CONTRACTILE TISSUES 449 



know from other lines of investigation, and must be content with referring the reader to 

 the lecture itself. 



There is one point that requires some attention. We know from the work 

 of Ryffel (1909) and others that, in considerable muscular work in man, lactic 

 acid appears in the urine. This must be due to the fact that the oxygen supplied 

 in the blood is insufficient to replace in the muscle system the whole of the lactic 

 acid formed by the vigorous contractions before a part of it is washed away by 

 the blood current. Now this part is lost to the muscle system and must be 

 replaced from some source. It can readily be formed, as we have seen, from 

 glucose or from alanine, but its actual origin is unknown. The fact that the 

 excretion of nitrogen is not increased in muscular work appears to be against 

 the latter possibility. It seems possible that experiments might be made to 

 see whether removal of lactic acid by perfusion of active muscle would or would 

 not affect the final amount to be obtained in heat rigor, and whether there is 

 any evidence of more carbohydrate being used under these conditions. 



In excessive work, as opposed to normal vigorous work, there is evidence of a certain 

 amount of nitrogenous breakdown of the structure itself, as we have already seen (page 272). 



Food Used. Locke and Rosenheim (1904) found that glucose, added to 

 the perfusion fluid of a mammalian heart preparation, gradually disappeared ; 

 but, as Evans (1914, 1, p. 408) points out, this fact does not satisfactorily prove 

 that it was -consumed, since it might have been converted into glycogen or 

 some other substance of less reducing power than glucose. The proof was 

 afforded by Evans himself, in the paper referred to, by showing that the 

 respiratory quotient (see p. 279) was raised by the addition of glucose. Rohde 

 (1910) had shown that the respiratory quotient in the first period, after setting 

 up the preparation, was dependent on the previous diet of the animal, so that 

 it was lower after a flesh and fat diet than after one of carbohydrate. The 

 result was confirmed by Evans, and appears to show that the heart can also 

 utilise fat. Experiments of Palazzolo (1913) show that the fat content of 

 frog's muscles is diminished by tetanisation to exhaustion. If the muscle actually 

 has the power to use fat for energy purposes without previous conversion to 

 carbohydrate, we have a further argument that the oxidation reaction may 

 make use of various combustible materials indifferently, and therefore that the 

 contractile system so built up is not a chemical one. 



Campbell, Douglas, and Hobson (1914) show that muscular work is associated 

 with a rise in the respiratory quotient, which is not merely due to production of 

 lactic acid driving off carbon dioxide, since it remains raised during the per- 

 formance of work, and, on cessation of work, there is at first a further temporary 

 rise to nearly one. It subsequently falls to below normal. This seems to show 

 that carbohydrate is burned. The late fall to below normal might be due either 

 to the carbohydrate store having been exhausted or to formation of new carbo- 

 hydrate from other substances. 



Benedict and Cathcart also (1913, p. 94) find that there is a rise in the 

 respiratory quotient during work ; thus showing that there is an increased 

 consumption of carbohydrate. But the fact that the respiratory quotient very 

 rarely rises to 0-98, as they point out, prevents the conclusion being drawn from 

 these experiments that muscular work is performed exclusively at the expense of 

 combustion of carbohydrate. Moreover, the more severe the work, the heavier 

 is the draft upon the carbohydrate material of the body. So that, in the 

 subsequent resting period, a lower proportion of carbohydrate is burned for the 

 purpose of the total energy output of this period. These workers state that their 

 average results suggest that the energy for muscular work is afforded exclusively by 

 the oxidation of carbohydrate. They were unable to find any evidence of the 

 conversion of fat to glycogen during muscular activity (p. 146). 



Efficiency. We have already seen that the potential energy of tension can 

 practically be all converted to external work, a very small fraction only being 

 degraded to heat in the process. In other words, nearly the whole of the energy 

 is "free." In the engineer's sense, the "efficiency," that is, the proportion of 

 the work done to the total energy change, is nearly 100 per cent. This value 

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