ch 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
machine is the device for distributing this fuel to the various parts of
the machine where it is to be used as a source of energy, corresponding
in a sense to the fireman of a locomotive. This mechanism we call the
circulatory system. It consists of a series of tubes, or blood vessels,
running to every part of the body and supplying every bit of tissue.
Within the tubes is the blood, which, from its liquid nature, is easily
forced around the body through the tubes. At the centre of the system is
a pump which keeps the blood in motion. The tubes form a closed system,
such that the pump, or heart, may suck the blood in from one side to
force it out into the tubes on the other side; and the blood, after
passing over the body in this closed set of tubes, is finally brought
back again to be forced once more over the same path. As this blood is
carried around the body it conveys from one part of the machine to
another all material that needs distribution. While in the intestine, as
already noticed (Fig. 3), it receives the food, and now this food is
carried by the circulation to the muscles or the other organs that need
it. While in the lungs the blood receives oxygen, and this oxygen is
then carried to those parts of the body that need it. The circulatory
system is thus simply a medium by which each part of the machine may
receive its proper share of the supplies needed for its action.

Now in this circulation we have again to do with chemical and physical
forces. All of its general phenomena are based upon purely mechanica