INTRODUCTION 



15 



by uniting in the cells with sugar, fat, and all other sub- 

 stances 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 ob- 

 tain). The Blessings that follow Good Health. The Tissues Com-, 

 pared (function, proportion of cells, intercellular material and fibers, 

 activity, rate of change). 



See also pages 50, 1 16. Pupils should choose their own subjects. 



