Trypsin completes the digestion of proteids. Steapsin converts fats into fatty acids and glycerin. FIFTH--THE SMALL INTESTINES The intestinal juices secreted by the small intestines are _alkaline_ substances which change sugar and maltose into glucose, and perform the last step in the process of breaking up or subdividing food so fine that it will pass through the intestinal walls into the circulation. LESSON V CHEMISTRY OF DIGESTION [Sidenote: Alternation of digestive juices] The digestive juices of the human body are five in number, namely: Saliva, gastric juice, bile, pancreatic juice, and the several intestinal juices. Beginning with the saliva these juices alternate, first an alkali, then an acid. It is the opinion of the writer that this alternating plan is carried on throughout the entire intestinal tract, as the final dissolution of food matter takes place in the intestinal canal. These five juices are secreted from the blood by special cells or glands. Each of these juices contain one or more enzyms or digestive principles. These enzyms are highly organized chemical compounds which have the property of changing other chemical compounds without being destroyed or used up themselves except in minute quantities. [Sidenote: Malt and yeast-cells] Malt, which was studied in the last lesson, and which is produced by the sprouting of barley, is a true digestive enzym of the barley. Yeast-cells are minute plants which secrete an enzym that causes the fermentation of bread. It was formerly thought that the fermentation of yeast could not take place except in the presence of a living cell. This has now been disproved, as a German scientist has succeeded in grinding up yeast-cells and filtering off the chemical compound or true enzym which causes the fermentation of sugar. [Sidenote: Fermentation due to enzyms] It is now recognized by scientists that all processes of fermentation and digestion found in plant and animal life are due to definite chemical compounds known as enzyms. The action of digestion is truly a chemical one, and could take place without the body as well as within, if we could manufacture the proper enzym and could produce the exact conditions of temperature, moisture, etc., that are found in the human digestive economy. [Sidenote: Predigested foods] The manufacture of predigested foods depends upon various processes of fermentation, or upon the digestion that may be carried on by inorganic chemical agents, such as acids, or by the ferments of bacteria, or other forms of life. The following are illustrations of these processes of predigestion: 1 The manufacture of glucose from starch by the action of sulfuric acid 2 The malting of starch for the production of malt-sugar or of fermented liquors 3 The making of cheese by the action of the enzym rennet which has been extracted from the stomach of a calf