Absorption of food.—­The next function of this machine to attract our attention is the absorption of food from the intestine into the blood.  The digested food is carried down the alimentary canal in a purely mechanical fashion by muscular action, and when it reaches the intestine it begins to pass through its walls into the blood.  In this absorption we find engaged another set of forces, the chief of which appears to be the physical force of osmosis.  The force of osmosis has no special connection with life.  If a membrane separates two liquids of different composition (Fig. i), a force is exerted on the liquids which cause them to pass through the membrane, each passing through the membrane into the other compartment.  The force which drives these liquids through the membrane is considerable, and may sometimes be exerted against considerable pressure.  A simple experiment will illustrate this force.  In Fig. 2 is represented a membranous bag tightly fastened to a glass tube.  The bag is filled with a strong solution of sugar, and is immersed in a vessel containing pure water.  Under these conditions some of the sugar solution passes through the bag into the water, and some of the water passes from the vessel into the bag.  But if the solution of sugar is inside the bag and the pure water outside, the amount of liquid passing into the bag is greater than the amount passing out; the bag soon becomes distended and the water even rises in the tube to a considerable height at a(Fig. 2).  The force here concerned is a force known as osmosis or dialysis, and is always exerted when two different solutions of certain substances are separated from each other by a membrane.  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