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.

[Illustration:  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.]

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