WHEN the food which we ingest starts on its way along the path of the alimentary tract it is ordinarily regarded as having entered the body. It does, in truth, disappear from sight as soon as it has passed beyond the mouth and into the deeper recesses of the organism; but every one who is familiar with the structure of the long gastro-intestinal tube--the digestive canal--realizes that the walls of the latter offer a pronounced barrier to the ready transport of the swallowed food materials to the various tissues and organs where it may be needed. To follow the nutrients into the stomach and upper intestine is comparatively easy; far more difficult, however, is the task of tracing their passage through the thick walls of the alimentary tract into the lymph and bloodstreams wherein they are distributed far and wide in the body.
Insoluble food particles obviously cannot permeate the mucous membrane that lines the enteric tract. The older physiologists, who concerned themselves not at all with the problem of how such solid nutriment is made available, were content to assume that in some way it must become soluble so that it can filter or diffuse through the gastro-intestinal wall. Some sort of digestion was thus conceived to be essential to absorption in the case of insoluble products such as much of an ordinary meal represents. For a long time it was vigorously debated whether digestion in the