666 A MANUAL OF PHYSIOLOGY 



is the proportion of energy which appears as work, the smaller 

 the proportion which appears as heat. For every muscle, 

 under given conditions, there is a certain load which can be raised 

 more advantageously than any other ; but even in the most 

 favourable case, an excised frog's muscle never does work equal 

 to more than J of the heat given off. Generally the ratio is 

 much less, and may sink as low as -}-%. In the intact mammalian 

 body the muscles work somewhat more economically than the 

 excised frog's muscles at their best; for both experiment and 

 calculation show (p. 583) that in a normal man under the most 

 favourable conditions as much as one-third of the energy is 

 converted into work. According to Zuntz and Katzenstein, 

 35 per cent, of the total energy appeared as muscular work in 

 climbing a mountain, and in bicycling only 25 per cent. Move- 

 ments which have been much practised are more economically 

 performed than unaccustomed ones, and this explains the 

 superior efficiency of the muscles concerned in climbing, for 

 no movements can possibly be more familiar than those concerned 

 in locomotion. So far as this indication goes, it would seem that 

 in the treatment of obesity unfamiliar, and therefore physiologi- 

 cally expensive, forms of exercise should be recommended, in so 

 far, of course, as they do not injuriously react upon the general 

 condition, especially upon the circulation. 



As a muscle becomes fatigued it works more economically, 

 the heat-production diminishing more rapidly than its working 

 power. This is an illustration of the fact that the heat-pro- 

 ducing mechanism and the work-producing mechanism of muscle 

 are in some respects distinct, and a variation in the activity of 

 the one is not necessarily associated with a corresponding 

 variation in the activity of the other. 



(4) Chemical Phenomena of the Muscular Contraction. We know 

 but little regarding the chemical composition of living muscle, since 

 most chemical operations cause the immediate death of the tissue. 

 The composition of dead mammalian muscle of the striped variety 

 may be stated, in round numbers, as follows, but there arc consider- 

 able variations, even within the same species : 



Water 75 per cent. 



Proteins - 20 ,, 

 Fats, lecithin, and cholesterin- 2 ,, 



Nitrogenous ( Kreatin (' 2 to '4) 



metabolites I Xanthin - "1 Purin 

 [Hypoxanthin, etc. I bodies 



Carbo-hydrates (glycogen, dextrose, and maltose) 

 Lactic acid 

 Inosit 



Salts, chiefly carbonate and phosphate of potassium, less than 

 i per cent. Potassium is absent from the nuclei (Frontispiece) . 



intact body the fraction of the energy transformed into heat is greater in 

 hard than in moderate work. 



