The call for oxygen by the heart 



91 



The following data taken from the experiment in question 

 illustrate this point : 



It is perhaps even more clearly shown by the following chart : 



Time in periods of 20 minutes 



FIG. 54. Note the correspondence between the increments of oxygen used and of 

 the rate and maximal pressure of the pulse. 



To the work of Hill (8) on the heat given out by frog's muscle, I 

 have already referred ; I must return again to it, for he came to the 

 same conclusion about the heat given out as Rohde had done about 

 oxygen taken in, namely that with any one contraction it varied 

 directly with the maximal tension set up in the muscle. 



For any heart, the law that ^^ is constant, has of course its 



limitations : 



(1) If subjected to very high pressures the amount of oxygen 

 required begins to go up relatively, in other words the heart is losing 

 its efficiency, it is being over taxed. 



