RESPIRATION 247 



be made to contract many times in this oxygen-free atmo- 

 sphere, and to produce a correspondingly large quantity of 

 carbon dioxide. In mammals the muscles can also be made 

 to contract repeatedly when the dissociable oxygen has, as 

 far as possible, been got rid of from the blood by asphyxiating 

 the animal, although they lose their contractility much more 

 rapidly than the muscles of the frog. This leads us to the 

 very important conclusion that the carbon dioxide does not 

 arise, so to speak, on the spot, from the immediate union of 

 carbon and oxygen. Oxygen is essential to muscular life 

 and action. But a stock of it is apparently taken up by the 

 muscle, and stored in some compound or compounds which 

 are broken down during muscular contraction, and more 

 slowly during rest, carbon dioxide in both cases being one 

 of the end products. It is possible that there is an ascend- 

 ing series of bodies through which oxygen passes up, and a 

 descending series through which it passes down, before the 

 final stage is reached. 



When muscle goes into rigor (p. 585) 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 di- 

 oxide 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 sub- 

 stance (inogen), the decomposition of which yields carbon 

 dioxide in contraction, is also the substance which decom- 

 poses 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.* 



* Fletcher states that during tetanus there is no increase in the amount 

 of carbon dioxide given off by an excised muscle unless the stimulation is 

 so severe and prolonged as to hasten the onset of rigor. He therefore 

 supposes that in the contraction 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. 



