The substances in solution will, under these conditions, pass from the dense to the weaker solution. The process is a purely physical one. [Illustration: FIG. 1.--To illustrate osmosis. In the vessel _A_ is a solution of sugar; in _B_, is pure water. The two are separated by the membrane _C_. The sugar passes through the membrane into _B_.] [Illustration: FIG. 2.--In the bladder _A_ is a sugar solution. In the vessel _B_ is pure water. Sugar passes out and water into the bladder until it rises in the tube to a.] This process of osmosis lies at the basis of the absorption of food from the alimentary canal. In the first place, most of the food when swallowed is not soluble, and therefore not capable of osmosis. But the process of digestion, as we have seen, changes the chemical nature of the food. The food, as the result of chemical change, has become soluble, and after being dissolved it is _dialyzable_--i.e., capable of osmosis. After digestion, therefore, the food is dissolved in the liquids in the stomach and intestine, and is in proper condition for dialysis. Furthermore, the structure of the intestine is such as to produce conditions adapted for dialysis. This can be understood from Fig. 3, which represents diagrammatically a cross section through the intestinal wall. Within the intestinal wall, at _A_, is the food mass in solution. At _B_ are shown little projections of the intestinal wall, called _villi_ extending into this food and covered by a membrane. One of these _villi_ is shown more highly magnified in Fig. 4, in which _B_ shows this membrane. Inside of these villi are blood-vessels, _C_, and it will be thus seen that the membrane, _B_, separates two liquids, one containing the dissolved food outside the villus, and the other containing blood inside the villus. Here are proper conditions for osmosis, and this process of dialysis will take place whenever the intestinal contents holds more dialyzable material than the blood. Under these conditions, which will always occur after food has been digested by the digestive juices, the food will begin to pass through this membranous wall of the intestine into the blood under the influence of the physical force of osmosis. Thus the primary factor in food absorption is a physical one. We must notice, however, that the physical force of osmosis is not the only factor concerned in absorption. In the first place, it is found that the food during its passage through the intestinal wall, or shortly afterwards, undergoes a further change, so that by the time it has fairly reached the blood it has again changed its chemical nature. These changes are, however, of a chemical nature, and, while we do not yet know very much about them, they are of the same sort as those of digestion, and involve probably nothing more than chemical processes.