RESPIRATION BEYOND THE LUNGS 395 



shows that the contraction is not dependent upon oxidation, but that 

 the oxidation occurs after the contraction is over. The mechanism involved 

 in muscular contraction can not therefore be analogous with that by 

 which energy is liberated in a steam engine by the oxidation of the coal. 

 The mechanism must rather be like that of a spring, which becomes un- 

 wound during the muscular contraction and requires 2 for its rewinding. 



Interesting results corroborative of these conclusions have been se- 

 cured by observations on the heat production of isolated muscles. It 

 was found that heat production occurred after a single shock to the 

 muscles, not only during the contraction, but for a considerable period 

 after it, provided 2 was present. In the absence of 2 this recovery 

 was either greatly delayed or entirely abolished. Such results favor 

 the view that 2 is used largely in the processes whereby the muscles, 

 "like an engine charging an accumulator, synthesize substances con- 

 taining a considerable amount of potential energy, which again, like the 

 accumulator, it discharges when appropriate stimuli are applied" (L. 

 V. Hill, cf. 27). One immediately thinks of lactic acid in connection 

 with these interesting results, for, as has already been stated, Hopkins 

 and Fletcher 29 have shown that this acid is produced in the absence of 

 2 in excised frog muscles, but when 2 is present, it is either not pro- 

 duced or, if so, quickly disappears. 



Heart Muscle. Another- muscle that has been thoroughly investigated 

 in this connection is that of the heart. The gaseous exchange has been 

 studied both on isolated heart preparations and by examining the ex- 

 change in the lungs of a combined lung 'and heart preparation. The 

 most important investigations by the first of these methods are those of 

 Rohde (cf. 27), who arrived at the very important conclusion that the 

 2 taken in by the heart muscle varies directly with the maximal ten- 

 sion set up in the heart by the contraction. This tension was measured 

 by placing a rubber bag in the ventricle and distending it with water at 

 a known pressure. By altering the initial pressure and by observing the 

 pulse rate, it was found that the 2 used by the heart depends on the 

 product of the pulse frequency and the maximal increase in pressure 

 produced by each cardiac contraction; or, in the form of an equation: 



Q 



= a constant quantity ; where Q is the oxygen used, T the maximal 



NT 



increase of pressure at each beat, and N the frequency of the pulse. 



It should be pointed out, however, that constancy in the product of 

 the above equation does not hold under abnormal conditions of the heart- 

 beat. For example, when the pressure in the heart is very high, the 

 amount of 0, required begins to go up out of proportion, indicating that 



