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 mechanical
principles. The action of the heart--leaving out of consideration for a
moment its muscular power--is that of a simple pump. It is provided with
valves whose action is as simple and as easy to understand as those of
any water pump. By the action of these valves the blood is kept
circulating in one direction. The blood vessels are elastic, and the
study of the effect of a liquid pumped rhythmically into elastic tubes
explains with simplicity the various phenomena associated with the