RESPIRA TION 263 



ing series through which it passes down, before the final stage is 

 reached In a normal muscle with intact circulation, while 

 carbon dioxide is given off, certain of the other decomposition 

 products appear, in conjunction with oxygen and some substance 

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

 which breaks down in contraction. When oxygen is not avail- 

 able, as in an atmosphere of nitrogen, carbon dioxide is still 

 given off, but the other decomposition products are not re- 

 generated to contractile substance, but accumulate in the muscle, 

 producing the phenomena of fatigue, and eventually of rigor. 



When muscle goes into rigor (Chap. IX.) and this is most 

 strikingly seen when the rigor is caused by raising the tempera- 

 ture 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 obtain- 

 able during rigor is markedly less if the muscle has been pre- 

 viously tetanized. From this it has been argued that the 

 hypothetical substance, the decomposition of which yields 

 carbon dioxide in contraction, is also the substance which de- 

 composes 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 contraction 

 or in rigor, or partly in the one and partly in the other. Accord- 

 ing 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 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. 



The respiration of muscles in situ can be studied by collecting 

 samples of the blood coming to and leaving them and analyzing 

 the gases. The mere difference of colour between the venous 

 and arterial blood of a muscle, or other active organ, is 

 sufficient to show that oxygen is taken up and carbon dioxide 

 given out by it to the blood. This is the case in muscles at 

 rest, and even in muscles with artificial circulation after they 

 have become inexcitable. In active muscles more oxygen is used 

 up and more carbon dioxide produced than in the resting state. 

 Chauveau and Kaufmann, in their experiments on the levator 

 labii superioris muscle of the horse in feeding, found that the 

 consumption of oxygen and the production of carbon dioxide 

 might be many times as great in activity as in rest. 



Thus in one experiment the amount of oxygen taken in, ex- 

 pressed in c.c. per gramme of muscle per minute, was 0*0079 

 during rest, and 0*14 during work ; the corresponding quantities 



