[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 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. [Illustration: FIG. 4.--Diagram of a single villus enlarged. _B_ represents the membranous surface covering the villus; _C_, the blood-vessels within the villus.] [Illustration: FIG. 5.--An enlarged figure of four cells of the membrane _B_ in Fig. 4. The free surface is at _a_; _f_ shows fat droplets in process of passage through the cells.]