584 A MANUAL OF PHYSIOLOGY 



For example, the heart of an average man, \vhich weighs 280 

 grammes, contains about 60 grammes of solids, and among these not 

 more than i -5 grammes of glycogen. In twenty four hours it produces, 

 even on a low estimate, at least 190,000 calories of heat (p. 503), 

 equivalent to the complete combustion of a little more than 45 

 grammes of glycogen. To supply this amount, the whole store of 

 glycogen in the heart would have to be used and replaced every 

 40 minutes. But the accumulation of glycogen is immensely slower 

 in the muscles of a rabbit made glycogen-free by strychnia, and there- 

 fore we have to look around for some other source of energy 

 to supplement the glycogen. We have already brought forward 

 evidence (p. 468) that, under ordinary circumstances, not a great 

 deal, at any rate, of the energy of muscular contraction comes from 

 the proteids. Of carbohydrates, the only one except glycogen which 

 is at all adequate to the task of supplying so much energy is the 

 glucose 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 quantity of blood will contain at least 150 grammes of 

 glucose, and about 50 grammes will suffice to supply all the heat 

 produced by the heart. Of proteids a little less than 45 grammes 

 would be needed, of fat 20 grammes. We see, therefore, how 

 intense 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 two thirds 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 half its weight 

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

 T \5- of its weight. 



To sum up : It is universally admitted that carbohydrates 

 can yield energy for muscular work. It has been demon- 

 strated by Zuntz and his pupils and by others that fat can 

 do so. The experiments of Pfltiger, to which we have 

 already alluded (p. 469), have shown than when an animal 

 is fed on lean meat, the muscular work done is far too great 

 to have come from non-proteid substances. We must 

 conclude, therefore, that when carbohydrates 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 proteids, when the carbohydrates and the 

 fats are restricted, and the proteids plentifully supplied. 

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

 muscular energy in isodynamic relation. In other words, a 



