The soluble and insoluble portions of the intestinal contents become separated gradually, and the undissolved part arrives finally at the last stage of its journey along the alimentary canal, and is expelled as the solid excrement, or feces. This is made up of the undigested food and other matter, such as residues from the bile and other digestive juices, mucus, and more or less of the epithelial cells which have become detached from the walls of the stomach and intestines. Dead and living bacteria appear to constitute a considerable portion of the fecal matter. These organisms are not taken in with the food to any great extent, but are the result of their continuous growth in the lower intestines. Small quantities of fermentation products are present, which give to the feces its offensive odor. The incidental or waste products may properly be considered as belonging to the wear and tear of digestion.

Fig. 7. - Lacteals during digestion. (Collins.)

The ordinary conception of the fecal residue is that it is onty the part of the food that has resisted the action of the digestive fluids, but in fact it is much more than that. Not only does it include the Various waste products previously referred to, but also compounds that have been absorbed into the blood circulation and returned to the alimentary canal for excretion. It has been shown that when a phosphorus compound was injected subcutane-ously into a sheep, the phosphorus was excreted in the feces in another combination. It is also proven that mineral compounds absorbed from the intestinal tract may afterwards appear in the feces.