746 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



cerned in initiating and carrying out the mechanical effect is derived 

 from the decomposition of this precursor, the question would still 

 confront us, What are the materials at the expense of the energy 

 of which the muscle is restored to its original condition ready for 

 another contraction ? If the lactic acid is used over and over 

 again, it is indeed the metabolism of these substances which will 

 be chiefly represented in the waste products given off by the muscle ; 

 the lactic acid complex will merely represent a chemical machine 

 through which the energy of these other substances is transformed 

 into mechanical energy, and they will constitute the ultimate source 

 of energy of the muscular contraction. In this sense the muscular 

 glycogen, whether it yields lactic acid or not, is almost certainly 

 one source of energy for the active muscle, being converted into 

 dextrose, of course, before utilization. Dextrins and maltose, the 

 intermediate products of this decomposition, have been detected 

 in muscle, more maltose, indeed, than 'dextrose being present 

 (Osborne), since the dextrose is rapidly oxidized. Glycogen cannot 

 be the only source of muscular energy, for its amount is too small. 



For example, the heart of an average man, which weighs 280 grammes, 

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

 more than i gramme of glycogen. In twenty-four hours it produces 

 120 calories of heat (pp. 138, 663), equivalent to the complete com- 

 bustion of a little less than 30 grammes of glycogen. To supply this 

 amount, the whole store of glycogen in the heart would have to be used 

 and replaced every fifty minutes. But the accumulation of glycogen 

 is immensely slower in the muscles of a 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. 599) 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 the glycogen 

 of the heart muscle 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 quantity 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. There is no reason to suppose that 

 this dextrose must first be changed into muscular glycogen, which only 

 represents a certain amount of reserve carbo-hydrate. Of proteins a 

 little less than 30 grammes would be needed, of fat a little more than 

 12 grammes. We see, therefore, how intense must be the metabolism 

 that goes on in an actively contracting 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 ^ to ^ of its weight. 



The general conclusions to which physiologists have been led 

 as to the relative importance of the different food substances for 



