THE MECHANISM OF TISSUE RESPIRATION 167 



bearing upon this question is that obtained by Fletcher in his 

 studies upon the respiration of excised frogs' muscles. A slow 

 current of air or other gas was drawn over the muscle or muscles 

 placed in a suitable chamber, and the C0 2 given off absorbed by 

 baryta and estimated by titration. On tetanising the muscles 

 kept in air or in nitrogen for periods of 10 to 30 minutes, little 

 if any increase in the C0 2 output was observed, provided that 

 the muscles did not, as the result of fatigue, pass into a state of 

 rigor mortis. If this was the case, then the increase of C0 2 

 might be considerable, and the increase observed by Hermann 

 on tetanisation of muscle is probably to be ascribed to this 

 cause. When, on the other hand, the muscles were kept in a 

 current of pure oxygen, not only was the output of C0 2 during 

 the resting stage increased, but on tetanisation it became more 

 than doubled in amount. For instance, five gastrocnemii in a 

 current of nitrogen gave out '05 c.c. of C0 2 per half-hour during 

 rest, and "07 c.c. during tetanisation. Five other gastrocnemii 

 (taken from the opposite limbs of the same five frogs), when kept 

 in oxygen, had at first an output of *o8 c.c. of C0 2 during rest, and 

 •14 c.c. during activity, and later on an output of "095 c.c. during 

 rest and '23 c.c. during activity. It was also noted by Fletcher 

 that fatigue came on much more rapidly in muscles kept in air or 

 nitrogen than in those kept in oxygen, and that fatigued muscles, 

 if allowed to rest in oxygen, recovered much more rapidly and 

 perfectly than if allowed to rest in nitrogen. Still again, he 

 noticed that muscles kept in oxygen never passed into rigor 

 mortis at all. They survived many hours — sometimes as much 

 as fifty hours — longer than muscles kept in nitrogen or air, and 

 then underwent a gradual lengthening instead of shortening. 

 These and other results are most readily explained by supposing 

 that during the resting stage, and still more during activity, 

 there is a continual breaking down of complex molecules in the 

 muscle substance into simpler ones, but that these simpler 

 molecules do not reach their final C0 2 stage except in the 

 presence of free (or intramolecular) oxygen, and that within 

 certain limits the more plentiful the supply of oxygen the more 

 complete and rapid is the oxidation. In an excised gastroc- 

 nemius oxygen can only reach the tissue cells by diffusion from 

 the outside, and hence the inner layers of a muscle kept in 

 oxygen would be very much more adequately oxygenated than 

 those of a muscle kept in air. 



