*70 RESPIRATION 



the immediate um'on of carbon and oxygen, but that a stock of 

 it is taken up by the muscle, and stored in some compound or 

 compounds, which are broken down during contraction, and more 

 slowly during rest, carbon dioxide in both cases being one of 

 the end-products. In a normal muscle with intact circulation, 

 while carbon dioxide is given off, certain of the other decomposition 

 products are supposed, in conjunction with oxygen and some sub- 

 stance rich in carbon, like sugar, to be regenerated into the material 

 which breaks down in contraction. When oxygen is not available, 

 as in an atmosphere of nitrogen, carbon dioxide is still given off, but 

 the other decomposition products are supposed not to be regenerated 

 to contractile substance, but to accumulate in the muscle, producing 

 the phenomena of fatigue, and eventually of rigor. 



When muscle goes into rigor (Chapter XIII.) and this is most 

 strikingly seen when the rigor is caused by raising the temperature 

 of frog's muscle to about 40 or 41 C. there is a sudden increase in 

 the quantity of carbon dioxide given off. Moreover, in an isolated 

 muscle the total quantity of carbon dioxide obtainable during rigor is 

 markedly less if the muscle has been previously tetanized. From this 

 it has been argued that the hypothetical substance (' inogen '), the 

 decomposition of which yields carbon dioxide in contraction, is also 

 the substance which decomposes so rapidly in rigor; that a given 

 amount of it exists in the muscle at the time it is removed from the 

 influence of the blood; and that this can all ' explode ' either in con- 

 traction or in rigor, or partly in the one and partly in the other. 

 However, according to Fletcher, there is no increase in the amount 

 of carbon dioxide given off during tetanus by an excised frog's 

 muscle unless the stimulation is so severe and prolonged as to 

 hasten the onset of rigor. He therefore supposes that in the con- 

 traction the decomposition does not proceed quite to the formation 

 of carbon dioxide, which in the intact body is afterwards liberated 

 from some more complex carbon-containing waste-product. He 

 considers that the carbon dioxide yielded by excised muscles is 

 really performed carbon dioxide, already existing in a state of loose 

 combination, from which it is displaced by the lactic acid formed 

 after excision. There is no reason to suppose that any independent 

 new formation of carbon dioxide occurs within the isolated muscle 

 in the absence of a good supply of oxygen. However this may be, 

 there is good evidence that oxygen is used up in recovery processes 

 after the contraction is over, and that these recovery processes are 

 not completed when oxygen is lacking. Hill's work on the heat 

 production of muscle (Chapter XIII.) has tended to rehabilitate the 

 older conceptions, at least to this extent, that his results are in 

 favour of the view that oxygen is used largely during the recovery 

 after contraction in reactions ' whereby the molecular machine like 

 a steam-engine charging an accumulator builds up bodies contain 



