n the middle. Opposite ends are then
tied, making a loose-fitting loop around the limb. The knot is placed
directly over the blood vessel to be compressed and a short stick inserted
in the loop. The necessary pressure is then applied by twisting the
handkerchief with the stick. Time must not be lost, however, in the
preparation of a suitable bandage. The blood vessel should be compressed
with the fingers while the bandage is being prepared.

*Summary.*--The blood, to serve as a transporting agent, must be kept
continually moving through all parts of the body. The blood vessels hold
the blood, supply the channels and force necessary for its circulation,
and provide conditions which enable materials both to enter and to leave
the blood stream. The heart is the chief factor in propelling the blood,
although the muscles and the elastic tissue in the walls of the arteries
and the valves in the veins are necessary aids in the process. In the
capillaries the blood takes on and gives off materials, while the arteries
and veins serve chiefly as tubes for conveying the blood from one system
of capillaries to another.

*Exercises.*--1. Of what special value in the study of the body was the
discovery of the circulation of the blood?

2. State the necessity for a circulating liquid in the body.

3. Show by a drawing the general plan of the heart, locating and naming
the essential parts. Show also the connection of the large blood vessels
with the cavities of the heart.

4. Compare the purpose served by the chordae tendineae to that served by
doorstops (the strips against which the door strikes in closing).

5. Explain how the heart propels the blood. To what class of pumps does it
belong? What special work is performed by each of its divisions?

6. Define a valve. Of what use are the valves in the heart? In the veins?

7. By what means is pressure from contracting muscles in different parts
of the body made to assist in the circulation?

8. Of what advantage is the elasticity of the arteries?

9. How is blood forced from the capillaries back to the heart?

10. Why should there be a difference in structure between the two sides of
the heart?

11. Following Fig. 23, trace the blood through a complete circulation,
naming all the divisions of the system in the order of the flow of the
blood.

12. If the period of rest following the period of contraction of the heart
be as long as the period of contraction, how many hours i