In these ways the proteids, fats, and carbohydrates of the food are gradually digested, so far as conditions will admit, digestion being practically completed by the time the material reaches the ileocæcal valve at the beginning of the large intestine. Throughout the length of the small intestine absorption proceeds rapidly; water, salts, and the products of digestion passing out from the intestine into the circulating blood and lymph. At the ileocæcal valve, however, the contents of the intestine are practically as fluid as at the beginning of the small intestine, due to the fact that water is continually being secreted into the intestine. In the large intestine, the contents become less and less fluid through reabsorption of the water, and as the propulsive movements of the circular and longitudinal muscle fibres of the intestinal wall carry the material onward toward the rectum, the last portions of available nutriment are absorbed. Finally, in varying degree, certain putrefactive changes are observed in the large intestine involving a breaking down of some residual proteid matter, through the agency of micro-organisms almost invariably present, with formation of such substances as indol, skatol, phenol, fatty acids, etc. These processes, however, in health are held rigidly in check, and count for little in fitting the food for absorption. Digestion, on the other hand, extending as we have seen from the mouth cavity to the ileocæcal valve, is the handmaiden of nutrition, preparing all three classes of organic foodstuffs for their passage into the circulating blood and lymph, and thus paving the way for their utilization by the hungry tissue cells. CHAPTER II ABSORPTION, ASSIMILATION, AND THE PROCESSES OF METABOLISM TOPICS: Physiological peculiarities in absorption. Chemical changes in epithelial walls of intestine. Two pathways for absorbed material. Function of the liver as a regulator of carbohydrate. Absorption of proteid products. Assimilation of food products. Anabolism. Katabolism. Metabolism. Processes of metabolism. Older views regarding oxidation. Discoveries of Lavoisier. The views of Liebig. Theory of luxus consumption. Oxidation in the body not simple combustion. Oxygen not the _cause_ of the decompositions. Oxidation not confined to any one place. Intracellular enzymes. Living cells the guiding power in katabolism. Some intermediary products of tissue metabolism. Chemical structure of different proteids. Decomposition products of nucleoproteids. Relation to uric acid. Action of specific intracellular enzymes. Creatin and creatinin. Relation to urea. Proteid katabolism a series of progressive chemical decompositions. Intracellular enzymes as the active agents.