THE LUNGS AND RESPIRATION. 227 



of the tissues there is no analogy with the stove. In the 

 stove the fuel and the entering oxygen at once combine, 

 combustion occurs, and carbon dioxide is the result. But 

 in the muscle the food and the oxygen are not at once 

 burned, but are in a way, entirely unknown to us, built 

 into living tissue, much as in the manufacture of gunpow- 

 der all the various elements are built up without any com- 

 bustion taking place, until later, when it is purposely 

 ignited. Upon the ignition of the gunpowder it at once 

 breaks up into a number of burned products, not the least 

 one of which is, by the way here, carbon dioxide. The 

 carbon dioxide in the muscles is due to a disintegration of 

 parts of the muscle substance itself. Thus, the amount of 

 carbon dioxide formed in the muscles will vary directly 

 with the amount of muscular work done in the same way 

 as the amount of smoke in battle will vary directly with the 

 number of shots fired. 



To show that carbon dioxide may be formed in a muscle 

 when there is no oxygen present, the following experiment 

 will suffice: If a living muscle, say from a frog, be put in 

 a closed case and all the oxygen withdrawn it will, when 

 properly stimulated, contract as usual, and the production 

 of CO 2 follows. In fact, the absence of even traces of oxy- 

 gen does not seriously affect the muscle for a while, but it 

 keeps on contracting for some time and finally becomes 

 exhausted because its reserve material, not being replen- 

 ished, is gradually used up. If the CO 2 in the muscle were 

 the result of the direct burning of the oxygen as soon as it 

 enters the muscle, such an experiment with such results 

 would be impossible. 



As all living cells are continually at work, if not in 

 giving rise to motion at least in producing heat and in 

 keeping up the bodily temperature, so there is going on 

 continually in our tissues a production of CO 2 . This gas 

 will at once be absorbed by the surrounding lymph in con- 

 formity with the law of Daltoii. Thus there will come to 

 be in the lymph quite a little CO 2 pressure. In the blood 



