by uniting in the cells with sugar, fat, and all other substances in the body except water and salt. Oxygen burns or consumes the substances with which it unites, and the process is called oxidation. Hence the cells have to be continually growing and multiplying to repair the tissue and replace the material used up by oxidation. Sugar and flour and fat oxidize, or burn, outside of the body, as well as in it, as can be proved by throwing them into a fire. Water and salt are two foods that do not burn. Hence they can furnish no heat or energy to the body. Water puts out a fire instead of helping it, and so does salt. Throw salt into a fire or on a stove; it will pop like sand, but will not burn.
The cells need the oxygen of fresh air; they need food for the oxygen to unite with, but they are injured by many substances called poisons. Arsenic destroys the red blood cells. Strychnine attacks the nerve cells in the spinal cord. Alcohol attacks the epithelial cells lining the stomach and, when it is absorbed, attacks the nerve cells and other cells. Morphine attacks the nerve cells.
Written Exercises.—Draw a series of seven pictures to show the
seven tissues (Figs. 10, 14, 15, 18, 19). Write the "Autobiography"
of a White Blood Cell (see also pages 59 and 68). The Rewards of
Caring for the Health. Health and the Disposition. Which is more
important, a Thorough Knowledge of Geography or of Physiology?
Five Things which people Value above Health (and lose health to obtain).
The Blessings that follow Good Health. The Tissues Compared
(function, proportion of cells, intercellular material and fibers,
activity, rate of change).
See also pages 50, 116. Pupils should choose their own subjects.
