The stomach empties itself in about four hours. The food, in various states of digestion, is then carried forward by the intestinal peristalsis, which is much like the constrictive movement of the stomach. It is carried forward at a rate allowing time for the intermixture of intestinal, pancreatic, and liver juices, for the conversion of food into assimilable form, and for absorption, through the intestinal walls, into the blood. Finally, through the ileo-ca2cal valve, it reaches the large intestine, where absorption, except for water, largely ceases and the waste products are agglutinated with the mucous, bacteria, and epithelial debris from the entire intestinal tract in the form of faeces, a stool or mould of which is evacuated from the body by the act of defecation once, according to the immutable standard of modern civilization, in every twenty-four hours.
The essential process of digestion is a chemical change, a conversion of the complex foodstuffs into a form which can be absorbed into the blood and utilized by the body. The digestive juices which accomplish this change are many in number, each one fitted for the conversion of a particular sort of food – the lipase of the pancreas for fat, saliva for starches, gastric juice for proteins. They are all of the general chemical nature of enzymes. An enzyme is a catalyzer and exerts its properties not by uniting with a given substance to form a new substance, but by exerting an influence. It is a kind of exhorter, leading chemical compounds to turn themselves into something different, if not better. For instance, oxygen and hydrogen brought together will effect no union, but if brought together in the presence of spongy platinum they form water. The platinum does not change at all: it is a catalyzer.
The action of most of these enzymes is astonishingly rapid. You may test this on yourself by chewing a piece of cracker or bread. The saliva, through its digestive enzyme, ptyalin, is amylolytic ; that is, it digests starch exclusively. It does so by breaking down the complex starches into simple sugars.
So, almost five seconds after the bread has been in your mouth, it begins to turn sweet. That rapidly does an enzyme work. The highly complex starch of the bread has been converted into maltose. The digestive enzyme of the stomach is pepsin. It splits only proteins or albuminous foods: meats, eggs, etc. It is secreted by small glands all over the wall of the stomach. It acts only in an acid medium, so that the stomach glands also secrete hydrochloric acid.