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.]

_Circulation_.--The next piece of mechanism for us to consider in this