RESTIRA 77 ON. 1 1 1 



along a nerve fiber is slight. But it rouses a great amount of energy 

 that lay dormant in the muscle. It would seem to have "touched off 11 

 a lot of explosive material that was already there, rather than merely 

 started an action that depends on the comparatively slow process of 

 respiration at the time. We cannot follow this theory farther, as it 

 takes us too deep into the study of chemistry in its most difficult 

 branch, physiological chemistry. 



Summary of Respiration. The tissues need oxygen ; 

 air is pumped into the lungs ; this air gives oxygen to the 

 blood ; the blood carries it to the tissues. 



In oxidizing, the tissues produce energy (heat and mo- 

 tion) and give off waste matter (water, carbon dioxid, etc.); 

 these the blood carries to the lungs, the lungs give them 

 to the air, and the air carries them out of the body. 



The pumping of the air in and out may be called " me- 

 chanical respiration." The changes between the air and 

 the blood in the lungs we will call the " ventilation of the 

 blood," and the interaction of the blood and the tissues 

 the "real, or internal respiration." 



The Two Breaths. " Every time you breathe you 

 breathe two different breaths ; you take in one, you give 

 out another. The composition of these two breaths is 

 different. Their effects are different. The breath which 

 has been breathed out must not be breathed in again. "- 

 KINGSLEY. 



Breathing Expired Air. The air in the vesicles re- 

 ceives from the blood carbon dioxid, water vapor, and 

 other impurities above mentioned. It has been believed 

 for a number of years that the organic impurities consti- 

 tute the most dangerous element in expired air. Carbon 

 dioxid, though to some extent a poison, is not very injuri- 

 ous in such quantities as ordinarily exist in the air, even in 

 poorly ventilated rooms ; while the headache and drowsi- 



