CHAP. IL] RESPIRATION. 579 



On the other hand, it will be remembered that in speaking of 

 muscle, we drew attention ( 61) to the fact that a frog's muscle 

 removed from the body (and the same is true of the muscles of 

 other animals) contains no free oxygen whatever; none can be 

 obtained from it by the mercurial air-pump. Yet such a muscle 

 will not only when at rest go on producing and discharging a 

 certain quantity, but also when it contracts evolve a very con- 

 siderable quantity, of carbonic acid. Moreover this discharge of 

 carbonic acid will go on for a certain time in muscles under 

 circumstances in which it is impossible for them to obtain oxygen 

 from without. Oxygen, it is true, is necessary for the life of the 

 muscle : when venous instead of arterial blood is sent through the 

 blood vessels of a muscle, the irritability speedily disappears, and 

 unless fresh oxygen be administered the muscle soon dies. The 

 muscle may however, during the interval in which irritability is 

 still retained after the supply of oxygen has been cut off, continue to 

 contract vigorously. The supply of oxygen, though necessary for 

 the maintenance of irritability, is not necessary for the manifesto,* 

 lion of that irritability, is not necessary for that explosive decom- 

 position which developes a contraction. A frog's muscle will 

 continue to contract and to produce carbonic acid in an atmo- 

 sphere of hydrogen or nitrogen, that is, in the total absence of 

 free oxygen both from itself and from the medium in which it is 

 placed. 



Thus on the one hand the muscle seems to have the property 

 of taking up and fixing in some way or other the oxygen to which 

 it is exposed, of storing it up in its own substance in such a 

 condition that it cannot be removed by simple diminished pressure 

 (so that the pressure of oxygen in the muscular substance may be 

 considered as always nil), and yet has not entered into any distinct 

 combination which we can speak of as an oxidation, but is still 

 available for such a purpose. The idea has been put forward that 

 the oxygen in this condition is physically attached to and lies 

 between the molecules of the muscular substance without being 

 chemically combined with them, and hence has been spoken of as 

 " intra- molecular " oxygen ; but we have no exact knowledge as to 

 what its condition really is at this stage. On the other hand the 

 muscular substance is always undergoing a decomposition of such 

 a kind that carbonic acid is set free, sometimes, as when the 

 muscle is at rest, in small, sometimes, as during a contraction, in 

 large quantities. The oxygen present in this carbonic acid, as an 

 oxidation product, comes from the previously existing store of 

 which we have just spoken. The oxygen taken in by the muscle, 

 whatever be its exact condition immediately upon its entrance 

 into the muscular substance, in the phase which has been called 

 ' intra-molecular,' sooner or later enters into a combination, or 

 perhaps we should rather say, enters into a series of combinations. 

 We have previously urged ( 30) that all living substance may be 



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