670 A MANUAL OF PHYSIOLOGY 



rabbit made glycogen-free by strychnine, and therefore we have to 

 look around for some other source of energy to supplement the 

 glycogen. We have already brought forward evidence (p. 537) 

 that, under ordinary circumstances, not a great deal, at any rate, 

 of the energy of muscular contraction comes from the proteins. Of 

 carbo-hydrates, the only one except glycogen which is at all adequate 

 to the task of supplying so much energy is the dextrose of the blood. 

 The quantity of blood passing through the coronary circulation has 

 been estimated at 30 c.c. per 100 grammes of cardiac muscle per 

 minute (Bohr and Henriques), which would be equivalent for an 

 average man to about 120 litres in twenty-four hours. This quan- 

 tity of blood will contain at least 120 grammes of dextrose, and 

 about 32 grammes will suffice to supply all the heat produced by the 

 heart. Of proteins a little less than 30 grammes would be needed, 

 of fat a little more than 12 grammes. We see, therefore, how in- 

 tense must be the metabolism that goes on in an actively con- 

 tracting muscle. On any probable assumption as to the source of 

 muscular energy a quantity of material equal to half of its solids 

 must be used up by the heart in twenty-four hours. Or, to put it 

 in another way, the heart requires not less than two-fifths of its weight 

 of ordinary solid food in a day. The body as a whole requires 

 5\j to J ( -j of its weight. 



To sum lip : It is universally admitted that carbo-hydrates 

 can yield energy for muscular work. It has been demonstrated 

 by Zuntz and his pupils and by others that fat can do so. The 

 experiments of Pfliiger, to which we have already alluded (p. 538), 

 have shown that when an animal is fed on lean meat, the mus- 

 cular work done is far too great to have come from non-protein 

 substances. We must conclude, therefore, that when carbo- 

 hydrates and fats are plentiful in the food, the greater part of 

 the energy of muscular contraction comes from them. It comes 

 on the other hand from proteins, when the carbo-hydrates and 

 the fats are restricted, and the proteins plentifully supplied. 

 Not only so, but these three groups of food substances yield 

 muscular energy in isodynamic relation. In other words, a 

 given amount of muscular work requires the expenditure of 

 approximately the same quantity of chemical energy, whether 

 it comes almost entirely from protein, or chiefly from carbo- 

 hydrates, or chiefly from fat. Some observers have stated that 

 the taking of even a comparatively small quantity of sugar 

 vastly increases the capacity for muscular work as measured 

 by the ergograph (p. 649). The glycogen of the muscle is believed 

 to be converted into dextrose during muscular activity. Dex- 

 trins and maltose, the intermediate products of this decom- 

 position, have been detected in muscle, more maltose, indeed, 

 than dextrose being present (Osborne), since the dextrose is 

 rapidly oxidized. But although it is not to be doubted that 

 sugar is under normal circumstances one of the most important 

 substances used up in muscular contraction, the claim that 



