(2) Carbohydrates — starches, sugars, gums; e. g., potato-starch, cane or grape sugar. (3) Fats and oils; e. g., suet, marrow, olive oil. (4) Minerals; e. g., water, table salt, iron, phosphates.
Some few substances are absorbed without being digested at all; they do not need to be. Such are water and the minerals, though even many of these un dergo some change. Grape-sugar is ab sorbed and probably proteids too are of ten absorbed to some extent at least. Fat is profoundly modified during digestion, though not as the result of any digest ing ferment. The saliva, of which about 30 ounces are secreted during the 24 hours, contains a ferment termed ptya lin, which is capable of turning a starch into a soluble sugar called grape-sugar, or, according to other observers, into another soluble sugar termed maltose. When the food has reached the stomach and the acid gastric juice has mixed with it, the saliva is unable to act and is probably killed. Any digested starch is subsequently converted into sugar when the food reaches the small intes tines by the pancreatic juice.
When the food reaches the stomach it causes a reflex secretion of gastric juice. This is but slowly produced when insipid heavy food, such as coagulated white of egg, boiled meat, sago, etc., is eaten, but flows readily when soups, broths, and fluids containing salts and extractions in abundance are taken. The gastric juice, several pounds of which are secreted daily, is acid in reaction containing free hydrochloric acid. In addition lactic and butyric acids are formed during the progress of digestion.
Within the small intestines most of the food undigested by the stomach is ren dered fit for absorption. This takes place through the issue of the mucous membrane; much of the sugar and pep tones find their way into capillary blood vessels. Absorbed products and notably fat globules, pass into the lacteals, and thence into the blood, circulating through the veins at the root of the neck.