in is one of these and its power to throw off impurities
depends upon its being clean and active.

*Effect of Drugs.*--Certain drugs and medicines, including alcohol and
quinine,(15) have recently been shown to destroy the white corpuscles. The
effect of such substances, if introduced in considerable amount in the
body, is to render one less able to withstand attacks of disease. Many
patent medicines are widely advertised for purifying the blood. While
these may possibly do good in particular cases, the habit of doctoring
one's self with them is open to serious objection. Instead of taking drugs
and patent medicines for purifying the blood, one should study to live
more hygienically. We may safely rely upon wholesome food, pure water,
outdoor exercise and sunlight, plenty of sleep, and a clean skin for
keeping the blood in good condition. If these natural remedies fail, a
physician should be consulted.

*Summary.*--The blood is the carrying or transporting agent of the body. It
consists in part of constituents, such as the red corpuscles, that enable
it to carry different substances; and in part of the materials that are
being carried. The latter, which include food and oxygen for the cells and
wastes from the cells, may be classed as freight. Certain constituents in
the blood destroy disease germs, and other constituents, by coagulating,
close small leaks in the blood vessels. Although subject to rapid and
continuous change, the blood is able--by reason of the balancing of
materials added to and withdrawn from it--to remain about the same in
quantity and composition.

Exercises.--1. Compare blood and water with reference to weight, density,
color, odor, and complexity of composition.

2. Show by an outline the different constituents of the blood.

3. Compare the red and white corpuscles with reference to size, shape,
number, origin, and function.

4. Name some use or purpose for each constituent of the blood.

5. What constituents of the blood may be regarded as freight and what as
agents for carrying this freight?

6. After coagulation, what portions of the blood are found in the clot?
What portions are found in the serum?

7. What purposes are served by water in the blood?

8. Show how the blood, though constantly changing, is kept about the same
in quantity, density, and composition.

9. In the lungs the blood changes from a dark to a bright red color and in
the tissues it changes back to dark red. What