Thus the blood contains all the materials that are needed by the various parts of the body to make good the wastes that are constantly taking place.

But if the blood were all in one place it could do little good, as the new materials are needed in every part of the body.

There has been provided a wonderful system of tubes running through every part of the body.

By means of these tubes the blood is carried into every part where it is required.
These tubes are connected with the heart.

When the heart beats, it forces the blood through the tubes just as water is forced through a pipe by a pump or by a fire-engine.
~4.