~Movements in the Large Intestine.~--The movements in the large intestine or colon are much like those in other parts of the digestive tract. The small and large intestine are divided by a valve known as the ileocecal valve, and any food which passes through it cannot return, since the valve is a competent one. The cells in the walls of the larger intestine secrete fluids of a lubricating character, containing no enzymes of digestion but aiding in moving the fecal matter toward the rectum. ~Distribution of Secretory Cells.~--Secretory cells are distributed in each of the three regions of the stomach, but are more numerous in the middle region than at either end. The third region includes the pyloric vestibule through which all foods must pass before they can enter the small intestine, and terminates in the pylorus, the valve which shuts off the stomach from the duodenum and the rest of the intestinal canal. ~Chemical Digestion.~--The chemical changes in the food materials, after they are eaten, are brought about through the action of certain substances known as soluble ferments or "enzymes." These enzymes exist in every tissue of the body, and their province is first to break down the food materials themselves into simpler compounds, and then to reconstruct the simpler substances into those of a more complex character, which are more available for the various uses of the organism. ~Action of the Enzymes.~--As Sherman has stated, "all fermentation is brought about either directly or indirectly by the activity of animal or vegetable organisms or cells. When the organisms or cells act directly and the chemical changes occur only in their presence, the fermentation is said to be due to an organized ferment. When the action is not brought about directly by the cell itself, but by means of a substance secreted by the cell but acting apart from it, this substance is called a soluble or unorganized ferment or 'enzyme.'" The enzymes concerned in digestion and metabolism, their source and their action, may be found in the following table: TABLE OF ENZYMES[54] SOURCE AND ACTION OF ENZYMES