132 



HUMAN BIOLOGY 



the tissues affects the nerves. We are rested and strong 

 when these wastes are removed and the tissues are sup- 

 plied with fresh food and oxygen. Work causes the ac- 

 cumulation of carbon dioxid, which is nature s narcotic. 1 

 The drowsy feeling that ensues is more pleasant than the 

 drowsy feeling from alcohol or opium. Those who do 

 not employ nature's narcotic but free themselves of it by 

 hurried, anxious breathing become restless and crave arti- 

 ficial narcotics. 



Fatigue without work occurs with people who are idle. 

 The oxidation in their cells is not complete, and poisonous 

 products of the incomplete burning result. This is known 

 as self-poisoning (auto-toxemia). The poisons are taken 

 by the blood to the nerves and brain, and give a tired feel- 

 ing as effectually as does hard work ; or the food may fer- 

 ment in the food 



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tube and form poi- 

 sons which increase 

 the tired feeling. 

 Such persons are 

 usually irritable, 

 while persons who 

 are fatigued by use- 

 ful labor are likely 

 to be dull and 

 drowsy. 



Headaches are 

 caused by poisons 



in the blood or by pressure of blood congested in the head. 



Like all other pains they should be a source of benefit in 



1 It has been found that it is injurious to rebreathe expired air containing 

 one per cent of carbon dioxid, but a far greater percentage is harmless if intro- 

 duced into fresh air, thus indicating that the injury from poor ventilation 

 comes chiefly from the " crowd poison," or organic particles thrown off. 



Fig. 118. — The Situation of Headaches 

 with reference to their causes. 



