om a
single point, the heart, and to this point is continually returning.

18 The heart at first glance seems to bear little resemblance to the
pumps in common use. When it is remembered, however, that any
contrivance which moves a fluid by varying the size of a cavity is a
pump, it is seen that not only the heart, but the chest in breathing
and also the mouth in sucking a liquid through a tube, are pumps in
principle. The ordinary syringe bulb illustrates the class of pumps
to which the heart belongs. (See Practical Work.)

19 The contraction of the heart is known as the _systole_ and its
relaxation as the _diastole_. The systole plus the diastole forms
the so-called "cardiac cycle" (Fig. 18). This consists of (1) the
contraction of the auricles, (2) the contraction of the ventricles,
and (3) the period of rest. The heart systole includes the
contraction of both the auricles and the ventricles.

20 Martin, _The Human Body_.

21 The pressure maintained by the left ventricle has been estimated to
be nearly three and one half pounds to the square inch--a pressure
sufficient to sustain a column of water eight feet high. The
pressure maintained by the right ventricle is about one third as
great. In maintaining this pressure the heart does a work equal to
about one two-hundredth of a horse power.

22 The location of the heart in the thoracic cavity causes movements of
the chest walls to draw blood into the right auricle for the same
reason that they "draw" air into the lungs.

23 Active exercise through short intervals, followed by periods of
rest, such as the exercise furnished by climbing stairs, or by short
runs, is considered the best means of strengthening the heart.

24 Nosebleed in connection with any kind of severe sickness should
receive prompt attention, since a considerable loss of blood when
the body is already weak may seriously delay recovery.

25 Newton, _Practical Hygiene_.

26 On account of its position in the body, the lymph is not easily
collected for examination. Still, nearly every one will recall some
experience that has enabled him to see lymph. The liquid in a water
blister is lymph, and so also is the liquid which oozes from the
skin when it is scraped or slightly scratched. Swelling in any part
of t