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.
FIG. 3—Diagram of the intestinal
walls. A, lumen of intestine
filled with digested food. B,
villi, containing blood vessels.
C, larger blood vessel, which
carries blood with absorbed food
away from the intestine.
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.
Secondly, we notice that there is one phase of absorption which is still obscure. Part of the food is composed of fat, and this fat, as the result of digestion, is mechanically broken up into extremely minute droplets. Although these droplets are of microscopic size they are not actually in solution, and therefore not subject to the force of osmosis which only affects solutions. The osmotic force will not force fat drops through membranes, and to explain their passage through the walls of the intestine requires something additional. We are as yet, however, able to give only a partial explanation of this matter. The inner wall of the intestine is not an inert, lifeless membrane, but is made of active bits of living matter. These bits of living matter appear to seize hold of the droplets of oil by means of little processes which they thrust out, and then pass them through their own bodies to excrete them on their inner surface into the blood vessels. Fig. 5 shows a few of these living bits of the membrane, each containing several such fat droplets. This fat absorption thus appears to be a vital process, and not one simply controlled by physical forces like osmosis. Here our explanation runs against what we call vital power of the ultimate elements of the body. The consideration of this vital feature we must, of course, investigate further; but this will be done later. At present our purpose is a general comparison of the body and a machine, and we may for a little postpone the consideration of this vital phenomenon.
