136 SUMMARY OF RESPIRATION. 



plied by the respiration at the time. If we study the chemis- 

 try of explosion, we learn that it is a very rapid combustion. 

 In the explosives are materials that unite instantaneously, 

 instead of slowly burning, as in the case of ordinary com- 

 bustibles. 



Now, many physiologists hold that a sort of explosive com- 

 pound is formed in the muscles, and that when the muscle 

 acts it does so as the result of the explosion, so to speak, of 

 this material. And, to carry out the figure, the nerve is com- 

 pared to the match that ignites the explosive. A little heat 

 is enough to cause the most violent explosion. So the force 

 that passes 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 " 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. 



We may thus sum up 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 motion), 

 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 mechanical 

 respiration. The changes between the air and the blood in 

 the lungs we will call the ventilation of the blood, and the in- 

 teraction of the blood and the tissues the real, or internal 

 respiration. (Compare Figs. 38, 48, and 49.) 



The Two Breaths. " Every time you breathe you 



