88 THE VERTEBRATE ANIMAL: METABOLISM 



found (Fig. 49). Substances, in being absorbed, pass through 

 the cellular membrane and into the fluid of these vessels. 



Although certain foods, such as grape sugar, water, and the inor- 

 ganic salts, require no change before they are ready for absorption, 

 there is practically no absorption in the stomach. In the upper 

 part of the small intestine the exposed surface of the mucous mem- 

 brane is greatly increased by the projection of numerous finger- 

 like villi into the cavity. Each villus is covered by mucous mem- 

 brane and has a core of submucosa containing capillaries and 

 lymphatics. It is in this region that by far the greatest amount of 

 absorption occurs. Simple sugars, amino acids, and mineral salts 

 pass directly into the blood stream and are found there and trans- 

 ported as such. The presence of a great deal of ammonia in the 

 cells of the mucous membrane would seem to indicate that some 

 of the amino acids had undergone a chemical change there, but 

 the significance of this is noi clear. The fats and lipins are digested 

 into fatty acids and glycerol in the presence of bile, and the fatty 

 acids combine with certain alkaline constituents of the bile to 

 form soaps. In this form they enter the cells of the mucous mem- 

 brane along with the glycerol. There the soaps are broken down 

 and the fatty acids recombine with the glycerol to form neutral fat 

 droplets, the presence of which can be demonstrated in the proto- 

 plasm of the cells of the mucous membrane. This fat is dis- 

 charged into the lymph vessels of the submucosa. Water is 

 absorbed throughout the length of the small and large intestine. 



No satisfactory theory to account for absorption has yet been 

 formulated. Such a theory must explain the phenomena by which 

 the digestive juices are passed into the cavity of the tract while 

 solutions of the digestion products are passed in the opposite direc- 

 tion into the blood or lymph. Osmosis, or the passing of water 

 through a membrane to equalize concentration of solutions on the 

 two sides of the membrane, would account for the passing of 

 water into the digestive cavity to decrease the concentration of the 

 food mass, but not for the reabsorption of water in other regions 

 of the tract. Diffusion, which is the mixing of substances in such a 

 way that regions of higher and lower concentrations tend to become 

 equal in concentration, would explain the passage of digestion 

 products from the tract, where they are very concentrated, into 

 the circulating fluids which are constantly changing so that equi- 

 librium is not estabhshed. However, it has been found that if the 



