids, 27 Classification of proteids, 29 Chemical composition of some of the more prominent proteids occurring in nature, 31 Chemical constitution of proteids, 33 The presence of hemi- and anti-groups in all typical proteids, 34 Cleavage of the albumin-molecule with dilute sulphuric acid, 34 Hydration and cleavage of albumin by the action of superheated water, with formation of atmid-albumoses, etc., 37 Action of powerful hydrolytic agents on proteid matter, 39 Initial action of pepsin-acid on proteids, 40 Scheme of the general line of proteolysis as it occurs in pepsin-digestion, with a view to the structure of the albumin-molecule, 41 LECTURE II. _Proteolysis by pepsin-hydrochloric acid, with a consideration of the general nature of proteoses and peptones._ Proteolysis by pepsin-acid, 44 Formation of hydrochloric acid in the gastric glands, 45 Liebermann’s theory regarding the formation of the acid of the gastric juice, 46 Differences in the action of free and combined acid, 47 Proteolysis in the presence of combined acid, 49 The combining power of various forms of proteid matter with hydrochloric acid, 51 Quantitative estimation of the affinity of the products of digestion for acid, 53