Every nerve leaving these centres divides and sub-divides into little threads as the arteries divide and sub-divide into arterioles and capillaries, until every part of the body--every muscle and part of the skin--has its nerve. [Sidenote: Voluntary and reflex action] Every time you _choose_ to move your hand, your brain sends to the necessary muscles an instantaneous order to act. This is called _voluntary action_. If you put your finger against a hot stove you jerk it away before you could have time to _choose_ to do it. This happens as an order from a sub-station and is called _reflex action_. Messages travel both ways and it is necessary that the nervous connection with every part of the body remain unbroken, and important that the nervous condition be kept healthy. NOTES TO THE TEACHER A false delicacy has often prevented the teaching of vital lessons to growing children. The day is how at hand when foolish sentiment must no longer prevent the spread of any knowledge which is necessary to exterminate the plagues that have afflicted the race. In chapters 17, 18, 19 and 32, unpleasant facts are given in plain language. They are facts that parents do not teach their children and that most teachers will not frankly treat with classes in a school. Though they are disagreeable to discuss, they are essential for children of school age to know. Now that the dependence of public health upon personal hygiene is recognized, personal habits and sanitary conditions are more frankly dealt with than formerly. It is an excellent thing occasionally, to have one or two points of an assigned lesson answered in writing. Any topic that the teacher may think advisable to treat in this manner may be discussed on paper at the beginning of the recitation, five or six minutes being allowed for that purpose. Even a whole chapter may be assigned to be studied with a view of answering in writing the questions at the end of the chapter. The written test can then take the place of the usual oral recitation. This method is suggested to the teacher who hesitates to discuss orally certain plain but essential facts. Chapter 32 on the Spread and Prevention of Consumption can be treated with best results orally, if the pupils are first made to _feel_ the great danger of consumption and to realize the possibility of preventing the vast havoc wrought by that dread disease. The attitude of teacher and pupils should be that while the discussion may be disagreeable, the disgusting habits referred to in the text are so commonly practised that unless their dangers are taught this disease can never be wiped out. INDEX Adenoids, 49, 50 Air, amount necessary for health, 45; effects of impure, 45-48; fresh air, when needed, 45-48; how secured, 46, 47; how changed in body, 45; necessity of pure air, 45-48 Air cells, 175