266 RESPIRATION 



while carbon dioxide is given oft, 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. -^ /\/A^ 



When muscle goes into rigor (p. 751) and this is most strike^ 

 ingly 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. 

 Recent work, however, has tended to show that this famous 

 inogen theory has very little foundation. 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 contraction the decomposition does not proceed quite to 

 the formation of carbon dioxide, which in the intact body is after- 

 wards liberated from some more complex carbon-containing waste- 

 product. He considers that the carbon dioxide yielded by excised 

 muscles is really preformed 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. 



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. 



