168 SCIENCE’ PROGRESS 
As to the intermediate products of muscle activity, there is 
not much definite information, but undoubtedly sarcolactic acid 
is one of them. The recent results of Fletcher and Hopkins 
show that absolutely fresh muscle (of the frog) contains only 
traces of lactic acid (‘015 per cent. or possibly less). Fatigued 
muscle, on the other hand, may contain a considerable amount 
of lactic acid ("11 to ‘21 per cent.), and rigored muscle still more 
(‘24 to ‘40 per cent.). The amount of lactic acid any given muscle 
can develop seems practically constant, whether it be formed 
suddenly by exposing the muscle to a temperature of 45°, or 
more slowly by tetanising the muscle when placed in an atmo- 
sphere of hydrogen or oxygen, and finally inducing heat rigor. 
It is formed gradually in resting excised muscle, if this muscle 
be kept in an atmosphere of hydrogen or nitrogen, z.e. under 
anzrobic conditions, but if kept in air it is formed only very 
slowly, and if kept in oxygen it is not formed at all. On the 
contrary, as might be expected from Fletcher's results on the 
restorative power of oxygen on irritability, if muscles in which 
a considerable lactic acid formation has been induced by 
tetanistion are placed in oxygen, a good deal of this lactic acid 
disappears. Such disappearance of lactic acid was found to be 
dependent on the integrity of the muscle, no diminution showing 
itself in chopped-up muscle. 
Other evidence of lactic acid formation was obtained by 
Spiro, who found that blood collected from the carotid of a 
dog in which the muscles of the hind limbs had been tetanised 
for an hour and a quarter showed quite an appreciable increase 
of sarcolactic acid. Again, Max von Frey and Gruber found that 
if an isolated muscle of the dog were perfused with an artificial 
circulation of blood, and were made to contract, there was a 
considerable increase in the absorption of oxygen, but a by no 
means corresponding increase in the output of CO,. There was 
a Striking rise in the sarcolactic acid in the blood, however, which 
more than accounted for the deficiency of CO,. Presumably this 
acid was not oxidised further in the tissues owing to the abnormal 
conditions of experiment, but it is possible, as suggested by 
Von Frey, that even in the intact living animal the lactic acid 
formed in the muscles may be to some extent oxidised else- 
where. In support of this view may be quoted the observations 
of Meyer, who found that the diminution of CO, production 
induced by arsenic and phosphorus poisoning was accompanied 
