must of necessity undergo rapid and continuous change. The red corpuscles,
whose changes have already been noted, appear to be the most enduring
constituents of the blood. The plasma is the portion that changes most
rapidly. Yet in spite of these changes the quantity and character of the
blood remain practically constant.(14) This is because there is a
_balancing_ of the forces that bring about the changes. The addition of
various materials to the blood just equals the withdrawal of the same
materials from the blood. Somewhat as a vessel of water (Fig. 12) having
an inflow and an outflow which are equal in amount may keep always at the
same level, the balancing of the intake and outgo of the blood keeps its
composition about the same from time to time.

*Hygiene of the Blood.*—The blood, being a changeable liquid, is easily
affected through our habits of living. Since it may be affected for ill as
well as for good, one should cultivate those habits that are beneficial
and avoid those that are harmful in their effects. Most of the hygiene of
the blood, however, is properly included in the hygiene of the organs that
act upon the blood—a fact which makes it unnecessary to treat this subject
fully at this time.

From a health standpoint, the most important constituents of the blood
are, perhaps, the corpuscles. These are usually sufficient in number and
vigor in the blood of those who take plenty of physical exercise, accustom
themselves to outdoor air and sunlight, sleep sufficiently, and avoid the
use of injurious drugs. On the other hand, they are deficient in quantity
and inferior in quality in the bodies of those who pursue an opposite
course. Impurities not infrequently find their way into the blood through
the digestive organs. One should eat wholesome, well-cooked food, drink
freely of _pure_ water, and limit the quantity of food _to what can be
properly digested_. The natural purifiers of the blood are the organs of