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 CO, 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 CO, 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 CO, 
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 CO, 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 ‘o5 c.c. of CO, per half-hour during 
rest, and ‘o7 c.c. during tetanisation. Five other gastrocnemil 
(taken from the opposite limbs of the same five frogs), when kept 
in oxygen, had at first an output of ‘08 c.c. of CO, 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 CO, 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. 
